Food Biochemistry and Food Processing

22 Biochemistry of Fruit Processing 519

of storage lipids in fresh fruits, they still possess cell
membrane lipids. Lipids make up the surface wax,
which contributes to the shiny appearance of fruits,
and cuticle, which protects fruits against water loss
and pathogens. The degree of fatty acid saturation
establishes membrane fluidity, with greater satura-
tion resulting in less fluidity. Lipids play a signifi-
cant role in the characteristic aroma and flavor of
fruits. For example, the characteristic aromas and
flavors of cut fruits result from the action of the
enzyme lipoxygenase on fatty acids (linoleic and
linolenic acids), which eventually leads to the pro-
duction of volatile compounds. The action of lip-
oxygenase increases after fruit is cut because there
is a greater chance for the enzyme and substrate to
mix together. Lipoxygenase can also be responsible
for the off-flavor and off-aroma in certain plant
products (soybean oil).

VOLATILES

The specific aroma of fruits is due to the amount and
diversity of volatiles they contain. Volatiles are pres-
ent in extremely small quantities ( 100
g/g fresh
weight). Ethylene is the most abundant volatile in
fruit, but it has no typical fruit aroma. Characteristic
flavors and aromas are a combination of various
character-impact compounds, mainly esters, terpenes,
short-chain aldehydes, ketones, organic acids, and
so on. Their relative importance depends upon the
threshold concentration (sometimes1 ppb), and
interaction with other compounds. For some fruits
such as apple and banana, the specific aroma is deter-
mined by the presence of a single compound, ethyl-
2-methylbutyrate in apples, and isoamylacetate in
bananas.

WATER

Water is the most abundant single component of
fruits and vegetables (up to 90% of total weight).
The maximum water content of fruits varies due to
structural differences. Agricultural conditions also
influence the water content of plants. As a major
component of fruits, water has an impact on both
their quality and their deterioration. Turgidity is a
major quality determinant factor in fruits. Loss of
turgor is associated with loss of quality and is a
major problem during postharvest storage and trans-
portation. Harvest should be done during the cool

part of the day in order to keep turgidity at its opti-

mum.

ORGANICACIDS

The role of organic acids in fruits is twofold:

1. Organic acids are an integral part of many
   metabolic pathways, especially the Krebs
   (TCA) cycle.The tricarboxylic acid cycle (res-
   piration) is the main channel for the oxidation
   of organic acids and serves as an important
   energy source for the cells. Organic acids are
   metabolized into many constituents, including
   amino acids. Major organic acids present in
   fruits include citric acid, malic acid, and quinic
   acid.


2. Organic acids are important contributors to the
   taste and flavor of many fruits and vegetables.
   Total titratable acidity, the quantity and speci-
   ficity of organic acids present in fruits, and oth-
   er factors influence the buffering system and
   the pH. Acid content decreases during ripening,
   because part of it is used for respiration, and
   another part is transformed into sugars (gluco-
   neogenesis). The composition of organic acids
   in some fruits is given in (Table 22.3).

PIGMENTS

Pigments are mainly responsible for the skin and

flesh colors in fruits and vegetables. They undergo

changes during maturation and ripening of the fruits,

including loss of chlorophyll (green color), synthe-

sis and/or revelation of carotenoids (yellow and

orange), and biosynthesis of anthocyanins (red, blue,

and purple) (see Table 22.6, below). Anthocyanins

occur as glycosides and are water soluble, unstable,

and easily hydrolyzed by enzymes to free anthocy-

anins, which may be oxidized to brown products by

phenol oxidases after wounding.

PHENOLICS

Fruit phenolics include chlorogenic acid, catechin,

epicatechin, leucoanthocyanidins and anthocyanins,

flavonoids, cinnamic acid derivatives, and simple

phenols. The total phenolic content of fruits varies

from 1 to 2 g/100 g fresh weight. Chlorogenic acid

is the main substrate responsible for the enzymatic

browning of damaged fruit tissue. Phenolic compo-