Rapid Methods of Microbial Determination
and Enumeration for the Meat Industry
R.H. DAINTYl and R.A. EDWARDS2
Introduction
Soil and faeces on the external surfaces of live animals are major sources of the
surface microbial contamination which is inevitably found on finished carcasses
going into chill. Adherence to the strictest hygiene standards, even when combined
with the use of spray washing with hot/warm water or aqueous solutions contain-
ing an antimicrobial organic acid cannot prevent, but only minimize it. Going into
chill a wide range of numbers and types of microorganisms, reflecting the sources
of contamination, are typically found including micrococci, staphylococci, flavobac-
teria, Bacillus spp., Enterobacteriaceae, coryneforms, Campylobaeter spp., Pseudo-
monas spp. and many others.
Not all of these grow after processing and/or storage, characteristic microbial
associations being recognized for particular products and associated storage condi-
tions. Although species of public health concern are present in variable, and some-
times unacceptable numbers from time to time, they will not be of primary concern
in this presentation. Attention will be focused on the organisms which grow and
eventually cause spoilage and hence economic losses either directly, or perhaps even
more importantly, through loss of future customer confidence in the product.
Cured meat products develop a quite different kind of flora to noncured products
during storage and the flora on both kinds of product is drastically changed by the
use of vacuum packaged rather than aerobic storage conditions. Pasteurization,
commercial sterilization (canning), drying, fermenting etc. each imposes its own set
of conditions and results in a more or less unique microbial association. It is impor-
tant to bear this in mind when considering the development of new methods or
testing the applicability of methods developed for another food to meat or applying
tests for one meat product to another. And a test applicable at one stage of produc-
tion, e.g. the finished but still warm carcass, is not necessarily the right test for the
cool carcass or the stored retail product made from it. However, the majority of the
rapid methods developed for other foods, or indeed for other areas of microbiology,
e.g. medicine or environment, are applicable to meat at one or more stages of
production.
Developments in a selection of the well-established methods listed in Table 1
will first be considered, followed by a more detailed consideration of the use of
lMATFORSK Norwegian Food Research Institute, Osloveien 1, 1430 As, Norway
2Agricultural and Food Research Council, Institute of Food Research - Bristol Laboratory
Langford, Bristol BS187DY, UK
A. Vaheri et al. (eds.), Rapid Methods and Automation in Microbiology and Immunology
© Springer-Verlag Berlin Heidelberg 1991
534 R.H. Dainty and R.A. Edwards
chemical changes, a method little considered in other foods with the exception of
fish. Those methods in Table 1 not discussed are the immunological methods,
nucleic acid probes and the hydrophobic grid-membrane filter technique. Their
development in the last few years has dealt exclusively with the detection of
pathogens and their use will be covered in sufficient detail elsewhere in this sym-
posium.
Results and Discussion
Methods Based on Detecting Whole Organisms
Of the methods listed in Table 1 the direct epifluorescent filter technique (DEFT)
is one which, according to a recent survey, appears to have caught a lot of attention
within the food industry (Jarvis and Easter 1987). In the year of the survey a
detailed study of the Biofoss Automated System for enumerating total viable or-
ganisms on carcass and jointed meat, fresh and frozen, as well as on chicken meat
appeared (Shaw et al. 1987). With the exception of mechanically recovered meat
samples, with which background fluorescence was a problem, good correlations
with standard total viable plate counts were obtained within 35-45 min and the
authors were confident of its suitability. In the intervening years the technique has
been shown to be useful in the selective enumeration of coliforms, pseudomonads
and Staphylococcus aureus grown as microcolonies on membranes on appropriate
selective media (Rodrigues and Kroll 1988). Using a 3-5 h resuscitation step, the
same authors subsequently demonstrated the use of the same technique for
enumerating sub-lethally injured microbes from frozen meat and pre-cooked
vacuum packed ham (Rodrigues and Kroll 1989). Liberski (1990) also concluded
that DEFT offered a better prediction of microbiological problems for sliced, cUTed
products produced from chilled, canned cured meat products than traditional
counts because of the failure of the latter technique to count injured organisms
which might grow after prolonged storage.
Table 1. Rapid methods of potential use for the detection
and enumeration of microbes on meats and meat products
Direct epifluorescent filter technique
Thrbidometry
Hydrophobic grid-membrane filter
Immunological methods
DNA/RNA probes
Limulus amoebocyte lysate
ATP-bioluminescence
Dye reduction-electrical
Electrical measurements
Enzyme activities
Chemical changes
