Fundamental Food Microbiology, Third Edition - Fuad Fathir

MICROBIAL STRESS RESPONSE IN THE FOOD ENVIRONMENT 113
necessary. Time, temperature, and gaseous environment also have to be optimal for
the species. Generally, a longer repair time is required by spores damaged by heat,
radiation, and chemicals than by low temperature. 1,14,15
E. Injury in Yeasts and Molds
Very limited studies on injury and repair in yeasts and molds have been conducted.
Freezing, low heat treatment, and irradiation have been reported to cause injury in
vegetative cells of Saccharomyces, Kluyveromyces, Candida, Aspergillus, Penicillium,
and Rhizopus spp. Spores of the molds have been found to be damaged by
irradiation. The main characteristic of the injured cells and spores is their increased
sensitivity to many selective environments. The cell membrane seems to be the major
structure implicated in injury. Repair in nutritionally rich, nonselective media occurs
before multiplication. 1–3
F. Importance of Sublethally Injured Microorganisms in Food
Many of the physical and chemical treatments able to induce sublethal injury in
microbial cells and spores are used in the processing, storage, and preservation of
foods and the sanitation of facilities. Thus, it is quite likely that the foods and the
facilities will harbor injured microorganisms. Microbial injury is important in food
microbiology for several reasons. 14–16
1. Detection of Undesirable Microorganisms
Injured microorganisms are potentially capable of multiplying. Thus, injured pathogens
can cause foodborne disease, and injured spoilage microorganisms can reduce
the shelf life of a product. It is important that they be detected if present in a food.
For detection of many microorganisms in food, several types of selective liquid and
solid media are used (Appendix E). Injured microorganisms may not be detected in
these media. As a result, foods containing viable but injured pathogens and indicators
above the regulatory or acceptable limits, and high numbers of spoilage microorganisms,
can be sold. These products can be hazardous and have a short shelf life,
despite meeting the regulatory standards and specifications. To overcome these
problems, a short repair phase has been incorporated before the selective detection
procedures of important microorganisms in foods. This information is also important
in designing processing parameters to obtain proper reduction of undesirable microorganisms
in finished products (such as heating temperature and time).
2. Enhancing Shelf Life of Foods
Injured cells are susceptible to many physical and chemical environments. These
conditions, where possible, can be used in the preservation of foods (such as low
temperature, lower pH, or preservatives for low-heat-processed foods) to kill injured
cells and spores. In this manner, they will be unable to repair, and their potential
ability to grow and cause product spoilage can be reduced.
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