Carbohydrate Metabolism During. Fruit Spoilage. Neelima Garg and V.K.Singh. Consumption of fruit and vegetables has dramatically increased globally during ...
8 Carbohydrate Metabolism During Fruit Spoilage Neelima Garg and V.K.Singh
Consumption of fruit and vegetables has dramatically increased globally during the past a few decades. These are living perishable biological entities which perform a number of metabolic functions and have a short life span. Once the fruit is harvested, respiration and transpiration continue, but only for as long as the fruit can draw on its own food reserves and moisture. It is this limited ability to continue vital metabolic functions which define fruit as perishable. It is also estimated that about 20% of all fruits and vegetables produced are lost each year due to spoilage. The process of the fruit spoilage is just the decomposition of the fruit itself. Once harvested, this begins to deteriorate until eventually it becomes unfit for consumption. This deterioration is known as decay and that leads to spoilage. Latter may be defined as “any sensory change (physical, visual, olfactory or flavour) which the consumer considers to be unacceptable. The gradual changes which cause deterioration and decay in fruits are due to certain organisms and chemicals present in the food and outside the food. Carbohydrates play a major role in the fruit spoilage process (Prasanna et al., 2007) by way of depolymerization leading to decreased molecular size with concomitant increase in the levels of ripening inducing specific enzymes, whose target differ from fruit to fruit. The major classes of 179
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cell wall polysaccharides that undergo modifications during ripening are pectins, cellulose, starch, and hemicelluloses (Hallet et al., 2005). The focus of this chapter is to provide information about carbohydrate metabolism during fruit spoilage. Before understanding the carbohydrate metabolism, it is important to know about fruit cell wall structure. The cell wall is a complex structure, composed of polysaccharides, imposing cell shape and rigidity. These are carbohydrate polymers consisting of tens to hundreds to several thousand monosaccharide units (Hallett et al., 2005) . All of the common polysaccharides contain glucose as the monosaccharide unit. Primary cell walls of plants are considered a network of cellulose microfibrils embedded in a matrix of hemicellulose and pectin (Rose and Bennett 1999; Cosgrove 2001).The cell walls of fruits do not have secondary walls and contain very low amount of lignins. Instead, they contain a high level of pectins, which can be modified more easily at different stages of plant development (Rose et al., 2003; Harris, 2005), therefore allowing a fine-tuning of cell wall properties during fruit maturation.
Cellulose The major component in the rigid cell walls in fruits is cellulose(Updegraff,1969). Cellulose is a linear polysaccharide polymer with many glucose monosaccharide units with beta-1,4 linkage between glucose molecules (Fig 1).
CH2OH
CH2OH CELLULOSE
Glucose Unit
n Fig. 1 : Cellulose structure
Cellulose fibers usually consist of over 500,000 cellulose molecules. It is the H-bonding that is the basis of the high tensile strength of cellulose(Fig 2).
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cellulose molecules H-bonds
Fig. 2 : Arrangement of Cellulose molecules in fibre
Pectin Pectins are the common and major components of primary cell wall and middle lamella, contributing to the texture and quality of fruits. Their degradation during ripening seems to be responsible for tissue softening of a number of fruits (Barras et al., 1994). Structurally, pectins are a diverse group of heteropolysaccharides containing partially methylated D-galacturonic acid residues with side chain appendages of several neutral polysaccharides. The degree of polymerization/ esterification and the proportion of neutral sugar residues/side chains are the principal factors contributing to their (micro-) heterogeneity. Pectin substances are the negatively charged and acidic complex polysaccharide in which the primary chain is composed with -(1,4) linked molecules of -D-galacturonic acid(Fig 3). The side chains also contain molecules of L-rhamnose, arabinose, galactose and xylose that are connected to the main chain through their first and the second carbon atom. The carboxyl groups of galacturonic acid are partially esterified by methyl groups and partially or completely neutralized by calcium, potassium,magnesium, iron, ammonium or other ions. Some of the hydroxyl groups on the second and the third carbon atom can be acetylated.
Fig. 3 : Structural formula of the primary chain of pectin- the polygalacturonic acid
Pectic substances may be divided into four types: Protopectin: It is the water insoluble pectic substance present in intact tissue. Protopectin on restricted hydrolysis yields pectin or pectic acids. The structure of protopectin is shown in Fig. 4.
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Protopectin structure Fig.4 : Protopectin structure
Pectic Acid It is a major component of middle lamella but also found in primary walls is the soluble polymer of galacturonans that contains negligible amount of methoxyl groups (Fig 5). It forms salts and salt bridges with Ca++ and Mg++ that are insoluble gels.
Fig 5 : Pectic acid structure
Pectinic acids: is the polygalacturonan chain that contains >0 and
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