Food Biochemistry and Food Processing (2 edition)

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