Food Biochemistry and Food Processing (2 edition)

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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
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