Food Biochemistry and Food Processing (2 edition)

BLBS102-c01 BLBS102-Simpson March 21, 2012 11:8 Trim: 276mm X 219mm Printer Name: Yet to Come

10 Part 1: Principles/Food Analysis

Table 1.4.Locations and Major Functions of Myofibrillar Proteins Associated with Contractile Apparatus and

Cytoskeletal Framework

Location Protein Major Function

Contractile apparatus

A-band Myosin Muscle contraction

c-Protein Binds myosin filaments

F-, H-, I-Proteins Binds myosin filaments

M-line M-Protein Binds myosin filaments

Myomesin Binds myosin filaments

Creatine kinase ATP synthesis

I-band Actin Muscle contraction

Tropomyosin Regulates muscle contraction

Troponins T, I, C Regulates muscle contraction

β-,γ-Actinins Regulates actin filaments

Cytoskeletal framework

GAP filaments Connectin (titin) Links myosin filaments to Z-line

N 2 -line Nebulin Unknown

By sarcolemma Vinculin Links myofibrils to sarcolemma

Z-line α-Actinin Links actin filaments to Z-line

Eu-actinin, filamin Links actin filaments to Z-line

Desmin, vimmentin Peripheral structure to Z-line

Synemin, Z-protein, Z-nin Lattice structure of Z-line

Source: Eskin 1990, Lowrie 1992, Huff-Lonergar and Lonergan 1999, Greaser 2001.

(myocommata) that divide the long fish muscles into segments

(myotomes) corresponding in numbers to those of the vertebrae.

A fine network of tubules, the sarcoplasmic reticulum, sepa-

rates the individual myofibrils, and within each fibre is a liquid

matrix referred to as the sarcoplasm-containing enzymes, mito-

chondria (cellular powerhouse), glycogen (carbohydrate storage

form in animals), adenosine triphosphate (energy currency), cre-

atine (part of energy transfer in muscle) and myoglobin (oxygen

transport molecule). The basic unit of the myofibril is the sar-

comere, which is made up of a thick set of filaments consisting

mainly of myosin, a thin set containing primarily of F-actin and a

filamentous ‘cytoskeletal structure’ composed of connectin and

desmin. Meat tenderisation is a very complex, multi-factorial

process involving glycolysis and the actions of both endogenous

proteases (e.g. cathepsins and calpains) as well as intentionally

added enzymes. Table 1.5 lists some of the more common en-

zymes used in meat tenderisation. Papain, ficin and bromelain

are proteases of plant origin that efficiently break down ani-

mal proteins applied in meat tenderisation industrially or at the

household/restaurant levels. Enzymes such as pepsins, trypsins

and cathepsins cleave animal tissues at various sites of pep-

tide chains, while enteropeptidase (enterokinase) is also known

to activate trypsinogen by cleaving its Lys6-Ile7 peptide bond.

Plasmin, pancreatic elastase and collagenase are responsible for

the breakdown of animal connective tissues.

Chymosin (rennin) is the primary protease critical for the

initial milk clotting step in cheese making and is traditionally

obtained from calf stomach. Lactic acid bacteria (starter) grad-

ually acidify milk to the pH 4.7, the optimal pH for coagulation

by chymosin. Most lactic acid starters have limited proteolytic

activities, i.e. product proteins are not degraded fully as in the

case of GI tract breakdown of dietary proteins. The proteases and

peptidases breakdown milk caseins to smaller protein molecules

that, combined with milk fat, provide the cheese structure. Other

enzymes such as decarboxylases, deaminases and transaminases

are responsible for the degradation of amino acids into secondary

amines, indole,α-keto acids and other compounds that give the

typical flavour of cheeses (see Table 1.6 for enzymes and their

reactions).

Germinating seeds also undergo proteolysis, although in a

much lower amount relative to meat. Aminopeptidase and car-

boxypeptidase A are the main, known enzymes (Table 1.7) here

that produce peptides and amino acids needed in the growth of

the plant.

In beer production, a small amount of protein is dissolved

from the wheat and malt into the wort. The protein fraction

extracted from the wort may precipitate if present in the resulting

beer due to its limited solubility at lower temperatures, resulting

in hazing. Proteases of plant origin such as papain, ficin and

bromelain break down such proteins to reduce this ‘chill-haze’

problem in the brewing industry.

Protein Modifications

A protein’s amino acid sequence is critical to its physico-

chemical properties, and it follows that changes made to indi-

vidual amino acids may alter its functionality. In addition to the

many chemical alterations that may occur to amino acids during