Quality Factors of Fruits and Vegetables - American Chemical Society

Chapter 19

Chemistry and Processing of High-Quality Dehydrated Vegetable Products Joseph J. Jen, Gurmail S. Mudahar, and Romeo T. Toledo

Downloaded by CORNELL UNIV on October 2, 2016 | http://pubs.acs.org Publication Date: September 7, 1989 | doi: 10.1021/bk-1989-0405.ch019

Department of Food Science and Technology, The University of Georgia, Athens, GA 30602

The basis for s t r u c t u r a l collapse of vegetable tissues on dehydration and approaches to preserve texture and rehydration properties were reviewed. These p r i n c i p l e s formed the basis f o r producing high quality carrots and potatoes by a process of biopolymer infusion followed by high temperature short time f l u i d i z e d bed dehydration. Infused biopolymers was shown to penetrate i n t r a c e l l u l a r spaces and c e l l walls and may contribute to reduced cell collapse i n the dehydration process. Deposition of infused biopolymer within the c e l l s was elucidated using a convalently bound complex of biopolymer and colored dye which was v i s i b l e upon histochemical examinations under a microscope. The dehydration process was optimized with response surface methodology. The r e s u l t i n g products have excellent quality, high rehydration r a t i o and a puffed structure. The Need for High Quality Dehydrated

Vegetables

The current trend towards larger produce sections i n supermarkets underscores increasing consumer demand for fresh f r u i t s and vegetables and the willingness of the consumer to pay a premium price for quality products. The p e r i s h a b i l i t y of fresh vegetables, however, leaves a niche for processed products i n competition for the consumer d o l l a r . Quality conscious consumers demand quality products along with the convenience. Therefore, for processed foods to remain competitive, processes must optimized for quality. An understanding of the fundamental chemistry and physics involved i n food processing operations i s e s s e n t i a l i n optimization. The use of microwave ovens has increased the convenience of dehydrated foods to the same l e v e l as that of frozen foods. Dehydrated foods can be produced, packaged and distributed at a f r a c t i o n of the cost of frozen or canned foods. For these reasons, dehydrated soup mixes and pasta based mixes are one of the fastest 0097-6156/89/0405-0239$06.00/0 © 1989 American Chemical Society

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

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growing grocery products. These items, which used to be available only i n s p e c i a l i t y stores catering to the r e l a t i v e l y small group who go backpacking and camping, are now regular items on the supermarket shelves. The increasing popularity of dehydrated foods can be attributed to the improved quality of products now marketed i n comparison with what was available i n the past. Further growth of t h i s sector of the grocery products l i n e requires the a v a i l a b i l i t y of high quality dehydrated ingredients suitable for compounding into dinner entrees and soup mixes.


Physio-Chemical Changes on Dehydration and Effects on Texture Holdsworth (1) summarized the physico-chemical aspects of dehydrated food products which are important i n determining the o v e r a l l organoleptic quality. These include: structure and composition of the raw material, shrinkage during drying, loss of v o l a t i l e components, browning reactions and moisture absorption and rehydration. Nutrient loss, degradation of color pigments and o v e r a l l texture may also be added to this l i s t . Except for structure and composition of the raw material, these factors are d i r e c t l y influenced by the conditions used during the dehydration process. Structure, composition of raw material and shrinkage during dehydration influence rehydration and textural properties of the product on rehydration. According to Reeve (2), texture of fresh f r u i t s and vegetables i s determined by both h i s t o l o g i c a l structure, i . e . , c e l l s i z e , i n t e r c e l l u l a r components, c e l l w a l l thickness and structure and c e l l wall composition. The c e l l u l o s i c m i c r o f i b r i l s , which comprises the basic architecture of the c e l l w a l l , may include some pentosans, vary greatly i n orientation, degree of c r y s t a l l i n i t y and cumulative thickness. The spaces between the m i c r o f i b r i l s are occupied by substances which encrusts the c e l l u l o s e . These encrusting substances include primarily amorphous polysaccharides pectin and hemicellulose, and l i g n i n , suberins and cutins of varying chemical make up depending upon the type of tissue. In addition, pectic substances and a i r may be present i n middle lamella and i n t r a c e l l u l a r spaces of c e l l s . For example, the c o r t i c a l zones of apples consist of 20 to 30% of i n t r a c e l l u l a r spaces and i s responsible for the spongy texture of apples. It i s the reason apples are p a r t i c u l a r l y suitable to be i n f i l t r a t e d by syrups or water during processing C3, 4). The i n t e g r i t y of the c e l l wall and the r e l a t i v e mobility of c e l l w a l l content upon breakage of the c e l l w a l l determine the changes i n textural c h a r a c t e r i s t i c s of vegetables during processing. Texture, according to Brown (5), i s the combined effect of the mechanical properties of the c e l l s and contents and the manner of breaking up of the tissue during mastication. When raw, turgor, the i n t e r n a l pressure of the c e l l contents, allows the c e l l to r e s i s t mechanical deformation and r e f l e c t the perception of firmness. The sudden collapse of the c e l l wall at the b i o y i e l d point during the process of mastication results i n the perception of crispness. When starch and protein are present inside the c e l l s , they may bind water or hinder i t s release, therefore, maintaining a firm texture, although a crisp perception may not be



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exhibited. The application of heat weakens the c e l l walls due to dissolution of pectin, resulting i n c e l l break up and loss of turgor or a reduction of the b i o y i e l d point resulting i n a loss of crispness and the onset of mushiness. In the process of dehydration, i n t r a c e l l u l a r water i s removed, r e l i e v i n g turgor, resulting i n collapse of the c e l l walls. The collapse can be t o t a l and i r r e v e r s i b l e i n the case of high moisture vegetables such as celery, tomatoes and lettuce whose c e l l walls are very f r a g i l e , such that the rehydrated product only absorbs a f r a c t i o n of the o r i g i n a l moisture l e v e l . For these vegetables, the absorbed moisture of rehydration i s only i n t e r c e l l u l a r ; therefore, the texture of the rehydrated product i s mushy ( 6 ) . In a f l e s h vegetable such as carrots, the l i g n i f i e d c e l l walls of some of the c e l l s and starch, o i l and vacuoles i n some c e l l s l i m i t shrinking, and although some c e l l s shrink completely, a t o t a l collapse does not occur. The vacuoles and o i l droplets inside c e l l s and d i f f e r e n t degrees of shrinkage which occur among adjacent c e l l s having d i f f e r e n t c e l l w a l l structures, produces i n t e r c e l l u l a r spaces i n the dried product ( 7 ) . In the same study, the author also showed that i n a starchy material such as potatoes, the starch content of the c e l l s prevented t o t a l tissue collapse although a sizable reduction i n c e l l size occurred as the tissues shrunk during dehydration. Formation of I n t r a c e l l u l a r Voids i n Dried Vegetables Based on these studies i n microstructure and the mechanism of moisture removal on dehydration and moisture absorption during reconstitution, good quality dehydrated products which reconstitute well upon dehydration should have the following c h a r a c t e r i s t i c s : c e l l s must not be t o t a l l y collapsed, c e l l walls must remain intact and i n t e r c e l l u l a r spaces must be maintained i n the dried product. The l a t t e r w i l l allow c a p i l l a r y action to draw the water into the v i c i n i t y of the c e l l s during the process of rehydration. Water can then diffuse across the intact c e l l w a l l and into the c e l l s to reestablish turgor. The development of i n t e r c e l l u l a r spaces i n the dried product i s c l e a r l y manifested i n the porous structure of freeze dried foods. In general, products which have undergone a minimum of shrinkage during dehydration réhydrate faster because of the presence of well-defined i n t e r c e l l u l a r spaces. These products also have low bulk densities, both an advantage and a disadvantage. Consumers may perceive receiving more product with a higher bulk, but, on the other hand, more packaging i s required to deliver a given weight of product. Rate of moisture removal during dehydration has a strong influence i n the shape and h i s t o l o g i c a l structure of a dehydrated product. Toledo (8) showed a diagram of the relationship between the moisture content of apple s l i c e s and the drying rate i n conventional a i r drying (Fig. 1). At moisture content i n excess of 1.0 on a moisture free basis (MFB, g water/g dry matter), water exists as free water and the vapor pressure of water i n the product i s the same as that of pure water (a = 1). In this moisture range, dehydration rate occurs i n two stages as the moisture



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0

50

100 Scale Change-»

(

300

500

700

t-Scale Change

Moisture Content g H 2 O / 1 0 0 g Dry Matter Figure 1. Desorption isotherm and rate of drying of raw apple s l i c e s . (Reproduced with permission from r e f . 8. Copyright 1980 Van Nostrand Reinhold.)



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content decreases. An i n i t i a l constant rate stage occurs at moisture contents i n excess of 6.0 MFB where surface moisture i s removed and c a p i l l a r y flow of moisture from the i n t e r i o r of the s o l i d replenishes surface moisture as soon as i t i s removed. A f a l l i n g rate stage at moisture contents from 6.0 to 1.0 MFB occurs when the surface dries out and c a p i l l a r y moisture flow i s no longer s u f f i c i e n t for the i n t e r n a l moisture to replenish the surface moisture. During t h i s f i r s t f a l l i n g rate period of drying, the surface would appear dry and because of a reduction i n turgor with the removal of moisture from the i n t e r i o r c e l l s , a s l i g h t deformation may be exhibited. In diced vegetables, the corners dry f i r s t and establish r i g i d i t y and as the i n t e r i o r sections lose moisture, causing the c e l l s to shrink i n size, the faces of the dice are drawn i n exhibiting a concave appearance. When the moisture content drops below 1.0 MFB, most of the water i s bound. Vapor pressure of water i n the product decreases below that of pure water (a < 1) and a second f a l l i n g rate period of drying i s exhibited . If moisture removal from the surface i s slowed such that moisture gradients are r e l a t i v e l y low, the rate of shrinkage of c e l l size i s uniform throughout the s o l i d , and the whole s o l i d w i l l shrink into a very small s i z e . The dried s o l i d w i l l have no i n t e r c e l l u l a r spaces; i t w i l l be hard and the bulk density w i l l be very high. On the other hand, when drying i s rapid, p a r t i c u l a r l y i n the f i r s t f a l l i n g rate period of drying, a r i g i d outer layer f i r s t forms. As the moisture content i n the i n t e r i o r of the s o l i d i s reduced and c e l l size becomes smaller, the r i g i d outer layer r e s i s t s deformation, l i m i t i n g t o t a l shrinkage and i n t e r c e l l u l a r voids are formed. Once the c e l l walls have dried to form a r i g i d structure, continued moisture removal can occur without further deformation. The formation of i n t e r c e l l u l a r voids i n the dried product has been exploited by several investigators as a means of increasing rehydration rate of dehydrated foods. Techniques for imparting a porous structure into the dried product other than freeze drying include: high temperature short time pneumatic dehydration by f l u i d i z e d bed (9, 10, 11) or centrifugal f l u i d i z e d bed (12, 13, 14) and by conventional a i r dehydration followed by explosion puffing (15, 16, 17, 18). Sullivan et a l . (19) showed that rate of dehydration i n the second f a l l i n g rate period of drying i s much faster i n apples which was explosion puffed compared to convent i o n a l l y dried apples. Low bulk densities and rapid rehydration were c h a r a c t e r i s t i c s common to dehydrated products which were subjected to explosion puffing or very rapid high temperature short time dehydration. One problem with high porosity i n dehydrated foods, i n addition to increased packaging requirements, i s the p o s s i b i l i t y of rapid oxidation because of increased surface area of exposure to oxygen i f a i r i s present within the pores. An approach which could be used to solve the problem was f i r s t suggested by Sinnam et a l . (20) when they compressed explosion puffed carrots after dehydration and found that there were no differences i n the rehydration rate or rehydration r a t i o compared to the o r i g i n a l explosion puffed dehydrated carrots. This concept has been extensively exploited by the U.S. Army Natick Laboratories (21) i n the development of 7



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compact rations by freeze dehydration and compression of vegetables. The process i s based on the p r i n c i p l e that at a certain moisture content range, the c e l l wall structure becomes f l e x i b l e such that the c e l l s can be collapsed without breaking. If the c e l l walls are collapsed when they are r e l a t i v e l y dry, such that they do not s t i c k together, rehydration w i l l not be impeded and the s o l i d swells back to the o r i g i n a l shape upon rehydration. I n t e r c e l l u l a r voids promote rehydration rate, but i n t e g r i t y of the c e l l walls promotes moisture retention and re-establishment of turgor upon rehydration. Approaches used to strengthen the c e l l wall to prevent damage during processing include: addition of calcium s a l t s (22), control of pectic enzyme a c t i v i t y (23), addition of polyhydric alcohols (24), osmotic dehydration (4, 25) and i n s i t u polyacrylamide polymerization (26). These approaches result i n firmer texture on rehydration but could interfere with moisture absorption r e s u l t i n g i n increased rehydration time and reduced rehydration r a t i o . Osmotic dehydration p r i o r to high temperature short time f l u i d i z e d bed dedydration has been shown by Kim and Toledo (11) to result i n lower rehydration r a t i o and slower rehydration rate i n dehydrated blueberries. Considerable amount of sugar exchange for water was observed i n osmosis-vacuum dehydration of apple chips by Dixon and Jen (27). These previous works demonstrated the complexity of interactions among processing parameters i n establishing f i n a l product c h a r a c t e r i s t i c s . Improvements i n the process by optimization i s needed to create high quality dehydrated f r u i t and vegetable pieces. Process Development for High Quality Dehydrated Diced Vegetables In our laboratory, a combined biopolymer treatment and high temperature short time (HTST) f l u i d i z e d bed dehydration process was developed for diced carrots and potatoes. Carrots and potatoes were selected as experimental material due to commercial importance of these vegetables. Raw carrots (Pansus carota) of a hybrid c u l t i v a r and Russell Burbank potatoes (Solanum tuberosum) were purchased from l o c a l supermarkets. For carrots, the top and narrow bottom parts were removed after washing. The middle portion of the carrots were diced to three-eighth inch cubes i n a vegetable dicer (Dito Dean Model TR-22). For potatoes, the skins were peeled before d i c i n g . The carrot and potato dices were treated with a mixture of biopolymers. The treatments included vacuum i n f i l t r a t i o n , pressure cooking, blanching and dipping. The biopolymers included maltodextrins, pectins, polydextroses, gums and others, singly or i n combination. After treatment, the vegetable cubes were f i r s t dried i n a HTST f l u i d i z e d bed dryer at a set temperature for a certain time period. The vegetable pieces were dried to a f i n a l moisture content of 3-5% i n a tunnel d r i e r set at 70°C and at 4 m/sec a i r v e l o c i t y . Response surface methodology (RSM) analyses was used to optimize the following parameters: concentration of biopolymer, time of blanching, time and temperature of the HTST f l u i d i z e d bed dryer. Product quality factors evaluated as responses were good color, high rehydration r a t i o and low bulk density. The time and temperature were controlled to within ± 5 sec and ± 1°C i n a l l cases.


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Literature reports showed that pretreated dehydrated carrots can have rehydration r a t i o (RR, t o t a l mass of rehydrated carrots per unit weight of dry matter) i n the range of 5 to 7 (28, 29). The RR was calculated a f t e r b o i l i n g a known weight of dried carrots i n 100 ml d i s t i l l e d water for 30 minutes. Currently commercially available samples tested i n our laboratory possessed RRt—J

2

ι

4

6

BLANCHING TIME,MIN Figure 2. Superimposed plots of 4 contour plots at 145°C and 10 min for rehydration r a t i o (RR), bulk density (BD), nonenzymatic browning (NEB) and expressible f l u i d (EF) of dehydrated potato cubes.


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The dye-biopolymer complexes were seen at i n t r a c e l l u l a r spaces i n intact carrot c e l l s ( F i g . 3a) and adhered to c e l l walls i n broken c e l l s ( F i g . 3b). These pictures provided some evidence for our hypothesis that the biopolymers migrate on and around c e l l walls and their presence may have assisted i n preventing or reducing c e l l collapse during dehydration. Much work needs to be done to elucidate the role of biopolymers on quality improvement of dehydrated vegetable pieces, and to define the proper size of molecules that would accomplish the desired texture.

Figure 3. Microscopic pictures of (a) intact and (b) broken carrot c e l l s after biopolymer-dye complex treatment, dehydration and rehydration. Bar i n picture shows one micrometer i n length.

American Chemical Society Library 1155 16th St., N.W. Jen; Quality Factors of Fruits and Vegetables Washington, D.C. 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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Conclusion Basic p r i n c i p l e s i n physio-chemical changes occurring during dehydration provided an e f f e c t i v e approach to improve quality of dehydrated products. Carrot and potato dices infused with b i o polymers before dehydration and processed under optimal conditions had good texture, high rehydration properties, good color and puffed appearance. Infused biopolymers were deposited on c e l l walls and i n t r a c e l l u l a r spaces and may contribute to prevent c e l l collapse.


L i t e r a t u r e Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

19. 20.

21.

22. 23. 24.

Holdsworth, S. D. J . Food Technol. 1971, 6, 331. Reeve, R. M. J. Texture Studies 1970, 1, 247. Reeve, R. M. Food Res. 1953, 18, 604. Dixon, G. M.; Jen, J. J . ; Paynter, V. A. Food Prod. Develop. 1976, 10 (7), 60. Brown, M. S. J . Texture Studies 1977, 7, 391. Shipman, J . W.; Rahman, A. R.; Segars, R. Α.; Kapsalis, J . G.; West, D. E.; J . Food S c i . 1972, 37, 568. Reeve, R. M. Food Res. 1943, 8, 128. Toledo, R. T. Fundamentals of Food Process Engineering; AVI Publishing Company, Inc.: Westport, CT, 1980, p. 363. Jayaraman, K. S.; Gepinathan, V. K.; Ramanathan, L. A. J . Food Technol. 1980, 15, 217. Jayaraman, K. S.; Gopinathan, V. K.; Pitchamuthu, P.; Vijavaraghavan, P. K. J. Food Technol. 1982, 17, 669. Kim, M. H.; Toledo, R. T. J . Food S c i . 1987, 52, 890. Farkas, D. F.; Lazar, M. E.; Butterworth, T. A. Food Technol. 1969, 23 (11), 125. Lazar, M. E.; Farkas, D. F. J. Food S c i . 1971, 36, 315. Brown, G. E.; Farkas, D. F.; DeMarchena, E. S. Food Technol. 1972, 26 (12), 23. Eisenhardt, Ν. H.; Cording, J . ; Eskew, R. H.; S u l l i v a n , J. F.; Food Technol. 1962, 16 (5), 143. Cording, J . ; Eskew, R. H.; S u l l i v a n , J . F.; Eisenhardt, Ν. H. Food Engin. 1963, 35 (6), 52. S u l l i v a n , J . F.; Cording, J . ; Eskew, R. K.; Heiland, W. K. Food Engin. 1965, 37 (10), 116. Heiland, W. K.; S u l l i v a n , J . F.; Konstance, R. P.; Craig, J . C.; Cording, J . ; Aceto, N. C. Food Technol. 1977, 31 (11), 32. S u l l i v a n , J. F.; Craig, J . C.; Konstance, R. P.; E g o v i l l e , M. J . ; Aceto, N. C. J . Food S c i . 1980, 45, 1550. Sinnam, H. L.; Eskew, R. K.; Cording, J. Dehydrated Explo sion Puffed Carrots with High Density, U.S. Department of Agriculture, ARS 73-50, 1965. Toumy, J . M. Freeze Drying of Foods for the Armed Forces, Technical Report 70-43-FL, U.S. Army Natick Laboratory: Natick, MA, 1970. Shimazu, F.; S t e r l i n g , C. J. Food S c i . 1961, 26, 291. Bourne, M. C. J. Food S c i . 1987, 52, 667. Savage, J . P. U.S. Patent 3 337 349, 1967.


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33. 34.

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Ponting, J . D.; Waters, G. G.; Forey, R. R.; Jackson, R.; Stanley, W. L. Food Technol. 1966, 20 (10), 125. Schwimmer, S. Food Technol. 1969, 23 (7), 115. Dixon, G. M.; Jen, J . J . J . Food S c i . 1977, 42, 1126. Horn, G. R.; S t e r l i n g , C. J . S c i . Food Agric. 1982, 33, 1035. Mazza, G. J . Food Technol. 1983, 18, 113. Jen, J . J . J . Food. S c i . 1974, 39, 907. Murphy, E. E.; Criner, P. E.; Gray, B. C. J. Agric. Food Chem. 1975, 23, 1153. Rinderknecht, H.; Wilding, P.; Haverback, B. J . Experientia 1976, 23, 805. Ng, T. K.; Zeikus, J . G. Anal. Biochem. 1980, 103, 42. McCleary, Β. V. Carbohydrate Res. 1978, 213.

R E C E I V E D April 11, 1989


Quality Factors of Fruits and Vegetables - American Chemical Society

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