Fruit protopectin

FIG. 15 Texture modification (percentage of raw fruit texture) and protopectin content (mg/100 g wet weight) before (BF) and after (AF) freezing of strawberry slices not pretreated (NT) or air dried at 80 °C up to 60% weight reduction without (AD) or following 60-min osmotic dehydration (OAD) in 60% (w/w) sucrose solution at 25 °C at atmospheric pressure (Brimar, 2002). 

The pectin of commerce is the acid or enzyme hydrolyzate of protopectin—the insoluble parent polymer residing in the cell walls of higher plants, primarily apples and citrus fruits. The pectin is isolated by entrainment in polymeric aluminum hydroxide produced in situ by neutralization of an acidic aluminum salt in solution. The pectin isolate is freed of Al(III) by washing and dialysis in an acidified ethanol-water solvent. Advanced demethylation of extracted pectin with alkali or enzymes yields a series of low DE pectins. 

Protopectin Plants and nonmatured fruits Decomposes to pectins on plant maturation and cooking... 

Protopectin is composed of water-insoluble pectic substances, which are fixed to the middle lamella and primary cell walls of plant cells. The neutral sugar side chain of the pectin is attached to the xyloglucan residues, which are bound to the cellulose fibers. The protopectin includes polyvalent such as calcium (Fig. 11.4-8). Protopectin is present in unripe fruits. During the maturation process of fruits or after harvesting, the protopectin is converted to soluble pectin11851. The insolubility of protopectin may be due to the polymerization of the molecule and to the cross-linking with divalent cations11861. 

A better understanding of this subject is possible when the location and function of the pectic substances in plants are reviewed. The movement of water and plant fluids to the rapidly growing fruits and the retention of form and firmness of fruits are functions of pectin. This intercellular substance in plants is similar in action to the intercellular substance of the vertebrates—collagen (the precursor of gelatin). Protopectin, the water-insoluble precursor of pectin, is abundant in immature fruit tissues. Ripening processes involve hydrolytic changes of protopectin to form pectin and later, as maturity is passed, enzymic demethylation and depolymerization of pectin to form pectates and eventually soluble sugars and acids. 

Fruit nectars are produced from fruit slurries or whole fmits by homogenization in the presence of sugar, water and, when necessary, citric and ascorbic acids. The fruit content (as fresh weight) is 25-50% and is regulated in most countries, as is the minimum total acid content. Apricots, pears, strawberries, peaches and sour cherries are suitable for nectar production. The fruits are washed, rinsed, disintegrated and heated to inactivate the enzymes present. The fruit mash is then treated with a suitable mixture of pectinolytic and cellulolytic enzymes. The treatment degrades protopectin and, thus, separates the tissue into its individual intact cells ( maceration ). 

Pectins are found mostly in large flowering plants and are located in both the primary cell wall and the intercellular space called the middle lamella [68], This results in the cell-cell adhesion effect and provides consistency and mechanical resistance to plant tissue. In immature fruits, the most abundant pectic substance is protopectin, very insoluble with a high molecular weight. Pectin together with cellulose and hemicelluloses forms the backbone of the cell wall [50]. 

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