Indicators in Food Microbiology
Essay by Yingying Feng • April 9, 2019 • Study Guide • 3,912 Words (16 Pages) • 929 Views
Unit 2 summary- Indicators in Food Microbiology
Indicators
- Purpose: safety & shelflife
- Looking for a pathogen is like looking for a needle 🡪 look for indicators instead (if indicators are present, spoilage are present)
- Provide a gauge of product shelf life
- Highlight potential hazards
- An assessment of the previous Hx of food product (temperature abuse/ contamination)
- Evaluation of the efficacy of control measures to prevent and/or inactivate microbial activity (pasterization)
*Expiry date (for nutra/pharmaceutical) vs. best before date (min time that ensure quality)
Attributes to indicator summary:
- Always present when the pathogen is present
- Higher resistance/persistence than the pathogen of interest
- Shares the same niche as the pathogen
- Can be enumerated inexpensively and w. simple techniques
- Can be differentiated against background microflora (ease of detection)
- Non-pathogenic
Attributes for Quality indicators
+Negatively relate to food quality (when count is high🡪 low quality)
+Present in spoiled food
Attributes for safety indicators
+Present when pathogen is present & ideally, absent when pathogen is absent (avoid false pos & false neg )
- Spoilage (quality) indicators [pic 1]
Include:
Total Aerobic Count
Psychrotrophic Count 🡪 pseudomonas
Lactic acid bacteria
Yeast and molds
Types of counts will depend on the nature of the product
1) Total Aerobic Count (standard plate count)
- General assessment of bac; high #--> significant bac activity, conditions plates are incubated under temp that reflect food envir[pic 2]
>5 log cfu/g is very high 6-7 ordours but not visible 9-10 visible
Even Low count 🡪 dangerous for raw milk (lipid degradation produce ketone 🡪 off-flavor)
2) Psychrotrophic Count
- can grow at low temp, spoilage of refrigerated food, estimate shelf life
- Listeria & Pseudomonas; also incl. yeast and molds
- Important b/c psychrophs are easier to cultivate, foods are commonly refrigerated
3) Pseudomonas
- Potent spoilers, very versatile (grow on anything)
- Obligate aerobe 🡪 vacuum packaging/modified atm mainly to stop pseudomonas growth
- Grow in low temp & forms polysacc 🡪 form biofilm (difficult to remove and easy to contaminate)
Enzymes produced by pseudomonas
- Proteinases- break down proteins leading to generation of ammonia, sulfur and/or organic acids (butyric, acetic).
- lipolytic - hydrolyze triglycerides and accelerate lipid oxidation (rancidity).
- Pectolytic- breakdown plant cell walls leading to loss of turgor pressure.
*Proteases, lipases and pectinases associated with other bacteria and molds but Pseudomonas commonly implicated.
Pigmentation – Flourescent Pseudomonas release siderophores (pigmented/fluor’) to assimilate iron
*Meat & eggs have low Fe and enhance siderophore production in Pseudomonas.
Important features of pseudomonas: quorum sensing & biofilm formation
cell-cell communication to coordinate activities
gram neg & pos (cyclic peptides)
cell density affect
why quorum sensing important? (*enable bac population to act collectively🡪 more efficient use of cell resources🡪 can overcome defenses more effectively)
- Increase spoilage potential of populations.
- Stimulate biofilm formation.
- Increase virulence of pathogens
Representatives
- Opportunistic pathogens
- P. aueruginosa --lung disease
- Spoilage (typically not pathogenic)
- P. fluorescens --fresh produce
- P. putrefacines -- meat[pic 3]
- P. fragi
4) Lactic acid bacteria- Gram positive non-spore forming rods / cocci
Genera of significance
- Lactococcus Carnobactrium (resistant to irradiation)
- Lactobacillus
- Leuconostoc Bifidobacteria (probiotic, sensitive to O2 and stress)
- Pediococcus (probiotic)
- Streptococcus
Why difficult to control?
- Facultative anaerobes,
- very diverse (common: produce lactic acid🡪 phenotypic classification, not genetic)
- Widespread(plants & GI of animals) but difficult to culture
- Fastidious (complex nutritional demand)
- Tolerate low pH and high ethanol
- Since they tolerate many stress, refrigeration is the key (<10C) (sorbic/benzoic acid can’t eliminate them)
- High lactic acid bac indicate temperature abuse problem
- Can be beneficial or spoiling depending on fermentation products & food type
Metabolism:
- Homofermentative (ferment carbohydrates to predominantly lactate)
- Lactobacillus planetarium & Lactobacillus delbrueckii
- Facultative homofermenters (used extensively in dairy fermentation) (prefer homolactic fermentation but can perform heterolactic)
- Lactococcus lactis subsp lactis
- Heterofermentative (real problems!) (ferment carbohydrates to>1 products i.e. lactate, acetate and ethanol, CO2) Kefir sourness: acetate
- Leuconostoc paramesenteroides & Lactobacillus brevis
[pic 4][pic 5][pic 6]
Probiotics – live microbes that when administered in sufficient amt confer a health benefit to the host.
Characteristics
- Safe
- Viable * (10^9 CFU/serving)
- Provide benefits (vague definition)
- Bile resistant (survive bile salt in GI tract🡪 viable)
- Acid resistant
Microbes used for human consumption
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