Fundamental Food Microbiology, Third Edition - Fuad Fathir

110 FUNDAMENTAL FOOD MICROBIOLOGY
Table 9.2 Effect of Freezing and Thawing on Viability Loss and Injury of Esc. coli
NCSM
Enumeration
Mediaa Media CFUs/ml Subpopulations
93.7% of cells die. The survivors form colonies in the nonselective TS agar (they
repair and multiply), but 80.1% of the survivors fail to form colonies in the selective
TSD agar (because of their injury and developed sensitivity to deoxycholate). Among
the survivors, however, 19.9% are normal or uninjured cells, inasmuch as they grow
equally well both in the TS and TSD agar media and are no longer sensitive to
deoxycholate. Many food systems can reduce both the cell death and injury incurred
from a specific stress.
Bacterial spore injury has been observed following heating, UV and ionizing
radiation, treatment by hydrostatic pressure, and some chemicals (e.g., hypochlorite,
H 2O 2, ethylene oxide, and probably nitrite) that are important in foods. The injured
spores develop sensitivity to NaCl, low pH, NO 2, antibiotics, redox potential, gaseous
atmosphere, and temperature of incubation. They also have delayed germination and
longer lag for outgrowth, and they develop a need for some specific nutrients. These
manifestations also vary with the nature of stress to which the spores are exposed.
C. Sites and Nature of Injury
Before
Freezing
After
Freezing
TS agar Nonselective 276 ¥ 10 6 17.5 ¥ 10 6
TSD agar Selective 267 ¥ 10 6 3.5 ¥ 10 6
Original population
— dead: 276 ¥
106 –17.5 ¥ 106 (93.7%)
Among survivors
— uninjured: 3.5
¥ 106 (19.9%);
injured: 17.6 ¥ 106 – 3.5 ¥ 106 (80.1%)
a Esc. coli cells in water suspension were enumerated before and after freezing (–20�C for
16 h) and thawing simultaneously in TS (tryptic soy) agar and TSD (TS + 0.075%
deoxycholate) agar media by pour plating followed by incubation at 37�C for 24 h.
Altered physiological characteristics of the injured bacterial cells have been used to
determine directly the site of damage in the cellular structural and functional components.
There is evidence that some cell components are damaged by almost all
types of stresses studied. In addition, specific components can be damaged by
specific stresses. The structural and functional components known to be damaged
by sublethal stresses are the cell wall (or outer membrane, OM), cytoplasmic membrane
(or inner membrane, IM), ribosomal RNA (rRNA) and DNA, and some
enzymes (see Figure 2.2). Damages in the cell wall (or OM) and cytoplasmic
membrane (or IM) are more evident in injury caused by freezing and drying, whereas
damage to rRNA is more extensive in sublethal heating and DNA damage following
radiation of cells. 1–3,14