Food Biochemistry and Food Processing (2 edition)

BLBS102-c27 BLBS102-Simpson March 21, 2012 13:25 Trim: 276mm X 219mm Printer Name: Yet to Come

536 Part 5: Fruits, Vegetables, and Cereals

fatty acids such as palmitic and stearic acids can decrease the

fluidity. Other membrane components such as sterols, and degra-

dation products of fatty acids such as fatty aldehydes, alkanes

and so on can also decrease the fluidity. Based on the physiolog-

ical status of the tissue, the membrane can exist in either a liquid

crystalline state (where the phospholipids and their acyl chains

are mobile) or a gel state where they are packed as rigid ordered

structures and their movements are much restricted. The mem-

brane usually has co-existing domains of liquid crystalline and

gel phase lipids depending on growth conditions, temperature,

ion concentration near the membrane surface and so on. The

tissue has the ability to adjust the fluidity of the membrane by

altering the acyl lipid composition of the phospholipids. For in-

stance, an increase in the gel phase lipid domains resulting from

exposure to cold temperature could be counteracted by increas-

ing the proportion of fatty acyl chains having a higher degree

of unsaturation, and, therefore, a lower melting point. Thus, the

membrane will tend to remain fluid even at a lower temper-

ature (Whitaker 1991, 1992, 1993, 1994). An increase in gel

phase lipid domains can result in the loss of compartmentalisa-

tion. The differences in the mobility properties of phospholipid

acyl chains can cause packing imperfections at the interface be-

tween gel and liquid crystalline phases, and these regions can

become leaky to calcium ions and protons that are highly com-

partmentalised. The membrane proteins are also excluded from

the gel phase into the liquid crystalline phase. Thus, during ex-

aminations of membrane structure by freeze fracture electron

microscopy, the gel phase domains can appear as regions devoid

of proteins (Paliyath and Thompson 1990).

Proteins

Fruits, in general, are not very rich sources of proteins. During

the early growth phase of fruits, the chloroplasts and mitochon-

dria are the major organelles that contain structural proteins.

The structural proteins include the light-harvesting complexes in

chloroplast or the respiratory enzyme/protein complexes in mito-

chondria. Ribulose-bis-phosphate carboxylase/oxygenase (Ru-

bisco) is the most abundant enzyme in photosynthetic tissues.

Fruits do not store proteins as an energy source. The green fruits

such as bell peppers and tomato have a higher level of chloroplast

proteins.

Organic Acids

Organic acids are major components of some fruits. The acid-

ity of fruits arises from the organic acids that are stored in the

vacuole, and their composition can vary depending on the type

of fruit. In general, young fruits contain more organic acids,

which may decline during maturation and ripening due to their

conversion to sugars (gluconeogenesis; eg. conversion of malic

acid into glucose during ripening of apple). Some fruit families

are characterised by the presence of certain organic acids. For

example, fruits of oxalidaceae members (e.g., starfruit,Averrhoa

carambola) contain oxalic acid, and fruits of the citrus family,

rutaceae, are rich in citric acid. Apples contain malic acid and

grapes are characterised by the presence of tartaric acid. In gen-

eral, citric and malic acids are the major organic acids of fruits.

Grapes contain tartaric acid as the major organic acid. During

ripening, these acids can enter the citric acid cycle and undergo

further metabolic conversions.

l-(+)tartaric acid is the optically active form of tartaric acid

in grape berries. A peak in acid content is observed before

the initiation of ripening, and the acid content declines on a

fresh weight basis during ripening. Tartaric acid can be biosyn-

thesised from carbohydrates and other organic acids. Radiola-

belled glucose, glycolate and ascorbate were all converted to

tartarate in grape berries. Malate can be derived from the citric

acid cycle or through carbon dioxide fixation of pyruvate by

the malic enzyme (NADPH-dependent malate dehydrogenase).

Malic acid, as the name implies, is also the major organic acid

in apples.

FRUIT RIPENING AND SOFTENING

Fruit ripening is a physiological event that results from a very

complex and interrelated biochemical changes that occur in the

fruits. Ripening is the ultimate stage of the development of

the fruit, which entails the development of ideal organoleptic

characters such as taste, colour and aroma that are important

features of attraction for the vectors (animals, birds, etc.) re-

sponsible for the dispersal of the fruit, and thus the seeds, in the

ecosystem. Human beings have developed an agronomic sys-

tem of cultivation, harvest and storage of fruits with ideal food

qualities. In most cases, the ripening process is very fast, and

the fruits undergo senescence resulting in the loss of desirable

qualities. An understanding of the biochemistry and molecu-

lar biology of the fruit ripening process has resulted in devel-

oping biotechnological strategies for the preservation of post-

harvest shelf life and quality of fruits (Negi and Handa 2008,

Paliyath et al. 2008a).

A key initiator of the ripening process is the gaseous plant

hormone ethylene. In general, all plant tissues produce a low,

basal, level of ethylene. Based on the pattern of ethylene produc-

tion and responsiveness to externally added ethylene, fruits are

generally categorised into climacteric and non-climacteric fruits.

During ripening, the climacteric fruits show a burst in ethylene

production and respiration (CO 2 production). Non-climacteric

fruits show a considerably low level of ethylene production. In

climacteric fruits (apple, pear, banana, tomato, avocado, etc.),

ethylene production can reach levels of 30–500 ppm (parts per

million, microlitre/L), whereas in non-climacteric fruits (orange,

lemon, strawberry, pineapple, etc.) ethylene levels usually are

in the range of 0.1–0.5 ppm. Ethylene can stimulate its own

biosynthesis in climacteric fruits, known as autocatalytic ethy-

lene production. As well, the respiratory carbon dioxide evolu-

tion increases in response to ethylene treatment, termed as the

respiratory climacteric. Climacteric fruits respond to external

ethylene treatment by accelerating the respiratory climacteric

and time required for ripening, in a concentration-dependent

manner. Non-climacteric fruits show increased respiration in re-

sponse to increasing ethylene concentration without accelerating

the time required for ripening.