Chemical Basis of Quality Factors in Fruits and Vegetables - American

Chapter 1

Chemical Basis of Quality Factors in Fruits and Vegetables An Overview

Downloaded by 185.14.192.68 on October 2, 2016 | http://pubs.acs.org Publication Date: September 7, 1989 | doi: 10.1021/bk-1989-0405.ch001

Joseph J. Jen Department of Food Science and Technology, The University of Georgia, Athens, GA 30602 Color, flavor, texture, and n u t r i t i v e value are generally recognized as the four quality factors of f r u i t s and vegetables. The natural pigments, chlorophylls, carotenoids and anthocyanins, form the chemical basis of color. Enzymatic and non-enzymatic browning contribute to coloring of certain processed f r u i t s and vegetables. Various v o l a t i l e aroma and nonvolatile compounds give f r u i t s and vegetables special flavors. C e l l wall components and turgor pressure are the two e n t i t i e s that provide the texture of f r u i t s and vegetables. Pectic substances and pectic enzymes are closely related to firmness and softening of many f r u i t s and vegetables. Celluloses and l i g n i n s are associated with toughness and woody texture. The roles of hemicelluloses and extinsins i n f r u i t s and vegetables are not clear. Vitamin C and minerals are the major nutrients of f r u i t s and vegetables. Processing often a l t e r s the quality of f r u i t s and vegetables but does not change the chemical basis underlining the factors.

F r u i t s and vegetables constitute an important part of the human d i e t . I t i s one of the four major groups of food our body needs to ingest d a i l y . The current consumer trend i s towards fresh, natural, minimally processed, and, yet, convenience foods. This situation increases the opportunities for this segment of diet to play an increased role i n our health and well-being. Consumers purchase foods on the basis of quality. To improve quality of f r u i t s and vegetables, we must understand the chemical basis of the quality factors. Many books and chapters have been written on food quality although few have been devoted solely on f r u i t s and vegetables (1, 2). Brief summaries of the subject matter can usually be found i n food chemistry books (3). The purpose of this chapter i s to provide an overview of the chemical bases of quality 0097-6156/89/0405-0001$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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QUALITY FACTORS OF FRUITS AND VEGETABLES factors i n f r u i t s and vegetables. Subsequent chapters w i l l provide detailed information on many s p e c i f i c topics. The flavor and n u t r i t i v e value of foods are often the subject of many symposia and books. This volume w i l l emphasis color and texture of f r u i t s and vegetables. Color


With few exceptions, the coloring matters of f r u i t s and vegetables belong to one of the following four categories of chemical compounds (4): 1. 2. 3. 4.

Tetrapyrrole derivatives: chlorophylls, pheophytins Isoprenoid derivatives: carotenoids, xanthophylls Benzopyran derivatives: anthocyanins, flavonoids, and related compounds Certain a r t i f a c t s : caramels, melanins, etc.

The f i r s t three groups are natural pigments while the l a s t group i s reaction products of enzymatic and non-enzymatic browning reactions from natural physiological changes or from processing of f r u i t s and vegetables. The chlorophylls are responsible f o r the green color of nearly a l l f r u i t s and vegetables. The isomers, chlorophyll a. and chlorophyll b^, exist i n a 3:1 r a t i o i n higher plants. They have s l i g h t l y d i f f e r e n t v i s i b l e spectra and color shades. Chlorophylls are magnesium-chelated tetrapyrroles with an e s t e r i f i e d 20 carbon alcohol, phytol. The phytol gives the pigments a unique a b i l i t y to be embedded i n the c e l l organelle chloroplasts. The naturally existing chlorophyllases can convert chlorophylls to water-soluble chlorophyllides, without the phytol, but do not s i g n i f i c a n t l y a l t e r the green color. Acidic conditions can cause the replacement of hydrogen for magnesium and make the chlorophylls into pheophytins. The pheophytins are brown i n color and are normally undesirable i n most foods. The loss of green color i n green vegetables i s an important problem i n certain thermal processing operations. The chlorophyll chemistry has been the subject of several investigations. The conversion of chlorophylls to pheophytins follows f i r s t - o r d e r reaction k i n e t i c s (5). The photooxidation of chlorophylls have slow reaction rates (6). Organic acids released from the destruction of intact c e l l s can cause conversion of chlorophylls to pheophytins i n processed plant foods. Acids are also formed during processing of certain green leafy vegetables (7). Various methods developed to preserve the green color of processed food have undesirable effects of one form or another. Green metallocomplexes of chlorophyll derivatives formed during thermal processing o f f e r a possible solution to some vegetables. A chapter dealing with this subject i s included i n this book. The carotenoids are a group of l i p i d - s o l u b l e pigments responsible for the yellow, orange, and red color of many f r u i t s and vegetables. Most plant foods contain a variety of carotenoids that d i f f e r mainly i n their content of double bonds and oxygen atoms. The mevalonic acid pathway derived isoprenoids may also occur i n combination with reducing sugars v i a glycosidic bonds.



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Carotenoids are the major source of pro-vitamin A i n our d i e t . Fortunately, they are r e l a t i v e l y heat stable and do not suffer extensive loss during thermal processing. However, most carotenoids are sensitive to photoxidation and may lose their bright color under long exposure to l i g h t and oxygen. In i n t a c t fresh tomatoes, l i g h t can promote carotenoid biosynthesis (8). Anthocyanins are a group of reddish water-soluble pigments that are widespread i n plant foods. Anthocyanins, together with flavonoids, are the largest group of natural pigments due to their conjugation with glycoside compounds. The destruction of anthocyanins are pH and temperature-dependent, making i t d i f f i c u l t to preserve during thermal processing operations. The unique feature of anthocyanins i s the display of d i f f e r e n t colors at a c i d i c and a l k a l i pH conditions. Sometimes metal complexes and dimerization of anthocyanins may lead to d i f f e r e n t color shades from the free benzopyran bases. The use of anthocyanins as natural color additives to foods has only been p a r t i a l l y successful due to the unstable nature of the pigments (9). Other natural pigments e x i s t i n g i n selected p i mt foods include tannin, b e t a l a i n , leucoanthocyanin, quinone and xanthone. Table I l i s t s the color and s t a b i l i t y of natural pigments i n f r u i t s and vegetables (10).

Table I.

Summary of Characteristics of Natural Pigments i n F r u i t s and Vegetables

Pigment Group

Chlorophylls Carotenoids Anthocyanins Flavonoids Betalains Leucoanthocyanins Tannins Quinones Xanthones

No. Compounds

25 300 120 600 70 20 20 200 20

Colors

Green, Brown Yellow, Red Red, Blue Yellow Red Colorless, Yellow Yellow Yellow Yellow

Stable To

Alkali Heat

— Heat

—

Sensitive To

Heat, Acid Light, Oxidation pH, Heat Oxidation Heat

Heat

—

Heat Heat Heat

— — —

Enzymes known as polyphenol oxidases cause enzymatic browning. Other names of the enzyme include phenolases and tyrosinases. The enzymes catalyze the conversion of monophenols and diphenols to quinones. The quinones can undergo a series of non-enzymatic reactions to produce brown, gray and black colored pigments, c o l l e c t i v e l y known as melanins (11). M a i l l a r d reactions, caramelizations and ascorbic acid oxidations can produce s i m i l a r types of colored compounds (12). For some food processing


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operations, browning reactions are desirable, such as i n potato chip manufacturing. However, i n most f r u i t and vegetable products, browning reactions, whether enzymatic or non-enzymatic, are undesirable. This i s to maintain the natural coloration of the plant foods. S u l f i t e s , a very e f f e c t i v e browning i n h i b i t o r , i s experiencing declined usage due to health reasons to asthmatic consumers. Alternative browning i n h i b i t o r s are described i n a subsequent chapter. Research projects i n basic enzymology of polyphenol oxidase and the chemistry of non-enzymatic browning may lead to new browning i n h i b i t o r s . Reviews on polyphenols oxidase (13) and on Maillard reactions (14) are available. The interest i n enzymatic browning i s aided by skin cancer research as melanins are the pigments involved in the coloring of human skin. Flavor In recent years, the instrumentation analyses of flavor compounds i n foods have given us a rather comprehensive knowledge on the chemical compounds that give the flavor sensation of f r u i t s and vegetables (15). The biosynthetic pathway and enzymes involved i n biogenesis of flavor compounds of many f r u i t s and vegetables have also made progress (11). It i s beyond the scope of this chapter to discuss i d e n t i f i c a t i o n and biosynthesis of f r u i t and vegetable flavor compounds. The techniques involved i n the prevention of o f f - f l a v o r formation during processing and storage periods i s of interest to food chemists. The enzymes involved i n o f f - f l a v o r formation i n f r u i t s and vegetables seem to be peroxidases and lipoxygenases. These enzymes have the a b i l i t y to form highly reactive free radicals and hydroperoxides. L i p i d and unsaturated fatty acids are the substrates for the oxidase a c t i v i t i e s . It has been an industry standard for years to blanch vegetables to t o t a l destruction of peroxidase i n shelf-stable foods. In recent years, some researchers have raised the question of whether blanching to peroxidase free condition may be over-processing. Lowering of blanching time or processing temperature can lead to superior quality products and can save energy i n operation. Research on lowering the heat treatment to inactivate lipoxygenase rather than peroxidase has yielded promising results i n selected vegetables. Details of t h i s research are discussed i n another chapter i n t h i s book. Using flavor enzyme to regenerate l o s t fresh food flavor i n processed f r u i t s and vegetables has received limited attention (16). Flavorase can catalyze flavor regeneration of banana, raspberry, peanuts and a number of vegetables, e.g., cabbage, onions, carrots, green beans and tomatoes (11). With consumer interest i n high quality products, renewed interest i n the concept of flavorase i s l i k e l y to occur. The rapidly spreading biotechnology techniques for mass production of high valued ingredients, including food flavor, w i l l undoubtedly soon accelerate flavor research. Currently, s c i e n t i s t s are working on f r u i t and vegetable tissue culture research (17). A chapter on the application of biotechnology to improve the quality of f r u i t s and vegetables i s included i n t h i s book.


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Texture Texture i s probably the most elusive quality factor of f r u i t s and vegetables. S c i e n t i s t s have d i f f e r e n t d e f i n i t i o n s for food texture (18, 19). In food chemistry, texture of f r u i t s and vegetables represent the biomolecules involved i n the c e l l u l a r structure of c e l l walls. The degradation during natural physiological transitions or a r t i f i c i a l processing operations may a l t e r textural properties of foods. The size and shape of the c e l l s and the turgor pressures are factors i n determining textural parameters of fresh f r u i t s and vegetables. Some of the most important parameters are hardness, firmness and crispness. The classes of macrobiomolecules i n c e l l walls of f r u i t s and vegetables that have received the most attention are pectic substances and pectic enzymes. A recent book provided an i n depth review of the chemistry and function of pectin (20). Considering the importance of pectins and their close relationship with textural properties of f r u i t s and vegetables, more information on the subject w i l l continue to be gathered. Pectic substances are the glues to plant c e l l s and exist predominately i n middle lamella between c e l l walls. The American Chemical Society defined the terms protopectin, p e c t i n i c acid and pectic acid (21). The native protopectin i s a long chain, alpha-1, 4-D-galacturonic acid polymer, interspersed with alpha-1,2-Lrhamnose residues. Shorter polymers, galactans and arabinans exist and are bonded covalently with the main chain (22). Many types and d i f f e r e n t amounts of side chains exist i n various plant tissues. In apple pectin, hairy regions and smooth regions were observed (23). The hairy region has a backbone of rhamnogalacturonan carrying arabinogalactan and xylogalacturonan side chains. The smooth regions have homogeneous galacturonan chains of 70-80% methylation. Other f r u i t or vegetable protopectin fractions probably have similar but different structures. The nature of bonding between pectin molecules and other polysaccharides and s t r u c t u r a l protein, extinsins, have been proposed i n cultured plant tissue (24). The s o l u b i l i z a t i o n of insoluble protopectin to water-soluble p e c t i n i c acid i s a phase of the natural ripening process i n many f r u i t s and vegetables. Besides several polysaccharides and structural proteins, other substances such as calcium and magnesium may play a role i n the firmness of raw and processed f r u i t s and vegetables. The divalent ions can form a bridge between free carboxyl groups of pectic chains to give r i g i d i t y to the c e l l structure. Many plant c e l l walls contain pectins with high degree of methylation and few free carboxyl groups f o r such bonding to occur. Polygalacturonases (PG) are enzymes that can break down the free pectic acid chain into lower molecular weight pectins. Highly methylated pectins must be de-esterified by pectin methyl esterase (PME) before PG action. For certain vegetables, such as tomatoes, a direct correlation between pectic substances or pectic enzyme a c t i v i t i e s and textural softening do not exist (25). Since the blossom end of a tomato ripes faster than the stem end of the f r u i t , a non-uniform sample exist i n each tomato f r u i t . In one experiment carried out i n our laboratory, the tomatoes were cut


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into three equal sections and analyzed as separate samples. Linear correlations between PME a c t i v i t i e s and texture were obtained for each of the three sections ( F i g . 1). However, i n the same experiment, no correlation of textural properties of tomato f r u i t s with d i f f e r e n t fractions of pectins or PG a c t i v i a t i e s were observed. The d i v e r s i f i e d f r u i t and vegetable c e l l s make direct c o r r e l a t i o n between c e l l wall compositions and textural properties d i f f i c u l t to achieve. However, within a single f r u i t or vegetable, many such correlations are possible. These correlations can provide useful information for process optimization to produce high quality products with desirable textural properties. Other c e l l w a l l polysaccharides, the c e l l u l o s e s , hemicelluloses and l i g n i n s , have received less attention i n comparison with pectins. Celluloses and l i g n i n s are normally associated with woody, tough and other undesirable textural q u a l i t i e s . The hemic e l l u l o s e s are d i f f i c u l t to study i n f r u i t s and vegetables. The s t r u c t u r a l proteins of the c e l l walls, the extinsins, have received l i t t l e or no attention i n f r u i t s and vegetables. However, the extinsins could very well provide information needed to f u l l y explain many phenomena observed i n the ripening and senescence of f r u i t s and vegetables. More basic information on extinsins and their i n t e r r e l a t i o n s h i p s with f r u i t and vegetable c e l l wall polysaccharides are needed. N u t r i t i v e Values The most important n u t r i t i v e values of f r u i t s and vegetables are their content of vitamin C, minerals and f i b e r . Various amounts of other vitamins and nutrients exist i n f r u i t s and vegetables which contribute s i g n i f i c a n t l y to the balance of our diet (26). It i s a general b e l i e f that fresh, raw f r u i t s and vegetables contain the highest amount of vitamins and minerals. This b e l i e f i s not always true. The n u t r i t i v e value of f r u i t s and vegetables vary due to many factors. Some of the factors are v a r i e t i e s , maturities, c u l t u r a l practices, climate and location of the growth of the plant foods. Home storage time and method of preparation before consumption has s i g n i f i c a n t bearing on nutrient retentions (27). Processing does have some destructive effect on the n u t r i t i v e value of foods. This topic i s reviewed i n book form (28). Most national brand processed foods have tight ingredient s p e c i f i c a tions. Thus, most frozen and canned f r u i t s and vegetables contain as much nutrients as fresh, raw products. Other processed foods contain enriched or f o r t i f i e d vitamins and minerals. Ascorbic acid i s often higher i n processed foods than i n non-processed products. Consumers are interested i n fresh, natural, raw and minimally processed foods. F o r t i f i c a t i o n and enrichment of nutrients are more d i f f i c u l t i n these foods. Effect of Processing on Quality Factors The food industry consistently seeks new and improved processing technology to produce high quality products at a low cost. The


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Chemical Basis of Quality Factors in Fruits and Vegetables - American

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