41 Detecting the Presence of Bacteria or Chemical Residue
41 Detecting the Presence of Bacteria or Chemical Residue
This chapter covers outcome 9: Describe feels, smells, or visual clues that might indicate the presence of bacteria or chemical residue. (We also look for physical contaminants)
The three main types of hazards we are concerned with are biological (microbial), physical (glass, metal, wood, etc.) and chemical (allergens, excessive chemicals, etc.).
We look at ingredients and finished products. It is often referred to as organoleptic testing or sensory evaluations. These tests are done for safety and quality. We look for standard appearance, off odours, colour changes, bone fragments in meat, bruising, etc.
Lab tests are usually done to determine the presence of bacterial contamination. However, spoilage and indication of contamination can be indicated by appearance changes, as well.
Microbial testing provides the detection and quantification of pathogens and spoilage microorganisms in a variety of products. Not only do these analyses allow processors to address food safety issues if pathogenic bacteria are detected, but they also predict the shelf life of a product when evaluating spoilage microorganisms.
Culture techniques
Culture techniques, which are commonly used in microbiology, rely on the growth of microorganisms under specific conditions (ex: time, temperature, oxygen content, pH and pressure) in liquid or on solid media containing nutrients. Culture methods using standard media require between 1 to 7 days for detecting the presence of microorganisms. Often, an enrichment step and/or a confirmation procedure is required before and after microbial detection, respectively.
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Biochemical and enzymatic methods
Rapid methods for microbial testing assess the microbial population through the metabolic activity of the cells. Such techniques include chromogenic media, colorimetry, ATP (adenosine triphosphate) determination, protein detection and biochemical kits.
- Chromogenic media for culture techniques. This method relies on the detection of enzymatic activities of target microorganisms by the addition of dyes or organic compounds. Microorganisms growing in the presence of these compounds may fluoresce under UV light or produce distinctive colours. Compared to a standard media analysis, this method enables more sensitivity and rapid analysis (1) (within 24 hours). Chromogenic media to identify Listeria monocytogenes and E. coli are commercially available (ex: Bio-Rad, Weber Scientific, etc).
- Colorimetry or optical systems measure microbial growth by monitoring pH and other biochemical reactions that generate a color change as microorganisms grow and metabolize in a culture broth. Test for the detection of coliforms, E. coli, yeast, molds, lactic acid bacteria, Listeria spp., and Enterobacteriaceae are commercially available (ex: Neogen Corporation).
- ATP Bioluminescence. ATP, which is the energy source in all living cells, produces light in the presence of luciferin-lucifera