Fundamental Food Microbiology, Third Edition - Fuad Fathir

46 FUNDAMENTAL FOOD MICROBIOLOGY
Flavobacterium, Alcaligenes, and some coliforms and Bacillus spp. They can affect
the acceptance quality of raw milk (e.g., by making flavor and texture undesirable).
Some of them can produce heat-stable enzymes (proteinases and lipases), which can
also affect the product quality, even after pasteurization of raw milk (see Chapter
20). Psychrotrophic pathogens (Lis. monocytogenes and Yer. enterocolitica) can
multiply in refrigerated raw milk during storage.
Microbiological quality of raw and pasteurized milk is monitored in many
countries by regulatory agencies. In the U.S., the standard plate counts of raw milk
for use as market milk are 1–3 ¥ 10 5 /ml, and for use in product manufacturing are
0.5–1 ¥ 10 6 /ml. Grade A pasteurized milk can have standard plate counts of
20,000/ml and £10 coliforms/ml.
Microorganisms present in pasteurized milk are those that survive pasteurization
of raw milk (e.g., the thermodurics) and those that enter after heating and before
packaging (e.g., postpasteurization contaminants). Thermodurics surviving pasteurization
include Micrococcus, some Enterococcus (e.g., Ent. faecalis), Streptococcus,
some Lactobacillus (e.g., Lab. viridescens), and spores of Bacillus and Clostridium.
Postheat contaminants can be coliforms as well as Pseudomonas, Alcaligenes, and
Flavobacterium. Some heat-sensitive pathogens can also enter pasteurized milk
following heat treatment. Psychrotrophs can grow during refrigerated storage. 1,2,5
IV. SHELL EGG AND LIQUID EGG
Shell eggs are contaminated with microorganisms on the outer surface from fecal
matter, nesting materials, feeds, air, and equipment. Each shell, depending on the
contamination level, can have 10 7 bacteria. Washing helps reduce bacterial level
considerably. Eggshells can harbor different types of bacteria, namely Pseudomonas,
Alcaligenes, Proteus, Citrobacter, Esc. coli, Enterobacter, Enterococcus, Micrococcus,
and Bacillus. They can also have Salmonella from fecal contamination. Infected
ovaries of laying hens can be the source of Salmonella Enteritidis in the yolk. Liquid
egg can be contaminated with bacteria from the shell of washed eggs as well as
from the breaking equipment, water, and air. Pasteurization can reduce the numbers
to 10 3 /ml. Bacteria, especially motile Gram-negative bacteria, can enter through
pores of eggshells, particularly if the shells are wet. Several antimicrobial factors
present in egg albumin, such as lysozyme, conalbumin (binds iron), avidin (binds
biotin), or alkaline pH (8.0 to 9.0), can control bacterial growth. However, if the
storage temperature is favorable, they can grow in yolk that is rich in nutrients and
has a pH of 7.0. Pasteurization of liquid egg has been designed to destroy pathogens
(especially Salmonella) and other Gram-negative rods. Thermoduric bacteria,
namely Micrococcus, Enterococcus and Bacillus, present in the raw liquid egg
survive pasteurization. 1,2,6
V. FISH AND SHELLFISH
This group includes finfish, crustaceans (shrimp, lobster, crabs), and mollusks (oysters,
clams, scallops) harvested from aquatic environments (marine and freshwa-