Fundamental Food Microbiology, Third Edition - Fuad Fathir

108 FUNDAMENTAL FOOD MICROBIOLOGY
ingested are killed in the stomach, but a few survive, enter the GI tract, and set up
infection. However, if a pathogenic strain in a food is stress adapted, even consumption
of a much lower number will enable it to survive in the stomach and cause
infection in the GI tract. An attempt should be made to eliminate or reduce the
presence of stress-adapted pathogens in a ready-to-eat food.
3. Enhancing Viability of Starter Cultures and Probiotic Bacteria
Commercial starter cultures are normally frozen or freeze-dried before their use by
the food processors with an intention of having high levels of survivors. Viability
of these cultures, especially freeze-dried cultures, is generally low. Similarly, many
probiotic bacteria are normally susceptible to stomach pH and low-pH food products
(e.g., yogurt containing Lactobacillus acidophilus). However, first exposing the
cultures to a mild stress to release stress proteins may enable the cells to survive
subsequent freezing, freeze-drying, or exposure to low pH in the stomach or in food
products. Genetic engineering techniques can also be used to develop new strains
capable of producing cryoproteins and different stress proteins and surviving better.
III. SUBLETHAL STRESS AND INJURY
A. Definition and Observations
Sublethal injury occurs following exposure of bacterial cells to unfavorable physical
and chemical environments (beyond the growth range but not in the lethal range)
that cause reversible alterations in the functional and structural organizations of the
cells. 1–3 As early as 1932, Rahn 12 suggested that death of microbes exposed to
sublethal stresses is a gradual process that can be reversed under proper conditions
if the reactions have not progressed too far. In 1959, Strake and Stokes 13 showed
that bacterial cells exposed to cold temperature developed characteristics that were
different from those of the normal population; the cells also temporarily lost the
ability to multiply. However, in an appropriate environment, they repaired their injury
and initiated multiplication. Later studies revealed that cells of yeasts and molds
and spores of bacteria also incur reversible injury following exposure to sublethal
stresses.
Many of the treatments include those used directly during food processing and
storage as well as microbial detection from foods. Treatments include low heat (such
as pasteurization), low temperature (freezing, refrigeration, and chilling), low A w
(different types of drying or adding high solutes such as sugar or salt), radiation
(UV or x-ray), high hydrostatic pressure, electric pulse, low pH (both organic and
inorganic acids), preservatives (sorbates or benzoates), sanitizers (chlorine or quaternary
ammonium compounds), hot microbiological media (especially selective
agar media above 48�C), and different selective enumeration and detection methods.
This phenomenon is observed in many species of bacterial cells and spores, yeasts,
and molds that are important in foodborne diseases, food spoilage, and food bioprocessing,
and as indicators of sanitation (Table 9.1). From this list it becomes