Food Biochemistry and Food Processing (2 edition)

(Wang) #1

BLBS102-c38 BLBS102-Simpson March 21, 2012 14:17 Trim: 276mm X 219mm Printer Name: Yet to Come


728 Part 7: Food Processing

Table 38.1.Summary of Zero-, First-, and Second-Order Reaction Rates
(M-molar Concentration)

Rate Law/Order Differential Form Integral Form

Linear Plot to
Determinek

Units of Reaction
Constant (k)

Zero

dA
dt

=−k [A]=[Ao]−kt [A]vs.t

M
s

First

dA
dt

=−k[A]^1 ln[A]=ln[Ao]−kt ln[A]vs.t

1
s

Second

dA
dt

=−k[A]^2

1
[A]

=

1
[Ao]

+kt

1
[A]

vs.t

1
M.s

Common Food Microorganisms

Foods by their very nature involve complex biological molecules
that strongly support the survival and growth of food microor-
ganisms. One of the major causes for food deterioration is the
growth and activity of microorganisms. Microorganisms can
contaminate the food before and after processing from various
sources. The major categories of microorganisms involved in
food spoilage and deterioration are fungi and bacteria. Each mi-
croorganism has its own optimum temperature, pH, and oxygen
level to grow. Broadly, food microorganisms can be categorized
as pathogenic and spoilage ones. Food pathogenic microorgan-
isms are those groups that can cause illness in human being or
animals due to food poisoning or infection. Food poisoning and
food infection are different, although the symptoms are similar.
True food poisoning or food intoxication is caused by eating
food that contains a toxin or poison due to bacterial or fungal
growth in food. The bacteria or fungi that produced and ex-
creted the toxic waste products into the food may be destroyed
during processing, but the toxin they produced can cause the
illness or digestive upset to occur.Staphylococcus aureusand
Clostridium botulinumare two common species of bacteria that
cause food poisoning. Food infection is the second type of food-
borne illness. It is caused by eating food that contains certain
types of live bacteria, which are present in the food. Once the
food is consumed, the bacterial cells themselves continue to
grow and illness can result.Salmonellae, C. perifringens, Vibiro

parahaemolyticus, Yersinia enterocolitica, and Listeria mono-
cytogenesisare common infectious bacteria of food.
The other category includes food spoilage microorganisms.
Food spoilage can be defined as the process or change leading to
a product becoming undesirable or unacceptable for consump-
tion. This can be chemical, physical, or microbial spoilage or
the combination of these factors. Food spoilage is more of an
economic concern than safety issue.
Food microorganisms can be grouped based up on their oxy-
gen and temperature requirement for survival and growth (Table
38.2). In foods that are packaged under vacuum, low oxygen
levels are intentionally achieved. Therefore, the prevailing con-
ditions do not support the growth of microorganisms that require
oxygen (obligate aerobes). Likewise, the thermophilic microor-
ganisms require temperatures much higher than 30◦C for their
growth.
Acidity level of a food is another intrinsic food factor that de-
termines the survival and growth of food microorganisms. From
thermal processing standpoint, foods are divided into three pH
groups: (i) high-acid foods (pH<3.7), (ii) acid or medium-acid
foods (3.7<pH<4.5), and (iii) low-acid foods (pH>4.5).
The most important distinction in the pH classification, with
reference to thermal processing, is the dividing line between
acid and low-acid foods. Most laboratories concerned with ther-
mal processing have devoted attention toC. botulinum,which
is a highly heat-resistant, rod-shaped, spore-forming, anaerobic

Table 38.2.Classification of Microorganisms and Bacteria Based on Their Oxygen Demand and Optimum
Temperature for Growth

Classification Groups of Microorganisms Examples of Microorganisms

Oxygen-based classification
Obligatory aerobes (require oxygen) Most molds,Micrococcus, Serratia marcescens, Mycobacterium
tuberculosis
Obligatory anaerobes (require absence of oxygen) C. botulinum, C. sporogenes, C. thermosaccharolyticum
Facultative anaerobes (tolerate some oxygen) Bacillus coagulans, Staphylococcus aureus

Optimum temperature for growth
Thermophilic (55–35◦C) B. coagulans; S. thermophilus; C. thermosaccharolyticum, C. nigrificans
Mesophilic (40–10◦C) C. pasteurianum, B. mascerans, C. sporogenes, B. licheniformis
Psychrophilic (35–< 5 ◦C) Pseudomonas, Micrococcus, C. botulinumType E, S. aureus
Free download pdf