Food Biochemistry and Food Processing (2 edition)

BLBS102-c09 BLBS102-Simpson March 21, 2012 11:15 Trim: 276mm X 219mm Printer Name: Yet to Come

200 Part 2: Biotechnology and Enzymology

sterilization, blanching, UHT, high temperature short term treat-

ment, pasteurization, and related thermal treatments to curtail

undesirable enzymatic reactions in food and protect foods from

enzyme-induced postharvest and post-processing spoilage that

could arise from enzymes naturally occurring in food materi-

als, or from enzymes deliberately added to foods to bring about

particular transformations. Nevertheless, thermal processing of

foods has the disadvantage of destroying heat-labile essential

components in foods such as some vitamins and essential oils,

thus the need for effective nonthermal techniques.

Low Temperature Treatments

A decrease in temperature slows down the average kinetic en-

ergy of biomolecules such as enzymes and their substrates. The

molecules are sluggish at low temperatures and collide less fre-

quently and effectively with one another, thus reaction rates are

relatively slow. Hitherto, advantage is taken of lower thermal

energy in approaches such as refrigeration, iced storage, refrig-

erated sea water storage, and frozen storage to slow down the

undesirable effects of enzymes in foods after harvest. Low tem-

perature treatments are generally used to slow down the deleteri-

ous effects of enzymes in fresh foods (vegetables, eggs, seafood,

and meats); however, other processed food products such as

high-fat food spreads (margarine and butter), vacuum-packaged

meats and seafood products, pasteurized milk, cheeses, and yo-

ghurt also benefit from the desirable effects derived from low

temperature treatments.

It must be noted here that there are some enzymes that

are quite stable and active in extreme temperatures (high- or

low-temperature-adapted enzymes—known as extremophiles),

which may survive the traditional temperature treatments and

induce autolysis and spoilage in foods. For these enzymes, other

techniques are needed to stop their undesirable effects.

Low-temperature treatments (refrigeration, chilling, and

freezing) all slow down enzyme activity but do not completely

inactivate the enzymes. Enzymatic activity still takes place in

foods thus treated, albeit at much reduced rates. Once the food

material is out of the source of the low temperature, once the

frozen food material is thawed, enzyme activity may restart and

cause undesirable autolytic changes in foods. Refrigerated and

chilled storage in particular may not be considered as effective

methods for long-term storage of fresh foods in high quality.

Effect of pH

Enzyme activity in foods is pH dependent (Table 9.3). In gen-

eral, enzymes tend to be destabilized and irreversibly inactivated

at extreme pH values, and advantage is taken of this property

of pH to control enzymatic activity in foods. For example, the

enzyme PPO is known to cause undesirable browning in fresh

fruits and vegetables. By adding acids such as citrate, lactate,

or ascorbate, acidic conditions are created (around pH 4) where

the PPO enzyme is inactive. A similar effect is achieved when

glucose/catalase/GOX cocktail is used on raw crustacea (e.g.,

shrimp); the GOX oxidizes the glucose to gluconic acid and this

is accompanied by a drop in pH, which inactivates the PPO.

Table 9.3.pH Stabilities of Selected Enzymes

Enzyme pH Stability Optimum

Pepsin 1.5

Malt amylase 4.5–5.0

Invertase 4.5

Gastric lipase 4.0–5.0

Pancreatic lipase 8.0

Maltase 6.5

Catalase 7.0

Trypsin 8.0

In household food preparations, vinegar or lemon juice is of-

ten sprinkled on fresh foods (e.g., vegetable salads) to prevent

dark discolorations. Lowering pH of citrus juice with HCl has

been used to achieve irreversible inactivation of pectin esterase

(PE; Owusu-Yaw et al. 1988), and LOX activity in soy flour is

drastically reduced at pH≤5.0 (Thakur and Nelson 1997).

Effect of Inhibitors

Enzyme inhibitors are substances that slow down or prevent

catalytic activities of enzymes. They do this by either binding

directly to the enzyme or by removing co-substrates (e.g., O 2 in

enzymatic browning) in the reaction catalyzed by particular en-

zymes. Enzyme inhibitors fall into several categories, such as re-

ducing agents that remove co-substrates such as O 2 (e.g., sulfites

and cysteine), metal chelators that bind or remove essential metal

cofactors from enzymes (e.g., EDTA, polycarboxylic acids, and

phosphates), acidulants that reduce pH and cause enzyme inac-

tivation (e.g., phosphoric acids, citric acid, sorbates, benzoates),

and enzyme inhibitors that bind directly to the enzymes and pre-

vent their activity (e.g., polypeptides, organic acids, and resorci-

nols). Food-grade protein inhibitors from eggs, bovine/porcine

plasma, and potato flour are all used to control undesirable prote-

olytic activity in foods. Egg white contains ovomucoid, a serine

protease inhibitor; bovine and porcine plasmas have the broad

spectrum protease inhibitorα 2 -macroglobulin; and potato flour

has a serine protease inhibitor (potato serine protease inhibitor

or PSPI). Fractions containing these inhibitors have been used to

prevent surimi texture softening due to proteolysis (Weerasinghe

et al. 1996).

Effect of Water Activity

Enzymatic activity depends to the availability of water. Wa-

ter availability, and hence water activity (Aw), in foods may

be modified by using procedures that remove moisture, such as

drying, freezing, addition of water-binding agents or humectants

(such as salt, sugar, honey, glycerol, and other polyols), and by

lyophilization (Tejada et al. 2008). Most enzyme-catalyzed reac-

tions require moisture to progress effectively for several reasons:

(i) a thin film of moisture (bound water) is required to maintain

proper enzyme conformational integrity for functional activity,

(ii) to solubilize substrates and also serve as the reaction medium,