1 Food Biochemistry—An Introduction 9tissues. The structure will break down slowly after
the animal is dead. The desirable postmortem situa-
tion is meat tenderization, and the undesirable sit-
uation is tissue degradation/spoilage.
In order to understand these changes, it is impor-
tant to understand the structure of animal tissues.
Table 1.4 lists the location and major functions of
myofibrillar proteins associated with contractile ap-
paratus and cytoskeletal framework of animal tis-
sues. Schematic drawings and pictures (microscopic,
scanning, and transmission electronic microscopic
images) of tissue macro- and microstructures are
available in various textbooks and references. Chap-
ter 13 in this book, Biochemistry of Raw Meat and
Poultry, also shows a diagram of meat macro- and
microstructures. To avoid redundancy, readers not
familiar with meat structures are advised to refer to
Figure 13.1 when reading the following two para-
graphs that give a brief description of the muscle
fiber structure and its degradation (Lowrie 1992,
Huff-Lonergan and Lonergan 1999, Greaser 2001).
Individual muscle fibers are composed of myofib-
rils 1–2 m thick and are the basic units of muscular
contraction. The skeletal muscle of fish differs from
that of mammals in that the fibers arranged between
the sheets of connective tissue are much shorter. The
connective tissue is present as short transverse sheets
(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 separates the individual myo-
fibrils. Within each fiber is a liquid matrix, referred
to as the sarcoplasm, that contains mitochondria, en-
zymes, glycogen, adenosine triphosphate, creatine,Table 1.3.Changes in Carbohydrates in Cheese ManufacturingAction, Enzyme or Enzyme System Reaction
Formation of lactic acid
Lactase (EC 3.2.1.108) Lactose H 2 O →D-glucose D-galactose
Tagatose pathway Galactose-6-P →lactic acid
Embden-Meyerhoff pathway Glucose →pyruvate →lactic acid
Formation of pyruvate from citric acid
Citrate (pro-3S)lyase (EC 4.1.3.6) Citrate →oxaloaceate
Oxaloacetate decarboxylase Oxaloacetate →pyruvate CO 2
(EC 4.1.1.3)
Formation of propionic and acetic acids
Propionate pathway 3 lactate →2 propionate 1 acetate CO 2 H 2 O
3 alanine →propionic acid 1 acetate CO 2 3 ammonia
Formation of succinic acid
Mixed acid pathway Propionic acid CO 2 →succinic acid
Formation of butyric acid
Butyric acid pathway 2 lactate →1 butyrate CO 2 2H 2
Formation of ethanol
Phosphoketolase pathway Glucose →acetylaldehyde →ethanol
Pyruvate decarboxylase (EC 4.1.1.1) Pyruvate →acetylaldehyde CO 2
Alcohol dehydrogenase (EC 1.1.1.1) Acetylaldehyde NAD H→ethanol NAD
Formation of formic acid
Pyruvate-formate lyase (EC 2.3.1.54) Pyruvate CoA →formic acid acetyl CoA
Formation of diacetyl, acetoine, 2-3 butylene glycol
Citrate fermentation pathway Citrate →pyruvate →acetyl CoA →diacetyl →acetoine
→2-3 butylene glycol
Formation of acetic acid
Pyruvate-formate lyase (EC 2.3.1.54) Pyruvate CoA →formic acid acetyl CoA
Acetyl-CoA hydrolase (EC 3.1.2.1) Acetyl CoA H 2 O →acetic acid CoA
Sources:Schormuller 1968; Kilara and Shahani 1978; Law 1984a,b; Hutlins and Morris 1987; Kamaly and Marth
1989; Eskin 1990; Khalid and Marth 1990; Steele 1995; Walstra et al. 1999; IUBMB-NC website (www.iubmb.org).