Quality Factors of Fruits and Vegetables - ACS Publications

Chapter 9

Applications of Chemical Kinetic Theory to the Rate of Thermal Softening of Vegetable Tissue Malcolm C. Bourne

Downloaded by MONASH UNIV on June 11, 2016 | http://pubs.acs.org Publication Date: September 7, 1989 | doi: 10.1021/bk-1989-0405.ch009

New York State Agricultural Experiment Station, Cornell University, Geneva,NY14456

The rate of thermal softening of vegetable tissue is consistent with two simultaneous pseudofirstorder kinetic mechanisms. From 80-97% of thefirmnessis lost by a rapid softening mechanism. The other 3-20% is lost by a distinctly different kinetic mechanism that occurs at1/30thto1/170ththe rate of thefirstmechanism. It is well known that a low temperature blanch activates the pectin methylesterase system in vegetables and gives afirmertexture. Kinetic studies show that this increase is caused primarily by an increase in the amount of the slow softening substrate and not by changes in the apparent first order rate constants. We are using the two substrate, two kinetic mechanism theory to define the optimal thermal regime for producing thefirmesttexture in canned vegetables. The edible tissue of fruits and vegetables is composed predominantly of parenchyma cells. Mature parenchyma cells are generally 50 to 500μ across, although, in some cases, they may be considerably larger (1,2). The cell walls are comprised largely of pectin, hemicellulose, cellulose, and sometimes, lignin and protein (3). The individual cells are cemented together by an amorphous layer external to the primary cell wall, called the middle lamella, which consists primarily of the calcium salts of polymers of galacturonic acid that have been partially esterified with methyl alcohol, and is known as the pectic material (4). The texture of fruit and vegetable tissue derives primarily from the cell wall and the middle lamella. Most of the volume of the interior of the cell is comprised of an aqueous solution of sugars and salts that does not impart structural strength to the tissue. The hydrostatic pressure generated by the physiological processes within the living cell (turgor pressure) impartsrigidity,turgidity and crispness to fresh produce. Processing techniques such as heating and freezing loll the cells with the result that turgor is absent in processed products. Therefore, in processed products, the chemical composition and amount of cell wall and middle lamella material controls the texture. Changes in texture that occur during processing resultfromchanges in the chemistry of this hydrophilic polymeric material that affect the physical properties (5). 0097-6156/89/0405-0098$06.00/0 c 1989 American Chemical Society

Jen; Quality Factors of Fruits and Vegetables ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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Rate of Thermal Softening ofVegetable Tissue

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The chemistry of these changes is complex. For example, pectin can be depolymerized and deesterified by heat or enzymes; it can be crosslinked by means of divalent cation salt bridge formation between thefreecarboxylic acid groups on the polygalacturonic acid chains and cations (primarily Ca""") naturally present in the tissue or added during processing. Depending on which of these reactions occur, vegetable tissue may become softer or firmer during processing. Softening processes usually predominate during processing. Processed fruits and vegetables are softer, often very much softer than the raw material. Because of the consumer's desire for the firm, crisp and succulent textures of raw vegetables, there is great interest in developing new processing technology that will retain more of the textural character of the raw produce. The primary textural property of fruits and vegetables is firmness (6). Three principles are used to measure firmness. 1) The puncture test measures the force required to push a probe into the product. 2) The extrusion test measures the force required to make the product flow through one or more slots or holes. 3) The deformation test measures the distance the product compresses under a small force. All three test principles are used onfreshproduce, but only the first two (puncture and extrusion) are used on processed material (7).


1 1

Kinetics The study of the rates at which chemical reactions occur has been a powerful tool for understanding the nature of these reactions ever since the pioneering work of Wilhelmy in 1850. Most of this work has been performed in homogeneous systems with pure reactants. However, the chemical reactions that occur when vegetable tissue is heated are complex, and a number of different reactions can be occurring simultaneously. In addition, the cell wall and middle lamella are not homogeneous. Further, the linkage between these chemical reactions and firmness is not clear, and is probably non linear. Nevertheless, it has been found that applying the theories of chemical kinetics to the rate of thermal softening of vegetable tissue can provide useful insights into the softening mechanisms, and point the way to developing technologies that produce firmer textured processed products even though the progress of the reaction is measured by a physical test (firmness) instead of a chemical test. The physical test is usually either an extrusion test or a puncture test. A first-order reaction is one in which the rate of the reaction at anytimeis directly proportional to the concentration of the reactant present at that time. It is described by the well known equation: -dX/dt = K.X

(1)

where X is the concentration of the reactant, t is the time, and Κ is the rate constant. Integrating and transposing gives: lnX = In Xo - K.t

(2)

where Xo is the concentration of the reactant at zerotime.A plot of the lnX versus time will be linear for a first order reaction. The slope of the line gives the rate constant K, and the intercept on the ordinate at to gives In Xo. A number of processes that have been studied show a linear relationship between the log of the amount of substance present versus the time. Many of these processes do not arise from simple first-order chemical reactions, or the chemistry of the process may be unclear. Nevertheless, since they empirically fit the model for a first-order chemical reaction, they are commonly called pseudo first order processes. The slope of the log (concentration) versustimeplot is called the apparentfirstorder


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QUALITY FACTORS OF FRUITS AND VEGETABLES

rate constant. The modifying adjective "apparent" is used to signify that the process is not truly first order in the chemical sense, but that the experimental data give a good fit to thefirstorder chemistry model. The temperature dependence of afirstorder rate constant is given by the wellknown Arrhenius equation:


InK = InA - Ea/RT

(3)

where Κ is the rate constant, A is a constant, Ea is the activation energy, R is the gas constant and Τ is the absolute temperature. A plot of InK versus 1/T is rectilinear. The value of Ea can be calculated since the slope of the line is Ea/R, and the value of R is known. Many pseudo first order processes also follow the Arrhenius relationship. In these cases the apparent activation energy can be calculated because of the excellent fit of the data to the Arrhenius equation. A number of researchers who studied the rate of thermal softening of vegetable tissue have found a goodfitof their empirical data to thefirstorder kinetic model because a plot of log firmness versus time is rectilinear. Probably the first of these were Nagel and Vaughn (8) who measured changes infirmnessof cucumber pickles by means of a puncture test as they were pasteurized at 160F, 180F and 200F. Nicholas and Pflug (9) confirmed that pickles soften by a pseudofirstorder process. Many other reports on heating-softening effects show pseudofirstorder kinetics when the data are plotted on semilog axes, although not many authors presented their results in this form at thattime.Paulus and Saguy (10) found pseudo first order softening kinetics for carrots when cooked and they listed the apparent rate constants and activation energies for three carrot cultivars. Loh and Breene (11) also found the first order kinetic model was adequate to describe the changes in firmness of 11 kinds of fruits and vegetables. Other reports of apparentfirstorder kinetics for thermal softening have been published for cowpeas (12), red kidney beans (13) and potatoes (14). Most of the above reports on softening studies used either short cooking times, or low temperatures which gave slow rates of softening. Some of these reports inferred that the softening rate seemed to deviate from thefirstorder kinetic model with longer processtimes,but this aspect seems not to have been explored any further. Large quantities of vegetables and fruits are preserved by canning. This technology requires enclosing the product in hermetically sealed containers and heating at a specified temperature for a specified time to destroy all microorganisms inside the container. Products with a pH above 4.5 require a substantial heating regime to obtain commercial sterility. For example, green beans packed in brine in 1 lb cans require 22 minutes at 240F or 13 min at 250F (15). This heavy heat treatment cannot be compromised because microorganisms of public health significance, such as Clostridium botulinum, require this degree of heat treatment to be destroyed. Unfortunately, this amount of heat causes great damage to the texture. Most canned vegetables have a softer than desirable texture. Long heatingtimekinetics Huang and Bourne (16) studied the rate of softening of green beans, dry white beans, carrots and three varieties of beets for times up to 120 minutes using the extrusion principle to measure firmness and found a similar pattern in the softening curves for every product. Figure 1 plots log extrusion force versus processtimefor diced beets processed at 220F. The solid circles and solid line show the experimental data. The softening curve is characterized by an initial rapid decrease in firmness that is almost linear. This is consistent with the findings of apparentfirstorder



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101

Rate of Thermal Softening ofVegetable Tissue

softening kinetics in earlier reports. But then rate of firmness loss decreases; the firmness curves off into a second straight line with a shallow slope at longer process times. Since a first order process is represented by a rectilinear plot on a seimlogarithmic scale, it is quite clear that simple apparent first order kinetics cannot apply to lengthy process times. The general shape of this curve is typical for all vegetables that we have studied. Fruits show thermal softening curves similar to those for vegetables but the initial rate of softening is more rapid than for vegetables. Nevertheless, we have been able to obtain thermal softening data for several apple cultivars by cooking at temperatures in the range of 80-99C (Lacroix and Bourne, submitted for publication). The striking features of Figure 1 are the initial linear relationship with a steep slope and the later linear relationship with a shallow slope. Although inconsistent with a simple apparent first order process, this relationship is entirely consistent with two pseudo first order processes with different rate constants occurring simultaneously. One process is rapid and the other process is slow. From analogy with kinetic theory for two apparent first order processes we can postulate that the firmness of vegetabletissueis composed of two substrates, "a" and "b" and that substrate "a" softens rapidly by mechanism 1 while substrate "b" softens slowly by a different mechanism (mechanism 2). When the linear portion of mechanism 2 is extrapolated back to zero process time (dotted line in Figure 1) and this extrapolated line is subtracted from the solid line above it, a second line is obtained as shown by the open circles and dashed line in Figure 1. The derived dashed line represents mechanism 1 and the slope is its apparent rate constant. The linear portion of the solid line represents mechanism 2 and the slope is its apparent rate constant. The chemical kinetics theory for two simultaneous first order processes is as follows: assume there are two substrates, "a" and "b" -da/dt = K .a

and

a

-db/dt = Kb-b

(4) M

where "a" is the firmness contributed by the first component and "b the firmness contributed by the second component; t is the time, K is the apparent rate constant for substrate "a" and Kb is for substrate "b". K > Kb since substrate "a" decays more rapidly than substrate "b" when the tissue is heated. Integrating and transposing in the usual manner gives: a

a

lna = lnao-K .t a

and

In b = In b - Kb.t

(5)

0

where ao and b are the amount of substrate "a" and "b" respectively at zero process time. At any processtimethe total firmness = (a + b) and this value can be calculated when a , K , b and Kb are known. Since "a" has a high softening rate, its contribution to firmness becomes inconsequential after a long heating time. Experimentally determined values for the apparent first order rate constants for vegetables processed at 230F (16) are given in Table I below. The values for K are 28-fold to 170-fold greater than the values for Kb. Both apparent rate constants, K and Kb, give an excellent fit to the Arrhenius temperature relationship. Figure 2 plots the apparent rate constant K for dry white beans versus 1/T and is typical of the data obtained with the other vegetables. Table Π lists the apparent activation energies for the two substrates (16). 0

0

a

0

a

a

a


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Table L Apparent First Order Rate Constants for Vegetables Processed at 230F Vegetable

Ka(min-l)

Green beans Dry white beans Green peas, size 3 Green peas, size 5 Beets, Detroit dark red Beets, yellow Beets, Winter keeper Carrots

Kb (min-1) 0.0016 0.0039 0.0043 0.0041 0.0013 0.0013 0.0013 0.00082

0.149 0.109 0.211 0.337 0.171 0.195 0.199 0.139

Table Π. Apparent Activation Energy for Thermal Softening ofVegetables Vegetable Dry white beans Green peas, size 3 Green peas, size 5 Beets, Detroit dark red Beets, Winter keeper Carrots

Substrate "a" Kcal/mole

Substrate "b" Kcal/mole

24.9 27.1 35.0 22.6 27.1 15.2

12.9 24.4 22.1 12.9 18.4 5.1



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Rate of Thermal Softening of Vegetable Tissue

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PROCESS TIME - MIN

Figure 1. Softening of diced beets cooked at 220F. (Reproduced with permission from Ref. 16. Copyright 1983 Food and Nutrition Press.)

1.0 c

2

CD 2 2 LU

Ο .07 I 1 bî .00256

'

»

.00260

'

1

'

«

»

.00264 .00268

Figure 2. Arrhenius plot for apparent softening rate constant for dry white beans. (Reproduced with permission from Ref. 16. Copyright 1983 Food and Nutrition Press.)


104


One more valuable piece of information can be extractedfromFigure 1. The intercept of the extrapolated line on the ordinate gives the amount of substrate "b" at zero time. The difference between this point and the total firmness gives the amount of substrate "a" at zero time. Table ΠΙfiststhe proportion of total firmness of the raw vegetable supplied by substrate "b" (16). Table ΠΙ. Contribution of Substrate "b" to Raw Firmness


Vegetable

Total Firmness kN

Substrate "b" kN

Substrate "b" Total Firmness

11.8 7.2 8.5 15.3 15.8 17.1 17.0

0.80 1.14 1.23 1.53 2.04 1.60 0.50

0.066 0.159 0.145 0.100 0.129 0.094 0.029

Green beans Green peas, size 3 Green peas, size 5 Beets, Detroit dark red Beets, yellow Beets, Winter keeper Carrots

Note that substrate "b" contributes between 3% and 15% of the firmness of the raw vegetable. While this may appear to be a minor contribution to raw firmness, it is the major contributor to the firmness of the canned product. Figure 3 shows the softening curve for cut green beans processed at 115.6C (240F). It is a typical thermal softening curve (17). The National Food Processors Association recommended process for this product packed in #303 cans is 22 min at 115.6C. The vertical line in Figure 3 shows thistime.It is evident that substrate "a" has practically disappeared by thetimea commercial process has been completed. Substrate "a" provides more than 90% of the firmness in the blanched bean before canning but less than 5% of the firmness after canning. Hence, the amount of substrate "a" has a minor effect in determining the firmness of the canned product. Since its rate constant is so high, even substantial changes in the apparent rate constant for substrate "a" would have a minor effect on firmness after canning. What really determines the firmness after canning is the amount of substrate "b" present. Therefore, we have named the intercept of the extrapolated line on the ordinate, (which measures the amount of substrate "b" at zerotime)the "thermal firmness value" because this is the firmness component that can withstand a heavy heating regime with little degradation (17). It is well known that a low temperature blanch at 170-175F before canning gives a firmer texture in vegetables than a conventional blanch at 200-212F (5). The chemistry underlying this process is clear: The low temperature blanch activates the enzyme pectinmethylesterase, which demethylates the esterified carboxylic acid groups on the polygalacturonic acid polymer and allows salt bridge formation with divalent cations such as C a that are naturally present in thetissue(18). The two substrate theory of thermal softening provides a kinetic explanation of this phenomenon (17). Figure 4 shows the softening curves for diced carrots blanched at 74C and 100C and then cooked at 100C. Both curves show the typical rapid initial softening followed by a much slower rate of softening. The slopes for substrate "a" derived by subtraction of the extrapolated substrate "b" linefromthe experimental curve and the intercepts on the ordinate are shown in the figure. The apparent rate constants and thermal firmness values are given in Table IV below. ++




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Figure 3. Softening curve for cut green beans cooked at 240F. (Reproduced with permission from Ref. 17. Copyright 1987 Institute of Food Technologists.)


106


Table IV. Kinetic Parameters for Thermal Softening of Carrot Blanch Temp °C 100 74

1

1

K (min- ) Kb (min" ) a

0.052 0.030

0.00051 0.00096

Thermal Firmness (N) 201 505

A comparison of Table IV and Figure 4 shows that the low temperature blanch reduces K by 57%. However, since substrate "a" has practically all disappeared by the time a commercial process has been completed, the change in K is of little consequence. Kb for the 74C blanch is 88% higher than for the 100C blanch. Since both these rates are very slow, they too have little effect on the firmness after canning. The dominant factor is the change in thermal firmness, the amount of substrate "b". It is 201 for the 100C blanch and 505 for the 74C blanch—an increase of over 150%. The kinetic approach clearly shows why the low temperature blanch gives a firmertextured canned product. a


a

Future directions The two substrate, two pseudo first order softening mechanisms described above point the direction our research should be taking if we are to produce canned vegetables with significantly firmer texture. Shortening the process time by increasing the rate of heat transfer can only promise a moderate increase in firmness because reducing the processtimeby a few minutes is not sufficient to retain any appreciable amount of substrate "a" (see Figure 3). Searching for a way to slow the apparent rate constant K can also provide only small gains because K is so fast that even reducing it by 50% will have little net effect on firmness after a commercial process. The real gains in firmness in commercially canned vegetables will be found by studying the factors that affect the thermal firmness value (see Figure 4). Future research should concentrate on studying genetic factors and processing variables that increase the thermal firmness value because these increases can withstand the heat treatment of a commercial process and deliver firmer textured vegetables to the consumer. An example of this approach is shown with carrots (19). Seventeen cultivars of carrots were sliced transversely and blanched 4 min at 212F or 165F, then canned and processed 23, 30,45, 60,75 min at 250F. The firmness of the phloem tissue and xylem tissue (core) were separately measured by a puncture test. Figure 5 shows log firmness versus processtimefor the cultivar Dominator Sunseed together with the equations for the lines of best fit and the correlation coefficients R. Similar plots were obtained for the other sixteen cultivars. Sufficient data was obtained to measure substrate "b" and the thermal firmness value. No data were collected to measure substrate "a" because it is not needed. However, the firmness at zero process time was measured and these points are plotted on the ordinate. The correlation coefficients range from r = 0.98 to r = 1.00 which shows the excellent fit of the empirical data to the kinetic model. The thermal firmness values for the twotissuesand two blanch temperatures for the seventeen cultivars are shown in Table V. The data for the phloem and xylem tissues for every cultivar and both blanch temperatures are consistent with the two a

a


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9.


107


Figure 5. Substrate "b" softening for sliced carrots, cultivar Dominator Sunseeds.


i

I

1

Ο

GO

S

ο

H* Ο 00

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substrate theory of thermal softening. For every cultivar, the thermal firmness of both phloem and xylem tissue is higher for the 165F blanch than for the 212F blanch. The mean increase in thermal firmness is 85% for phloem tissue and 149% for xylem tissue. This example shows how the kinetic approach can identify processes that provide a firmer textured product. Table V.

Thermal Firmness of Canned Carrot Slices (newtons force) 165F Blanch xvlem phloem


Cultivar

212F Blanch xvlem phloem

Goldwine Sunseeds Cellobunch, Asgrow Dominator Sunseeds PY60 Peter Edwards XPH5120 Asgrow XPH 985 Asgrow 3011 Sunseeds Oranza Bejo XPH 875 Asgrow Norman Bejo Desdan Arco Larando Bejo W243 Crookham Aux 8160 Sunseeds XPH 5119 Asgrow Arco 154 Barnum Bejo

0.703 0.381 0.960 0.778 0.891 0.658 0.682 0.830 0.735 0.528 0.851 0.852 0.656 0.526 0.749 0.849 0.844

0.864 0.736 0.936 0.914 1.105 0.913 1.331 0.995 0.992 0.645 1.133 1.255 0.946 0.823 1.116 1.529 1.934

0.442 0.365 0.464 0.403 0.411 0.435 0.350 0.353 0.433 0.305 0.416 0.482 0.357 0.291 0.357 0.423 0.457

0.529 0.423 0.552 0.363 0.536 0.446 0.421 0.300 0.504 0.219 0.463 0.473 0.401 0.384 0.436 0.444 0.405

mean value

0.734

1.069

0.397

0.429

There is considerable variation in thermal firmness among cultivars. For xylemtissueblanched at 165F it rangesfroma low of 0.381 for the Asgrow Cellobunch cultivar to 0.960 for Dominator Sunseeds cultivar, and for the phloem tissue blanched at 165F it rangesfrom0.645 for Norman Bejo cultivar to 1.934 for the Barnum Bejo cultivar. It is likely that a large part of the variation is genetically controlled. If so, the kinetic analysis will provide plant breeders with a test procedure that will enable them to select breeding lines that will give a firmer textured product after canning. Literature Cited 1. Reeve, R. M. Food Res. 1953, 18, 604-617. 2. Reeve, R. M. J. Texture Studies 1970, 1, 247-284. 3. Pilnik, W.; Voragen, A. G. J. In Biochemistry of Fruits and Their Products; Hulme, A. C., Ed.; Academic: London, 1970; Vol. 1,pxx. 4. Northcote, D. H. Biological Reviews 1958, 33, 53-102. 5. Van Buren, J. P. J. Texture Studies 1979, 10, 1-23. 6. Bourne, M. C. Hortscience 1980, 15 (1), 7-13. 7. Bourne, M. C. Food Texture and Viscosity: Concept and Measurement; Academic: New York, 1982. 8. Nagel, C. W.; Vaughn, R. H. Food Res. 1954, 19, 613-616. 9. Nicholas, R. C.; Pflug, I. G. Food Technol. 1962, 16(2), 104-108.


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10. Paulus, K.; Saguy, I. J. Food Sci. 1980, 45, 239-241, 245. 11. Loh, J.; Breene, W. M. J. Texture Studies 1981, 12, 457-471. 12. Sefa-Dedeh, S.; Stanley, D. W.; Voisey, P. W. J. Food Sci. 1978, 43, 18321838 13. Moscoso, W.; Bourne, M. C.; Hood, L. F. J. Food Sci. 1984, 49, 1577-1583. 14. Harada, T.; Tirtohusodo, H.; Paulus, K. J. Food Sci. 1985, 50, 459-462, 472. 15. National Food Processors Association, 1982, 12th ed. 16. Huang, Y. T.; Bourne, M. C. J. Texture Studies 1983, 14, 1-9. 17. Bourne, M. C. J. Food Sci. 1987, 52, 667, 668, 690. 18. Lee, C. Y.; Bourne, M. C.; Van Buren, J. P. J. Food Sci. 1979, 44, 615, 616. 19. Bourne, M. C. Abstract 225, Institute of Food Technologists Annual Meeting 1988. February 7, 1989


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