Fruit II
Composition of fruit
(a) components
(b) function
(c) importance in food science
Fruit consist of mainly parenchyma cells
Cell walls
(a)Thin. Cellulose microfibrils embedded in a matrix of non-cellulosic polysaccharides including pectins and xyloglucans.
(b)Provide structural support, resist turgor
(c)Dietary fibre. Texture.
Water
(a) 70-90% fresh wt.
In cell vacuole, cytoplasm and cell wall.
(b)Cell is a highly hydrated entity, aqueous environment for movement of organelles and metabolites.
(c)Freshness associated with crispness - fully turgid cells
Limp and withered indicate age
Freezing- formation of ice crystals & rupturing of cells
Drying - removal of water may cause cellular changes and make rehydration difficult (soup mixes)
Sugars
(a)predominantly sucrose, fructose and glucose.
(b)Respiratory substrates, source of carbohydrate post harvest.
(c)Attracted by sweetness which contributes to flavour
Utilisation of sugar in fermentation (wine)
Sugar may accumulate prior to ripening, these fruits can be harvested green (tomatoes)
Organic acids
(a)Malic acid- common - apples, pears plum, banana
Citric - common, oranges lemons, bananas
Oxalic - found as insoluble calcium oxalate crystals
Succinic - small amounts in several fruit
Chlorogenic acid - common in young apples, pears, peaches, plums
Quinic - common in apple, apricot, peach,banana
(b)Malate, citrate, succinate - respiratory pathways (TCA)
(c)Provide acidity, contribute to flavour.
Lemon juice added to fruit salads reduces browning reaction by lowering pH
Starch
(a)Crystalline starch found in amyloplasts.
Gelatinises on heating in the presence of water, loss of crystallinity - degraded by amylases to maltose (disaccharide) and glucose.
(b)Storage carbohydrate - bananas, cereal grains
Present in unripe fruit eg tomatoes, apples
Metabolised during development
(c)Component of flours
Corn starch manufactured into syrups
Phenolics
(a)Chlorogenic acid, (apples), ferulic acid, gallic acid
(b)Involved in plant defense mechanisms
(c)Substrates for polyphenol oxidase which causes browning in cut fruit (refer to images of bananas in lecture 1)

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