Development of Efficient Designs of Cooking Systems. III. Kinetics of

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Development of Efficient Designs of Cooking Systems. III. Kinetics of Cooking and Quality of Cooked Food, Including Nutrients, Anti-Nutrients, Taste, and Flavor Rekha S. Singhal,*,‡ Aniruddha B. Pandit,*,† Jyeshtharaj B. Joshi,*,†,|| Shirish B. Patel,*,†,§ Sanjay P. Danao,†,^ Yogesh H. Shinde,† Ajitkumar S. Gudekar,† Nisha P. Bineesh,‡ and Kavita M. Tarade‡ †

Department of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai
400019, India Department of Food Engineering and Technology, Institute of Chemical Technology, Matunga, Mumbai
400019, India § Land Research Institute, Second Floor, United India Bldg., P.M. Road, Mumbai
400001, India Homi Bhabha National Institute, Anushaktinagar, Mumbai
400094, India

)

‡

ABSTRACT: Part III of the series on cooking systems presents a qualitative description of cooking methods such as open pan cooking, pressure cooking, steam cooking, solar energy-based cooking, microwave cooking, etc. A large number of chemical and physical changes occur during the process of cooking. These changes have been comprehensively covered in published literature including some textbooks. An attempt has been made to discuss a brief coherent description regarding the changes occurring in starches, proteins, fats, etc. The kinetics of the cooking reaction has also been investigated. This information can be advantageously employed for developing a protocol for an optimum temperature
time program. Because the cooking process is practically thermally neutral, a good scope is available for the optimization of energy supply. It was also thought desirable to understand the kinetics of degradation of proteins, vitamins, anti-nutrients, and flavors in different cooking practices, including microwave ovens and pressure cookers. The mechanism of cooking of rice and lentils has been described. The cooking process involves first the transfer of water from bulk to the particle surface, where the resistance for transfer is provided by a thin film in the vicinity of grain (rice and lentils) surfaces. Second, water has to transfer from the external surface to swollen cooked mass to uncooked core. Finally, on the surface of the uncooked core, the cooking reaction occurs. All published literature regarding this mechanism has been systematically analyzed, and the procedure has been given regarding the rate controlling step(s) and the estimation of the overall rate of cooking. For this purpose, the mathematical models have been given and methods have been described for the quantitative evaluation of the model parameters. A substantial amount of additional work is needed on the mechanism of cooking and suggestions have been made for future research.

1. INTRODUCTION Part I1 of the series on cooking systems was related to the development of cooker design. Part II2 consisted of the design optimization of the cooker geometry using computational fluid dynamics (Section 2) and experiments on cooking of rice and lentils (Section 3). It also dealt with optimum selection of gas burners and first level selection of renewable energy sources. Thus, after considering the various aspects of the cooker design in Parts I and II, it was thought desirable to analyze the actual cooking process. Accordingly, Section 2 of this paper briefly overviews the various cooking practices that are commonly employed. Section 3 of this paper describes the phyisco
chemical changes occurring during cooking of some major constituents such as carbohydrates, proteins, and fats and also some minor constituents such as vitamins, minerals, anti-nutrients, pigments, etc. Section 4 of this paper is concerned with quality of cooked food including nutrients, anti-nutrients, taste, and flavor. The kinetics of change during the cooking process of these constituents has been reported in the published literature. Here, we have carefully analyzed the published information and presented it in a coherent theme. r 2011 American Chemical Society

Finally, Section 5 deals with the kinetic description of change occurring at the particle level of rice and lentils. The classical chemical engineering approach has been adopted for the determination of the rate controlling step(s) during the cooking process that includes external mass transfer, diffusion through the swollen cooked layer, and finally, the physico
chemical changes occurring on the surface of the uncooked core. All of the published literature has been analyzed, and recommendation have been made for future work.

2. COOKING PRACTICES Various heating methods ranging from three stone fires to electrical, microwave, and induction ovens have been used over the years for cooking. Depending on the method used, the efficiency of operation may vary from 10% to 15% for conventional cooking to levels up to 80% for thermally efficient cooking devices. Special Issue: Nigam Issue Received: November 11, 2011 Accepted: December 19, 2011 Published: December 19, 2011 1923

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Industrial & Engineering Chemistry Research Biomass-based cooking is a major method followed in rural areas. The inefficient cooking operation (along with poor combustion characteristics) gives rise to the problem of indoor air pollution. The SPM (suspended particulate matter) levels in a household are at much higher levels (4700 μg/m3) than the levels recommended (140 μg/m3) by the CPCB (Central Pollution Control Board, India). The highest impact is on the health of women and children under the age of 5 years as they tend to spend more time indoors. Following is a brief discussion of cooking practices other than roasting or frying. In developing countries, where 75% of the population resides in rural areas, the open pan cooking method is still the most widely used process for cooking. As discussed later, the thermal efficiency of fuel combustion in this method is much lower compared to that of combustion of LPG or natural gas. Pressure cooker development has improved thermal efficiency in cooking considerably. However, because of safety issues, its size is limited to cooking volumes of about 10 L, and only rarely are larger size pressure cookers available. Recently, in some locations, steam cooking has been used with steam generated either by conventional methods or by using a renewable source like solar energy. Steam use can be either live (directly introduced in the food to be cooked) or as an external heating medium using a jacketed vessel. The better option for large-scale cooking would be a continuous cooking process because it is always preferred over batch processes. 2.1. Open Pan Cooking. A mixture of rice and water is placed in a vessel open to the atmosphere, usually with a lot of excess water. This method is still a major way of cooking rice in rural areas. It is also used in various places like hostels, prisons, hospitals, commercial canteens, religious places, schools (midday meal program), hotels, etc. Rice during cooking typically absorbs water in a proportion of 2 or 3 times its own weight. However, most of the time, this ratio in open pan cooking is nearly 1:5. The excess water is then drained off by pouring the cooked rice into perforated vessels (loss of vital nutrients), and the cooked rice is spread to get a freeflowing character (if required). There is unnecessary wastage of fuel in this cooking method. The low efficiency of cooking is mainly due to two reasons, namely, loss of heat to the surroundings during the heat supply and use of excess water. The loss of heat to the surroundings is from many sources, such as the direct loss from the cooking vessel through its top liquid surface, side walls, and water evaporation losses. In almost all cases, the flame from the burner (or any other source) covers the whole bottom of the cooking vessel and in some cases comes up the sides as well. Thus, the flue gases leaving the vessel are at a quite high temperature. A substantial amount of heat can be recovered from these hot flue gases if a proper ratio of flame size to cooking vessel size is maintained to improve heating efficiency. 2.2. Pressure Cooking. The method of pressure cooking is common in urban areas and also to some extent in semi-urban areas. It uses the simple principle of the boiling point elevation of water. Also, the water quantity used is the same or marginally less as required by rice, unlike the open pan method that uses excess water. This results in a lower loss of water from the cooking material compared to the open pan method and hence results into higher thermal efficiency. Though there is a reduction in water loss, it is not prevented completely. Water loss occurs through the steam going out during the whistles at the end of the cooking process. Three whistles are most commonly used for normal cooking, while more are used for the cooking of hard to

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cook material such as lentils. Also, in the case of pressure cooking, heat losses can be high from the hot walls of the cooking vessel. For faster cooking, high ratios of combustion are employed, which further results in a loss of thermal efficiency as described previously. The performance of a pressure cooker can be improved through a few simple measures. Once the temperature of the cooking material reaches the required value, cooking takes place if it is maintained at that level for some more time. For this purpose, a relatively low amount of energy is required, otherwise heat supplied during this period is practically lost to the surrounding. Hence, ideally, one can switch off the gas at the commencement of the first whistle and cover the cooker with insulating material for the rest of the cooking period. In practice, switching off the gas may not work because of the difficulty of providing adequate insulation. Instead, a fuel savings can be obtained by reducing the burner flame to the simmer position after the commencement of the first whistle. 2.3. Steam Cooking. Steam cooking is used where the scale of cooking is large enough where the use of a boiler is justified. In the case of steam cooking, there are a few advantages over previously described methods. Most often an industrial boiler is used for steam generation. These are available with optimized high thermal efficiency (>80%). The heat transfer step from steam to the cooking vessel and then to the food may be avoided if the steam is directly fed to the cooking material (live steam); however, this would require use of better quality water and also maintaining good hygiene. Solar energy-based steam cooking is an important area of development in cooking and is described briefly in Section 4 of Part II.2 2.4. Continuous Cooking. Continuous cooking processes offer advantages over batch processes. However, for it to be practically feasible in the case of rice cooking, the scale has to be large. Though large scale cooking is not uncommon, the continuous process is seldom used in practice. Interestingly, various methods for cooking of grains (especially rice) on a continuous basis have been patented. These methods and some recent development have been described in Section 4 of Part II.2 2.5. Solar Energy-Based Cooking Devices. Among the renewable energy sources, solar energy is widely used for cooking purpose after biomass-based fuels. It eliminates hazardous emissions like carbon monoxide, nitrous oxides, hydrocarbons, and suspended particulate matter that are typical of combustion of fuels like coal and wood. Solar cooking also helps in reducing the rate of depletion of natural resources for combustible carbon (trees). Some details pertaining to this subject are given in Section 4 of Part II.2 2.6. Microwave Cooking. Almost all foods contain a substantial proportion of water. Structurally, water has a negatively charged oxygen atom that is separated from positively charged hydrogen atoms, which forms an electric dipole. On application of microwaves, dipoles in water and some ionic components in food such as salts attempt to orient themselves to the electric field. The rapidly oscillating electric field from positive to negative, several million times per second causes the dipoles to follow them, generating a lot of frictional heat of water in the process. The increase in temperature of water molecules heats the components surrounding the food by conduction and/or convection. The distribution of water and salt within a food has a major effect on the amount of heating, although the shape of food particles also plays an important role. Microwave penetration is a function of loss factor and frequency of microwave, and it 1924

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Industrial & Engineering Chemistry Research increases dramatically when water changes its phase to ice. This has implications in microwave thawing. Microwave energy has received considerable attention from researchers for a wide spectrum of applications. The fundamentally unique mode of heat transfer from the source to the sample is the main benefit of utilizing microwave energy; direct delivery of energy to microwave-absorbing materials minimizes and/or avoids conventional issues such as long heating periods, thermal gradients, and energy lost to the system environment.3 Dielectric properties determine the behavior of the materials when subjected to high frequency or microwave fields in dielectric heating processes or cooking. A review of the techniques used for measurement of the dielectric properties of food materials is a useful guide in this respect.4 Since the advent of microwave ovens and microwave cooking, significant efforts have been directed on documenting the nutritional profile of foods subjected to this processing. Microwave oven technology has been improved by the use of low power. With the utilization of low-power techniques, studies showed equal or better retention of nutrients for microwave reheating of foods compared to conventional reheated foods with respect to thiamin, riboflavin, pyridoxine, folacin, and ascorbic acid.5 Microwave cooking has been recommended for processing legumes, not only for improving nutritional quality (better retention rates of both B vitamins and minerals, reduction in the level of anti-nutritional factors as well as an increase in in vitro protein digestibility), but also for reducing cooking time. With respect to the whole soaking and cooking processes, the best conditions, which result in minimum vitamin loss, are 9 h soaking in 0.1% citric acid solution or in water and subsequent microwave cooking of the seeds.6 In conventional meat and fish processing, the smoke generated during cooking is a source of heterocyclic amines (HCA), which are known to be highly mutagenic and carcinogenic. Microwave cooking is believed to suppress HCAinduced breast cancers, and a school of thought suggests that these should be encouraged.7 However, from the microbiological safety point of view, microwave heating has been reported to be less effective than conventional heating because of non-uniform distribution of heat, resulting in cold spots and reduced heating time. Furthermore, microwave cooking instructions frequently are based on time but not necessarily on cooking temperature.8

3. PHYSICO
CHEMICAL CHANGES IN COOKING PROCESS The destruction of many vitamins, aroma compound, and pigments by heat follows a similar first-order reaction to microbial destruction, generally expressed as the values DR (decimal reduction time or the time required to destroy 90% of the organisms) and z (slope of the graph obtained from a plot of DR value versus temperature and defined as the numbers of degree Celsius required to bring about a 10-fold change in a DR value). The DR and z values are used to characterize the heat resistance of a microorganism and its viability on temperature. Heat processing is a major cause of nutritional and structural changes in foods. Gelatinization of starches, coagulation of proteins, and the consequent improvement in digestion and destruction of anti-nutritional compounds are generally observed during cooking. On the other hand, destruction of heat-labile vitamins, reduction in biological value of proteins due to the destruction of amino acids or Maillard reaction, and promotion of lipid oxidation is also a reality during heat processing and

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cooking. The nutritional significance of the loss of vitamins must be viewed in light of the level originally present in the food and the recommended daily allowance (RDA). Foods containing 10 times the RDA and showing 90% loss are as acceptable as 90% loss of a vitamin present in a food at very low levels, e.g., 2% of RDA. 3.1. Major Food Constituents. 3.1.1. Carbohydrates. Carbohydrates9 range from simple sugars such as monosaccharides and disaccharides to complex polysaccharides such as starch. Simple sugars such as monosaccharides and disaccharides leach out in significant amounts in the cooking water as observed in techniques such as blanching, boiling, and canning. During cooking, partial breakdown of starch to sugars or conversion of sugars to syrups (in the presence of water) or brown caramelized products (in the absence of water) is commonly observed. Starch is a semi-crystalline material consisting of a linear polymer (amylase) and branched polymer (amylopectin). Approximately 70% of the starch is regarded as amorphous and 30% as crystalline. The amorphous part contains mainly amylose and also a considerable portion of amylopectin, while the crystalline portions mainly consist of amylopectin. On suspending in cold water, starch granules swell in diameter by 10
40%. If this suspension is heated, irreversible changes occur at a certain temperature called the gelatinization temperature, which is characteristic of each type of starch and ranges from 50
70 C. This results in a steep rise in the viscosity of starch suspension due to absorption of water to the extent of 20
40 g water per gram of starch. On heating with water, starch undergoes an irreversible loss of crystalline regions of starch and is termed as “gelatinization”. The crystalline regions melt due to hydration. In this process, hydrogen bridges between the glucose chains in the crystalline regions of the starch are primarily disrupted, and some disruption also takes place in the amorphous regions. The swelling of the amorphous regions causes leaching of the amylase from the crystallites, which are then destabilized.10 High sugar concentrations decrease the rate of starch gelatinization. Lipids and emulsifiers complex with amylose and retard the swelling of starch granules.11 A food is a dispersion in which starch granules and/or granular remnants constitute the dispersed phase. The degree of gelatinization achieved by most commonly used food processes, however, is sufficient to permit the starch to be rapidly digested. Even food processes that result in a low degree of gelatinization (e.g., steaming and flaking of cereals) are digested in a manner similar to that with completely gelatinized foods. Starches, when cooked in water alone or in an emulsion, form in the presence of fat and are responsible for thickening a range of soups, sauces, and gravies in all cuisines. The stability and nature of the pastes and the exact demand of the product for a specific nature and type of paste is the motivation for the development of range of modified starches in the food industry. The effect of other ingredients in the food, for instance, salt, sugar, and/or acid, play a cumulative role and further complicates the rheological and textural behavior of starch pastes and hence the products derived. Molecular changes in starches occur during food processing. For instance, the extrusion processing parameters of pasta making is reported to affect the molecular size distribution of starch samples. In particular, the elution peak shifted toward lower fraction numbers with increasing extrusion temperature, showing a higher molecular size for starch after extrusion cooking. However, all the differences detected between starch 1925

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Industrial & Engineering Chemistry Research samples according to extrusion conditions were deleted during cooking.12 Factors such as the material
water ratio, cooking temperature, cooking time, soaking time, and heat dehydration time influence the digestibility of starches, as has been shown recently with pea starch. The digestibility was evaluated with respect to resistant starch, slowly digestible starch, and rapidly digestible starch.13 Processing conditions and techniques such as cooking, extrusion, and tempering alters the content of resistant starch (RS) of cereals and cereal products. In addition to processing, lipids, dietary fiber, and sugars, which may be present within the food matrix itself, impacts the starch digestibility.14 Besides, interaction of protein and carbohydrate-based materials gives the well-known Maillard reaction, which is responsible for flavors in a range of foods, color in fried and roasted foods, and loss of amino acids such as lysine which in turn diminish the nutritional value of the protein. Interactions of starch with lipids also show a significant effect on starch structure and consequently the quality attributes of products during and after cooking. Defatted flours tend to have more single starch granules, higher gelatinization enthalpy, more water absorption, longer pasting peak time, higher peak and hold viscosity, and lower setback and breakdown than those of the control. The defatted flours show improved cooking stability and better ability to withstand mechanical shear stress.15 Besides starches, other carbohydrates or polysaccharides that constitute “dietary fiber” such as cellulose, pectins, hemicelluloses, pentosans, and many other variants show differences on heat processing. These are generally associated with the cell wall architecture and may dissociate with each other or undergo varying degrees of depolymerisation, which in turn has an effect on the texture, rheology, palatability, and also the nutritional profile of the products. 3.1.2. Proteins. Proteins are chains of amino acids linked together by amide linkages and give a shape to the protein through weak hydrogen bonds. These bonds are broken on heating and cause denaturation of proteins. Denaturation denotes a reversible or irreversible change of native confirmation of proteins without cleavage of covalent bonds (except for disulfide bridges). Denaturation is reversible when the peptide chain is stabilized in its unfolded state by the denaturing agent and which can return to its native form on removal of the denaturing agent. Irreversible interaction, on the other hand, occurs when the unfolded peptide chains are stabilized by interaction with other chains, as seen in egg boiling. The other physico
chemical changes that occur during denaturation of proteins are aggregation of the peptide chains and destruction of a highly ordered structure, which are characteristic of each protein type. For more details, the readers are referred to standard textbooks in food chemistry.10 Proteins give a distinct physical structure to a number of foods such as the fibrous structure of muscle tissues, porous structure of bread, and gel structure of dairy and soy products. The denaturation of proteins that occur during cooking could harden the texture as in albumen of the egg white or make the texture softer and friable as in meat. Denatured proteins are more readily digested by proteolytic enzymes in the body. In some cases such as sorghum, cooking reduces digestibility of sorghum kafirins through disulfide-mediated polymerization principally among the gamma- and beta-kafirin proteins found at the periphery of the protein bodies. This can be addressed by certain pretreatments, for instance, application of high pressure (300 MPa for

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5 min) before cooking.16 Although proteins are not lost during cooking, overcooking foods containing protein can destroy heatsensitive amino acids (for example, lysine) or make the protein more resistant to digestive enzymes. Interactions of proteins with other constituents during cooking have a direct effect on the texture of food products. During nixtamalization (a process for the preparation of maize, corn, or other grain in which the grain is soaked and cooked in an alkaline solution, usually limewater, and hulled; the term can also refer to the removal of pericarp from grains such as sorghum via an alkali process), the combined effects of lime on starch, zein polymerization, and the formation of calcium
zein bonds during cooking yield a stronger and more elastic gel structure.17 Heating sugars with proteins or fats can produce advanced glycation end products called as glycotoxins. These have been linked to aging and health conditions such as diabetes. 3.1.3. Fats. During cooking, fats can reach temperatures beyond the boiling of water and serve to transfer heat by conduction to food, as observed in frying and saut
eing. Fats undergo deterioration during cooking (hydrolysis, splitting, etc.), even in techniques requiring less time, as in microwave cooking. The changes are most evident during deep-fat frying. Autoxidation resulting in the formation of peroxides and its subsequent breakdown to a range of compounds, isomerization of fatty acids, formation of cyclic monomers, dimerization, and polymerization of triglycerides, and formation of volatile compounds responsible for both desirable and off-flavours (depending on the type and product) are some of the changes that place during deep-fat frying. Although chemical analysis could be used to quantify these changes, they are better done with instrumental techniques such as differential scanniorimetry that are expressed on the basis of endo- and exothermic peak temperatures.18 3.2. Minor Constituents. Because the DR and z values are higher for minor constituents such as vitamins, aroma compounds, and pigments as compared to those for microorganisms and enzymes, nutritional and sensory properties are better retained at higher temperatures and shorter exposure times during heat processing. 3.2.1. Vitamins. The following brief description is based on the extensive work reported by Garrow et al.19 Vitamins are constituents required for normal physiological functioning that cannot be synthesized or partly synthesized in the body and whose requirements have to be met from food alone. They are either water (vitamins belonging to the B group and C group) or fat soluble (vitamins A, D, E, and K). The effect of heat processing is most deleterious on vitamins. 3.2.1.1. Fat-Soluble Vitamins. In the absence of oxygen and high temperature, which occurs during cooking or food sterilization, the main reactions of vitamin A and its precursor carotenoids are isomerization and fragmentation. In the presence of oxygen, oxidative degradation leads to the formation of a range of volatiles and parallels lipid oxidation. Dehydrated foods are particularly sensitive to oxidative degradation of vitamin A and carotenoids. In some instances, isomerization of the carotenoids is observed during cooking, as has been shown recently with pumpkin puree.20 However, cooking does enhance the absorption of carotenoids (precursors of vitamin A in foods) compared to raw foods. The amount of nutrients that can be released from food products (i.e., nutrient in vitro bioaccessibility) is often studied as it is a starting point for investigating nutrient bioavailability, an indicator for the nutritional value of food products. In a 1926

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Industrial & Engineering Chemistry Research study of carrots, the importance of mechanical breakdown and mastication and thermal process prior to the particle size reduction show structural changes that determine the all-E-β-carotene bioaccessibility.21 Vitamin D is also sensitive to oxygen and light, while losses of vitamin E occur mainly during deep-fat frying. On the other hand, vitamin K is destroyed by light and alkali. 3.2.1.2. Water-Soluble Vitamins. Most water-soluble vitamins leach into the cooking water and could further degrade by oxidation. The most sensitive vitamins are ascorbic acid (vitamin C), folates, and thiamine, but losses of other vitamins can occur under specific conditions. The stability of thiamine is influenced by pH, temperature, ionic strength, and metal ions. Thermal degradation of thiamine initially yields thiazole and pyrimidine derivatives that are responsible for the meat-like aroma in cooked foods. The thiazole ring is thought to further degrade to products such as elemental sulfur, hydrogen sulphide, a furan, a thiophene, and a dihydrothiophene. The reactions leading to these products are not very clear, but it is anticipated that an extensive degradation and rearrangement of the thiazole ring must be involved.22 It is also inactivated by nitrite, probably through interaction with the amino group attached to the pyrimideine ring. Losses caused by sulphite in foods are pH-dependent. Thiamine is known to be extremely stable under acidic conditions. Riboflavin or vitamin B2 is stable during normal food handling proceses but is light sensitive, more so in the visible spectrum from 420 to 560 nm. Light exposure cleaves the ribitoil from riboflavin and converts it to lumiflavin. Pyridoxine or vitamin B6 reacts with cysteine during milk sterilization to an inactive thiazolidine derivative. It is believed that this reaction may also account for its loss in other heattreated foods. Biotin, pantothenic acid, and niacin are comparatively quite stable during food processing. Vitamin B12 is stable at high temperatures but in the pH range of 4
6. However, under alkaline conditions or in the presence of reducing agents such as ascorbic acid or sulphites, the vitamin is destroyed to a great extent. Factors affecting degradation of ascorbic acid are enzymes, trace metals such as copper, heat, alkaline conditions, salt and sugar concentration, and atmospheric oxygen. Degradation beyond dehydroascorbic acid to 2-3-diketo gulonic acid leads to irreversible loss of this vitamin. In the presence of amino acids, ascorbic acid and its degration products enter into a Maillard reaction and give browning, as has been reported in citrus juices and dried foods. Ascorbic acid degradation is used as a general indicator of food processing. For a more in-depth mechanism, the readers to referred to textbooks in food chemistry.10 3.2.2. Minerals. Garrow. et al.19 have addresed the subject of minerals in detail. They have found that the heat treatments by themselves have little effect on minerals in foods. Losses can occur by leaching into the cooking medium, but the major effects are on organic components that may restrict the bioavailability of minerals such as iron, calcium, and zinc. Heat treatment may allow phytases to hydrolyze phytates and release the minerals that are bound to phytic acid. The bioaccesibility of certain toxic metals such as cadmium23 and mercury24 are lowered with cooking, although its content in mussel flesh and fishes does increase with cooking. This is a beneficial effect. The arsenic concentration in cooked rice is reported to depend on the cooking methods with parboiled rice being relatively prone to arsenic contamination compared to untreated rice if contaminated water is used for parboiling and cooking.25

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3.2.3. Anti-Nutrients. Anti-nutritional factors may be heat labile or heat stable.26 Heat stable components such as estrogens, saponins, phytate, polyphenols, and allergens may not be significantly affected by cooking, except possible leaching into cooking water to varying degrees. Even with heat labile ones, complete inactivation may not always be possible. An understanding of the mechanisms of toxic action of such components has propelled the development of novel approaches to minimize their adverse effects.26 3.2.4. Pigments. Many naturally occurring pigments are destroyed during heat processing, chemically altered by changes in pH, or oxidized during storage. These pigments may be oil soluble (for instance, carotenes, bixin, canxanthin) or water soluble (for instance, anthocyanins, betalains, chlorophyll, curcumin, xanthophylls, among many others) and may have varying degrees of stability to heat, light, oxygen, and pH. While chlorophyll and anthocyanins have high stability to heat, light, and oxygen, they have poor stability to pH. On the other hand, carotenes have moderate to low stability to heat, light, and oxygen but high stability to change in pH. There are four types of pigments founds in foods: (i) chlorophyll, the green pigment, (ii) carotenoids, the yellow, orange, and red pigments, (iii) anthocyanin, the red, blue, and purple pigments, and (iv) anthoxanthin, the white pigment.27 The anthocycnins and anthoxanthins are chemically described as flavonoids. It is important to preserve as much natural color as possible when cooking vegetables because favorable consumer acceptance of a product is on the basis of color. Hence, visual quality is as important as its flavor or nutritional value. Pigments are compounds that give vegetables their color. Different pigments react in different ways to heat and acids and to other elements that may be present during cooking. Chemically, chlorophyll is a porphyrin ring containing four pyrrole rings with magnesium in the center. One of the pyrrole rings is esterified with phytol alcohol and another one with methanol. Displacement of the magnesium ion causes irreversible changes in chlorophyll, which can happen in the presence of acidic conditions, whereas zinc and copper produces a dull olive green pigment called pheophytin. Alkaline conditions during cooking displaces the phytol and methanol group and converts it to water soluble bright green chlorophyllin. However, this causes extensive loss of texture, ascorbic acid, and thiamine. Many other reactions may occur due to the functional side group of the chlorophylls, the isocyclic ring that may be oxidized to form allomerized chlorophyll, and the rupture of the tetrapyrrole ring to form colorless end products. These reactions occur to a lesser extent than pheophytinization during heat processing28 Carotenoids are unsaturated molecules and may be classified as carotenes (no oxygen) and xanthophylls (containing oxygen). These readily undergo oxidation, causing not only loss of color but also producing ‘off flavours’ that are highly undesirable. Anthocyanins are derivatives of a basic flavylium cation and are composed of an aglycone (an anthocyanidin) esterified to one or more sugars. The flavylium nucleus is electron deficient and hence highly reactive, most of which result in decolourization of pigments. These are pH-sensitive and can also be altered during cooking in nonstainless preparing tools. The decolourization of anthocyanins is accelerated in the presence of ascorbic acid, amino acids, phenols, sugar derivatives and other constituents and is caused by condensation reactions with these compounds. The polymers and degradation compounds produced in these reactions are quite complex. An example of a such compound is 1927

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Industrial & Engineering Chemistry Research

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Table 1. Food Constituents and Sources Selected in the Study food constituent

source

justification

rate constant, k

activation

reference for

(min
1) at 100 C

energy kJ/mol

analytical method

0.0028

17.11

Nisha et al.33

0.060

18.37

Bineesh et al.36

39.54

Nisha et al.34

vitamins vitamin C

amla

has a pH ∼3.0 and is one of

drumstick

has a pH ∼6.5
6.7, and is a

the richest sources of vitamin C (Moringa thiamine

recognized source of vitamin C

olifera L.) leaves red gram splits (Cajanus cajan L.)

riboflavin

pH 4.5

a lentil consumed almost daily in the

0.0156

pH 5.5

form of cooked legumes in various

0.0167

44.77

pH 6.5

forms having a pH range of 4.5
6.5

0.0200

41.09

consumed in various forms such as boiled

0.0069

29.8

Nisha et al.37

0.0032

21.72

Nisha et al.38

provides significant quantities of niacin

0.0025

16.7

Nisha et al.42

an important legume consumed worldwide;

0.0057

15.74

Nisha et al.39

0.0019

27.44

Nisha et al.40

0.00187

37.57

Nisha et al.35

green gram (Vigna radiate L.)

dry bean, dhal curry (green gram splits boiled to porridge like consistency), sprouts, noodles, and fried dhal (green gram splits); contains highest riboflavin content among the legumes

spinach

popular leafy vegetable consumed all over the world; is a rich source of riboflavin

niacin

potato tubers (Solanum tuberosum L.)

folic acid

cow peas (Vigna catjang L)

is an important source of folic acid

natural food colors lycopene

tomatoes

sensitive to thermal and oxidative degradation;

(Lycopersicon esculentum)

also a very potent nutraceutical

puree chlorophyll

spinach (Spinacea

an important leafy green vegetable popular all over the world

oleracea L.) betalains

red beetroot

Nisha et al.40

a natural source of red color used worldwide;

(Beta vulgaris L)

also has nutraceutical benefits

texture 1

potato cubes (1 cm3)

2

green gram whole

3

red gram splits at pH (i)

commonly consumed food

0.224

items all over India

4.5, (ii) 5.5, and

79

0.1018

60

(i) 0.0823, (ii) 0.0871,

(i)28, (ii) 27,

(iii) 0.0864

Nisha. P.,40 Nisha et al.41

(iii) 34

(iii) 6.5 flavor 1

black pepper

(i) 0.0014 for piperine, (ii) (i) 66.8 for piperine, (ii) Nisha et al.43

a popular spice used all over the world and used in culinary practices

0.0104 for oleoresin

27.14 for oleoresin

0.0127

31.66

Tarade et al.45

(i) 0.0132, (ii) 0.0128

(i) 30.50, (ii) 22.93

Tarade et al.46

antinutrients and food additives saponins

soyabean flour

β-N-oxalyl-2
3-

Lathytus

(Glycine max.) diaminopropionic

satiivus at pH

acid (ODAP)

(i) 4.0, (ii) 9.2

have an ability to hemolyse red blood cells used as thickening agents in food pastes and sauces and additive as food satiely and delayed glycaemic responses 1928

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Industrial & Engineering Chemistry Research phlobaphens which is brownish-red in color (Francis28). Most changes in anthocyanins are seen in canned foods, and hence lacquered cans are preferred during heat processing such products. White pigments, called flavones, are the primary coloring compounds in potatoes, onions, cauliflower, and white cabbage and in the white parts of such vegetables as celery, cucumbers, and zucchini. These flavones remain white under acidic conditions and turn yellow under alkaline. Cooking for a short time, especially in a steamer, maintains color, flavor and nutrients. However, overcooking or holding too long in a steam turns white vegetables dull yellow or gray. Many naturally occurring pigments are destroyed during heat processing, are chemically altered by changes in pH, or oxidized during storage. These pigments may be oil soluble (for instance, carotenes, bixin, canxanthin) or water-soluble (for instance, anthocyanins, betalains, chlorophyll, curcumin, xanthophylls among many others), and may have a varying degrees of stability to heat, light, oxygen and pH. While chlorophyll and anthocyanins have high stability to heat, light and oxygen, it has poor stability to pH. On the other hand, carotenes have moderate to low stability to heat, light and oxygen but high stability to pH change. 3.2.5. Miscellaneous. These include constituents such as toxicants, flavors, and phytochemicals. Although present in minor amounts, they have a significant effect on nutritional quality. Pulses are a rich source of isoflavones that are implicated in many physiological effects. However, these are completely eliminated by soaking and cooking. In contrast, the saponins present in the same pulses are much less affected.29 Cooking has also been shown to modify the total antioxidant capacity of meats, a fact that has been explained on the basis of factors such as denaturation and exposure of reactive protein sites, degradation of endogenous antioxidants, and the formation of Maillard reaction products having antioxidant properties.30 Dioxins can be generated during some cooking processes, such as burning, or when cooking with reactive organic chlorides. The dioxins are more likely to be present in the smoke (gas phase) than the edible portion (solid and liquid phases). Thus, more attention should be given to cooking of raw foods and organic chlorine-containing flavorings at high temperature. Maintaining good ventilation during cooking is also necessary to reduce human exposure risk to dioxins.32 Toxic substances such as veterinary drug residues do not decrease to any significant extent during cooking, as has been shown recently with studies on beef processing.31

4. QUALITY OF COOKED FOOD, INCLUDING NUTRIENTS, ANTI-NUTRIENTS, TASTE, AND FLAVOR 4.1. Preamble. The main objectives of this study are as follows: 1 To determine the kinetic parameters of degradation33
47 of vitamins, color, and softening of texture in various food systems over a temperature range of 50
120 C (steady state temperature). The vitamins and the food sources chosen along with their justification are listed in Table 1. 2 To study the degradation kinetics of the constituents indicated in Table 1 following different cooking methods (unsteady state process). 3 To develop a mathematical model relating the kinetic data, to estimate the order of the degradation reaction and the required activation energies, to estimate the order of the steady state temperature and time
temperature profiles of

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different cooking methods (unsteady state process), and to predict the loss of the target compound. 4 To apply this model to predict the degradation of the constituents as indicated in Table 1 for an unsteady state heating process from the time
temperature data of the unsteady state heating process (during cooling) and compare it with the actual degradation values. 4.2. Food Values. 4.2.1. Vitamins. Most vitamins degrade to some extent during heat processing; the most liable are vitamin C and thiamine, which undergo losses during normal food processing conditions. The basic function of the thermal process wherein the temperature of the product is brought above ambient temperature is as follows: • To eliminate or reduce microorganisms (both pathogenic and spoilage) and/or deactivate the enzymes (polyphenol oxidases, lipoxygenases)47 • To develop an appropriate texture (attributed to starch gelatinization, protein denaturation, cellulose crystallization) and evolve flavor (e.g., caramelisation of sugars)48 • To destroy undesirable anti-nutrients The objective of the thermal process must be to maximize the retention of nutrients by optimizing the processing conditions. Generally fat soluble vitamins are more stable than water soluble vitamins, and hence, the latter were chosen for the experimental work. Besides, there are many constituents that accelerate the degradation of a vitamin and also some that are protective toward the same vitamin. Hence, the degradation kinetics of the same component varies from food to food. Degradation of vitamins during cooking is a function of many variables, one of which is pH. In order to understand the role of food components, degradation kinetics of the pure vitamin at the concentration found in the foods selected was also undertaken. 4.2.2. Colors. The color of a food is due to the presence of natural pigments present. The strength and perception of the color is affected not only by the actual concentration of pigments but also by the physical structure of the food and the way in which light is scattered from the surface. Heating can alter the physical structure, resulting in brightening of the color, as observed in the case of green vegetables when first brought to boil. However, on prolonged heating, the color starts degrading. Pigments are confined in the food stuff, within a physical system such as a membrane. Complexing with other biological constituents will slightly modify the color. Heat processing will alter the nature of this complex and in turn causes chemical changes that affect color. Food pigments can be divided into four groups, namely, tetrapyrroles, carotenoids, flavonoids, and betalains. The most important natural colorants include chlorophylls, carotenoids, betalains, anthocyanins, curcuminoids, etc. Apart from the sensory attribute, some natural colors such as carotenoids (lycopene) and anthocyanins are important nutritional antioxidants, and their presence in the diet may reduce the risk of cardiovascular disease, cancer, and other diseases associated with aging.60 Therefore. kinetic studies with respect to color degradation are important form both the sensory and nutritional point of view. In this work, we have chosen three important colors belonging to different groups, chlorophyll (tetrapyrrole), lycopene (carotenoids), and betalains for the study on degradation kinetics 1929

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Industrial & Engineering Chemistry Research 4.2.3. Texture. For a long time, texture has been recognized as a quality attribute contributing to the enjoyment and acceptance of foods. Texture can be defined as “the way in which the structural components of a food are arranged in a micro and macro structure of the food material and the external manifestations (perception) of this structure”. The characteristics of perceived texture are determined by different physical and physico
chemical properties of the food and by the unique and complex features of human sensory systems. Because texture is one of the most important food product quality parameters, it was thought desirable to study the texture development kinetics during thermal processing. Potatoes, green gram (whole), and red gram splits purchased from local market of Mumbai were washed and processed. The potatoes were cut into cubes of 1 cm3 size before processing in the same way. 4.2.4. Flavor. In many food processing techniques, spices are added prior to processing. They undergo similar heat exposure as the food constituents in the food to which they are added. An example for studying flavor degradation was that of black pepper. The constituents chosen for analysis were piperine, the main active principle in pepper responsible for pungency, and oleoresins, which is an indication of total volatiles and nonvolatiles that can be extracted by a suitable solvent. 4.2.5. Anti-Nutrients. Saponin glycosides present in a wide variety of plants have the ability to hemolyse red blood cells. They are known to be relatively heat stable. A kinetic model for the degradation of saponins in soybean flour (Glycine max.) was studied, and the parameters are listed in Table 1. A non-protein neurotoxic amino acid, b-N-oxalyl-L-2,3-diaminopropionic acid (ODAP), found in Lathyrus sativus (grass pea or chickling vetch) seeds is also known to be relatively heat stable. The present study aims at development of a kinetic model for the degradation of ODAP in Lathyrus sativus subjected to a defined set of processing conditions. This study was carried out at pH 4.0 and 9.2. The degradation of ODAP was adequately modeled by the Arrhenius type of equation, and related parameters are listed Table 1. 4.3. Heat Treatments. Heat treatments were carried out at different temperatures (50, 60, 70, 80, 90,100, and 120 C) for 0
60 min using a water bath as the heating device. For 120 C, an autoclave was used. The protocol for all the constituents chosen in the study was similar. In some cases, the degradation of the selected constituent in a under isolated condition was also studied. For instance, for ascorbic acid, the study was undertaken as follows: • A solution of 686 mg of ascorbic acid/100 mL at pH 3.0 (simulating amla) and studied as equivalent to amla shreds. • A solution of 200 mg of ascorbic acid/100 mL at pH 6.5
6.7 (simulating drumstick leaves) was prepared and studied as above. This was thought desirable because many times other constituents are known to change the heat stability of the target compound, i.e., ascorbic acid on its own is much more heat liable compared to the ascorbic acid present in amla with its other constituents. 4.3.1. Analytical Methods. The methodology used for the selection of constituents in the study (summarized in Table 1) are as reported in the appropriate references. 4.3.2. Cooking Studies. For cooking studies, normal open pan cooking (20 min), pressure cooking (15 min), and cooking in a newly developed fuel-efficient cooker (30 min on simmer corresponding to a gas flow rate of 4.5 mL/sec followed by a

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Figure 1. Temperature profile for different cooking method used for amla and drumsticks leaves.

30 min retention period) were selected as three different cooking methods. The time and flow rates for the cooker (described in Sections 2
7 in Part I and Section 3 in Part II) were selected as per the instructions given for its usage.56. The 1:4 (w/v) ratios of the samples (both amla and drumstick leaves, taken separately) to water were taken and cooked using the above-mentioned three different cooking methods. Samples were withdrawn periodically and analyzed for ascorbic acid. Time
temperature data for each cooking method were monitored using a thermocouple. 4.3.3. Estimation of Kinetics. Kinetic calculations were carried out on the basis of the following procedure: A general reaction rate expression for degradation kinetics can be written as follows5153
d½C=dt ¼ k1 ½Cn

ð1Þ

where C is the quantitative value of the concentration of the constituent selected for degradation study, k1 is the reaction rate constant, and n is the order of the reaction. For the case of n = 1, integration of eq 1 can be written as lnð½C0 =½Ct Þ ¼ k1 t

ð2Þ

where [C]0 is the concentration of the constituent under consideration at time 0, and [C]t is the concentration after reaction time t (heat processing time). The relationship of reaction rate to temperature was quantified by the Arrhenius relationship.
   ð3Þ k ¼ A0 exp ð
Ea = R  TÞ where Ea is the activation energy of the reaction, R is the gas constant, T is absolute temperature in K, and A0 is a preexponential constant. For the various constituents, the values of degradation rate constant and the activation energies are listed in Table 1. 4.3.4. Time
Temperature Data of the Three Modes of Cooking. To extend the results obtained from steady state experiments to the unsteady state encountered in the three modes of cooking, open pan cooking, pressure cooking, and the use of the present cooker, time
temperature data during the processing of each was recorded (Figure 1). 4.4. Degradation Profile and Half-Life Values of Food Constituents under the Three Modes of Cooking. This is exemplified by the degradation of ascorbic acid in this discussion. The logic and planning was similar for all the other constituents 1930

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Table 2. Degradation Profile and Kinetics of Vitamin C in Amla with Different Cooking Methods method of cooking

time (min)

vitamin C

rate constant, kb

T1\2

concentration

(min
1)

(0.693/k)

vitamin C method of

kb
1

T1\2 (0.693/k)

cooking

(min)

(mg/100 g)

(min ) (R2)

(min)

175 ( 05

0.041 (0.988)

17.00

open pan

5

175 ( 05

0.041 (0.988)

17.00

0.100 (0.953)

06.93

0.050 (0.932)

14.00

5

135 ( 04

20

076 ( 06 168 ( 08

cookinga

pressure

5

cookinga

10

120 ( 10

pressure

15

061 ( 15

cookinga

20 30

rate constant,

(min)

10

10

concentration

(R )

cookinga

cookinga,c

time

(mg/100 g)

open pan

2

Table 3. Degradation Profile and Kinetics of Vitamin C in Drumstick Leaves at Different Cooking Methods

184 ( 13

0.100 (0.953)

0.050 (0.932)

06.93

14.00

128 ( 17 066 ( 09

Values are mean ( SD of three of more individual determinations. b The ascorbic acid content of the amlas chosen in the study was 686 mg/ 100 g. c The readings were taken after a 30 min holding period.

cookinga,c

a

evaluated in the study. Ascorbic acid degradation was followed in each of these modes of cooking for amla and drumstick leaves under steady state conditions. In any cooking process, the total degradation is the overall effect of time at specified temperature and temperature. Table 1 lists the values of rate constant and activation energy. Using these values and knowledge of the temperature profile, the rate of degradation can be estimated with respect to time. Eventually, these rates can be integrated over the time of cooking to obtain the overall degradation. The results are listed in Tables 2 and 3. It is shown that overall degradation remains practically the same in all three methods of cooking, although they do differ for different food constituents.

5. KINETICS OF COOKING PROCESS 5.1. Preamble. For cooking of rice and lentils, an adequate amount of water is added to it, and then heat is supplied to this mixture in a such a way that the cooking occurs at desired rates. During the cooking process, physico
chemical changes occur that are described in Section 3. The minimum cooking temperature for rice is >74 C and that for lentils is >94 C. The cooking process at an individual particle level can be described by the shrinking core model54,55, which is applicable for fluid
solid reactions where the solid phase undergoes a chemical reaction. A similar model can be used for the cooking process with some modification to include particle swelling. The cooking process involves (a) the transfer of water from the bulk to the particle surface, where the resistance for transfer is provided by a thin film in the vicinity of grain (rice and lentils) surfaces. (b) Second, water has to transfer from the external surface through the swollen cooked mass to the uncooked core. (c) Finally, on the surface of the uncooked core, the cooking reaction occurs. These three steps are schematically shown in Figure 2. The knowledge of the rate controlling step(s) and the estimation of respective rates is very important for an efficient design of cooking devices. This subject has received some attention in the published literature, and the following is a brief summary of the previous research. 5.2. Previous Work. Sujuki. et al.56 studied the cooking rate of rice over the temperature of 75
100 C. They have also investigated the effect of raw rice soaking time on the cooking

10

135 ( 04

20

076 ( 06

5

168 ( 08

10

120 ( 10

15 10

061 ( 15 184 ( 13

20

128 ( 17

30

066 ( 09

a

Results are mean SD of three determinations. b Calculated from semilog plot of ln C0/Ct vs t. c Held for 30 min as per the protocol recommended for cooking with cooker.

rate of rice. For experimentation they have used the rice to water ratio as 1:1.4. Rice was presoaked for 30 min before each cooking experiments. Extent of cooking (degree of starch geletanisation) was measured by parallel plate plastometer (rheological method) compared to the chemical or enzymatic methods, as the plastometry is easy for quick measurements. They found that the cooking of rice follows first order kinetics. Further, the cooking process is controlled by the rate of geletanisation of starch below 110 C [step (c) described above] and beyond 110 C it is controlled by the diffusion of water through the cooked portion of rice grain [step (b) described above]. The rate of geletanisation reaction was found to be first order with a value of Activation energy 19 kCal/mol. For the process of diffusion through swollen cooked layer, the activation energy was found to be 8 kCal/mol. The authors have developed the cooking-core model and have derived the rate equations for the reaction and the diffusion controlled operations. They have found the values of rate constants and the diffusion coefficients. The latter values should have been used for estimation of activation energy in diffusion controlled operation, but they have not done so and hence this work gives an incomplete mathematical and/or physical description of the process in the form of their model. Sujuki et al.57 also investigated the soaking rate over a temperature range of 8
50 C and a cooking rate over 70
98.5 C. For all the experiments, they have used an excess amount of water. Though a rheological method is more convenient, it could not be used for the soaking rate measurement because of the hardness of rice grains. Thus, the weighing method was employed for quantitative monitoring of the soaking and cooking process. They found that the values of the diffusion coefficient at 8
50 C were much smaller compared than that of 110
150 C. This is obviously because of the difference in the diffusion rates of water between the uncooked rice grain and the cooked rice grain. Juliano and Perez58 repeated the same experiments as Sujuki et al.57 over the temperature range of 80
120 C. They have characterized the rice grains before experiments in terms of physical and chemical properties such as length, width, thickness, weight per 100 grains, amylose content, alkali spreading value, 1931

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Figure 2. Schematic of concentration profile for water: (A) external mass transfer, (B) chemical reaction, (C) diffusion of controlling operations.

and crude protein content. The rice to water ratio was chosen depending on the amylose content of the rice. The hardness of the grain was measured and converted to the extent of cooking. They have reported that the cooking process is controlled by the reaction rate below 90 C and by diffusion through the swollen layer above 90 C. The value of activation energies were found to be 18
28 kCal/mol in the temperature range of 80
100 C and 7
13 kCal/mol in the temperature range of 100
120 C. These findings are consistent with that of Sujuki et al. except that the transition temperature was found to be 90 C compared with the value of 110 C reported by Sujuki et. al.56 Chakkravarthi et al.59 have measured the cooking rates of the unsoaked and presoaked rice. They have the varied rice to water ratio over a wide range. They have not characterized the rice grain in terms of its physical and chemical properties. Moisture content was measured to determine the extent of cooking. They have reported that the cooking of rice follows first-order kinetics. When cooking is performed with presoaked rice and excess water, the kinetic data can be fitted with a single rate constant However, in the case of unsoaked rice, two rate constants were needed for describing the process of cooking. Further, the cooking was also performed in an electrical rice cooker, and the rate constants were estimated for both unsoaked and presoaked rice. The authors have noted an interesting observation with respect to the ratio of water used. When an optimum amount of water is used (just sufficient for cooking), the number of rate constants were found to be more by one compared with the number of rate constants in the presence of excess water. It was speculated that the mechanism during cooking with optimal water is different from that with excess water during cooking, and the observations hold for both cases of unsoaked and presoaked rice. Bakalis et al.60 have used the finite element method to study the cooking process of parboiled rice at 100 C. The moisture transfer rate in a single rice grain was simulated using Ficks second law of diffusion. They have used Cosmol (Stockholm, Sweden) commertial finite element software for the simulations. Experimental results were found to be in good agreement with the prediction of numerical simulation. The value of effective moisture diffusivity was found to be 7  10
7 m2/s for the case of the water ratio and temperature range. The foregoing discussion brings out a definite need for additional work. First, the step of external mass transfer needs

to be assessed as a possible rate controlling step. Further, the description of diffusion through the swollen mass needs some additional work. In view of this, it was thought desirable to develop rate equations for all three rate-controlling steps. The resulting rate equations will be used for the analysis of all the data reported in the published literature. 5.3. Shrinking Core/Swollen Particle Model. 5.3.1. External Mass Transfer Controlled. The rice particle has been assumed to be spherical to simplify the analysis; however, it can easily be extended to a cylindrical case. The concentration diagram for the diffusion controlled operation is shown in panel (A) of Figure 2. Radius of the rice grain increases and that of the uncooked core decreases as cooking progresses. Volumes of completely uncooked grain, uncooked core, and completely cooked rice can be calculated with the following equations V ¼ V0 þ ðVe
V0 ÞX

ð4Þ

VC ¼ V0 ð1
XÞ

ð5Þ

Above volumes can be converted to the particle radius (assuming the maintenance of sphericity) as follows ( RG ¼

 3 )1=3 RC Re 3
ðRe 3
R0 3 Þ R0

The cooking rate is also given by    d 5 4 4 3 3 Fw πRG
πRC RCR ¼ dt 7 3 3 Mw

ð6Þ

ð7Þ

In the above equation, the factor 5/7, represents the fact that cooked rice grain can absorb water that is 2.5 times the weight of the uncooked rice grain. If the cooking rate is limited by the transfer of water from bulk to the surface of the grain, then the resistance is offered by the thin film in the vicinity of the grain. The cooking rate in this case RCR can be expressed by the following equation RCR ¼ kSL CB ð4πRG 2 Þ

ð8Þ

Equating eqs 7 and 8 and integrating it from time 0 to t and from radius RG to Rc and using eq 6, we get the variation in the 1932

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Figure 3. Plot of t/τ verses time for a chemical reaction controlled operation (eq 22), where t/τ = ({Re3
(Re3
R03)(RC/R0)3}1/3
RC)/Re: (1) 75 C, (2) 80 C, (3) 90 C, (4) 100 C.

radius of the uncooked core with respect to time. 1 !0" #1=3 15Fw Re 3 ðRe 3
R0 3 Þ 3 3 @ t ¼ RC
R0 A Re
R0 3 21kSL CB Mw Re 3
R0 3

Figure 4. Plot of t/τ verses time, for diffusion through the swollen layer controlled operation (eq 17), where t/τ = {Rc2
R02/2 + 1/2(Re3
R03/R03)[(Re3
(Re3
R03)(RC/R0)3)2/3
(R03)2/3]}/{
R02/2 + 1/2(Re3
R03/R03)[Re3]2/3
(R03)2/3]}: (1) 110 C, (2) 120 C, (3) 130 C, (4) 150 C.

Simplification of eq 14 gives dRC ¼ dt

ð9Þ

9 8 > > > > > > > > > > > > = 7Mw Dm ðCC
CB Þ< 1 2( 3
 3 )  3 1=3 > > Re > > RC > 5Fw > > 5> R
Rc 2 4 Re 3
ðRe 3
R0 3 Þ > > R0 > > ; : c R

If τ is the time required for complete conversion (cooking) where Rc = 0, eq 9 takes the following form 0" 1 #1=3 3 3 t ðR
R Þ e 0 3 3 ¼ @ Re
RC
R0 A=ðRe
R0 Þ τ R0 3

0

ð15Þ

ð10Þ Integration of eq 15 gives

where

! 15Fw Re 3 τ¼ ðRe
R0 Þ 21kSL CB Mw Re 3
R0 3

 3 Re ( R C 2
R0 2 R0 t ¼ 7Mw Dm ðCC
CB Þ 2 5Fw

ð11Þ

2

5.3.2. Diffusion through Swollen Mass Controlled. If diffusion of water through the cooked swollen starch layer of the rice controls the overall rate of cooking (Figure 2B), then the cooking rate can be expressed as RCR ¼
4πRG 2 Dm

dC dRG

ð12Þ

Integrating the above equation between the Rc to RG and CC to CB, we get RCR ¼

4πDm ðCC
CB ÞRC   RC 1
RG

1

þ 2

R e 3
R0 3 R0 3

!4 Re 3
ðRe 3
R0 3 Þ

9



RC R0

3 !2=3

> 3> > =


ðR0 3 Þ2=3 5

> > > ;

ð16Þ If τ is the time required for complete conversion (cooking) where Rc = 0, eq 16 takes the following form 8 > > 2 > >  3 !2=3 2 R0 τ Re
R0 > > 2 > 3 : R0

ð13Þ

8 > > > >
> > > = 2 R 1 0 3 2=3 3 2=3 3 2=3 ! þ ½ðRe Þ
ðR0 Þ 
ðR0 Þ g=
> > 2 Re 3
R0 3 > > > > 2 > > : ; R0 3

equating eqs 7 and 13 we get    4πDm ðCC
CB ÞRC d 5 4 4 Fw 3 3   πRG
πRC ¼ RC dt 7 3 3 Mw 1
RG ð14Þ

ð17Þ 1933

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Table 4. Values of Rate Constants (KR) and Diffusivity through Swollen Layer (Dm)

where

 3 Re R0 τ¼ 7Mw Dm ðCC
CB Þ 9 8 > > > > > > > > = < R2 1 0 2=3 2=3 3 3 ! 
þ ½ðRe Þ
ðR0 Þ  > > 2 Re 3
R0 3 > > > > 2 > > ; : R0 3 5Fw

rate constant Sr no.

authors

1

Sujuki et al.

2

3

temperature

(m/min)

(C)

 10 5

(m2/min)  1010

56,57

Juliano et al.58

Chakkaravarthi et al.59

diffusivity

8




2.15

15




2.74

20




3.30

30




4.07

50




5.01

75 80

3.72 5.91






90

13.90

100

23.94




110




688

120




977

130




1410

150




2110

80 90

7.030 14.69





100

31.03




110




885

120




1520

100 (presoaked in

27.4




100 (unsoaked in

17.41




excess water) 100 (presoaked in

7.61




9.23




ð18Þ 5.3.3. Chemical Reaction Controlled. The process of geletanization occurs at the surface of the rice particle. As the time progress reaction front moves toward the center of the particle, the radius of the unreacted core decreases with time (Figure 2C). The rate of reaction is given by RCR ¼ kR CC ð4πRC 2 Þ

Equating eqs 7 and 14 and substituting for RG from eq 6, we get   20πFw Re 3 2 dRC 2 ð20Þ RC kR CC ð4πRC Þ ¼ 7Mw R0 dt Integrating the eq 19 from time 0 to t and radius from RG to Rc

1 !0(   )1=3  3  RC 3 15Fw Re 3 @ 3 3 t ¼
RC A Re
Re
R0 21kR CC Mw R0 3 R0

excess water)

limited water) 100 (unsoaked in limited water)

ð19Þ

ð21Þ If τ is the time required for complete conversion (cooking), where RC = 0, eq 21takes the following form 0( 1  3 )1=3 t RC ¼ @ Re 3
ðRe 3
R0 3 Þ
RC A=Re ð22Þ τ R0

Figure 5. Plot of t/τ verses time for diffusion through a swollen layer controlled operation (eq 17), where t/τ = {Rc2
R02/2 + 1/2(Re3
R03/ R03)[(Re3
(Re3
R03)(RC/R0)3)2/3
(R03)2/3]}/{
R02/2 + 1/2(Re3
R03/R03)[Re3]2/3
(R03)2/3]}. 1934

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where ! 15Fw Re 3 τ¼ ðRe Þ 21kR CC Mw R0 3

interface and diffusion through swollen cooked mass. These resistances need to be eliminated by varying the relative agitation between the particles and fluid, particle size, and number of particles. This will enable the establishment of the intrinsic kinetics of the cooking process. For this purpose, the approach proposed by Levenspile54 and Mundale et al.55 can be used. The kinetic parameters will depend upon the quality of rice, lentils, vegetables, water, etc. (c) During cooking, water molecules enter the particle, and physicochemical changes occur. (d) In order to establish the amount of heat actually used for the cooking reaction, systematic research is needed for estimating the heat of the cooking reactions for the variety of materials being cooked in practice.

ð23Þ

5.4. Discussion. Chakkavarathi et al.;59 Sujuki et al.56,57 and

Juliano and Perez58 have systematically investigated the rate controlling steps and the kinetics of the cooking (temperature higher than 75 C) and soaking56 (temperature in the range of 8 to 50 C). It was found thought desirable to bring all these studies on a single platform using the mathematical models developed in Section 5.3. Accordingly, the value of t/τ were plotted with respect to time as given in eq 10, 18 and 22 for mass transfer, diffusion through swollen layer and chemical reaction controlled operations, respectively. On the basis of quality of fit in terms of correlation coefficient, it was found that the overall cooking operation is reaction controlled below 100 C and diffusion controlled above 100 C . Typical plots for both the cases are shown in Figures 3 and 4, respectively. The value of rate constant and diffusivity are listed in Table 4. It can be seen that the value of rate constants and diffusivity are practically the same (under otherwise identical conditions) in the experiments performed by various authors. The value of activation energy was found to be 79.4 kJ/gmole for reaction controlled operation and 37.8 kJ/gmole for diffusion controlled operation. We would like to bring out one interesting observation. Sujuki et al.56 have performed soaking experiments in the temperature range of 8
50 C. As mentioned previously, the cooking of rice (geletanizations) begins at temperature exceeding 74 C. In the other words, no chemical reaction can occur below 50 C. Figure 5 clearly shows that the soaking process is diffusion controlled, and the values of diffusivity are also shown in Figure 5.

6. CONCLUSIONS (1) The kinetics of change in the quality of food (such as nutrients, anti-nutrients, etc.) was found to be first order. The values of activation energy have been reported. (2) In regard to the rate controlling step, the published data indicates that below 100 C, the rate of cooking is controlled by physico
chemical changes at the surface of the uncooked core. However, above 100 C, the cooking rate is controlled by the diffusion rate of water through the swollen cooked layer. In this paper, the exact need for additional experimental work have been identified, and a coherent framework has been suggested for the data analysis. 7. SUGGESTIONS FOR FUTURE WORK (a) The present review deals very briefly with the kinetics of the change in the quality of food during the cooking process. In reality, a variety of starting materials and their constituents need attention in future work. (b) Regarding the chemistry and engineering of the cooking process of individual particles,a good amount of published literature is available. However, in practically all the cases, the experiments56
63 have been performed at boiling temperatures. All the data have been analyzed assuming the cooking process does not have any external resistance of mass transfer for water. A systematic experimental investigation is needed for estimation of the rate of water transfer at the solid
liquid

’ AUTHOR INFORMATION Corresponding Author

**Phone: +91 22 33611111 (J.B.J). Fax: +91 22 33611020 (J.B.J). E-mails: [email protected] (J.B.J), [email protected]. in (A.B.P.), [email protected] (S.B.P.). Present Addresses ^

AISSMS College of Engineering, Pune
411001, India

’ 8. NOTATIONS A0 = pre-exponential constant in eq 3 (s
1) CB = concentration of water in bulk (mol/m3) CC = concentration of water at uncooked core (mol/m3) C = quantitative value of the product of degradation under consideration (mol L
1) [C]0 = value of the product under consideration at time 0 (mol L
1 ) [C]t = value of the product under consideration after reaction time t (mol L
1 ) ΔC = amount degraded during ti (mol L
1 ) D R = Decimal reduction time or the time required to destroy 90% of the organisms (min) Ea = activation energy of the reaction (J mol
1 ) k 1 = reaction rate constant (s
1 ) k i = rate constant at time t i (s
1 ) KSL = mass transfer coefficient (m/sec) k R = reaction rate constant (m/sec) M w = molecular weight of water n = order of the reaction R = ideal gas constant (Jmol
1 K
1 ) R CR = rate of cooking reaction (mol/sec) R G = radius of grain (m) R 0 = radius of grain at time t = 0 (m) R e = radius of cooked grain (m) T = temperature (K) t = time (s) V = volume of grain (m 3 ) X = extent of cooking (
) z = slope of the graph obtained from a plot of D R value versus temperature Greek letters

Fw = density of water (kg/m3) τ = time required for complete conversion (sec) Subscript

0 = at time zero 1935

dx.doi.org/10.1021/ie202596d |Ind. Eng. Chem. Res. 2012, 51, 1923–1937

Industrial & Engineering Chemistry Research C = core CR = cooking rate e = at equilibrium

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