(13) Microbial Methods Flashcards
the types of microbes the food industry regularly analyse
- desirable microbes
Produces beneficial changes related to organoleptic properties or because they inhibit pathogens and spoilage organism - Spoilage microbes
Bacteria or fungi that influences the quality of food for its intended purpose ( spoiling is the initial stage of pathogenic microbe initiation) - indicator microbes
Non-pathogens used to indicate the risk of contamination - Pathogenic microbes
disease causing bacteria, viruses and protozoa; usually present in low numbers
Purpose of food microbiology
designed to find out
- total number of microbes present
- Presence or absence of specific microbe and pathogen
- level of indicator organism
Traditional methods
CFU determined using plate counts (prepare special medium to grow MO (agar), incubate then examine CFU (each dot/colony can contain 1000s of cells)
Membrane filter techniques (quite often need to test for MO in water, but load in water is very low so cant analyse 50 ml or 100 ml or else will detect nothing; membrane filtration avoids this problem)
MPN (most probable number; based on statistics)
- based on inoculation of 3-5 tubes from each 10 fold difference dilution
- can be made specific for coliforms or other bacterial group by using specific broth medium and selective indicators
direct microscopy (dairy industry uses)
surface sampling
Problem - takes a long time (usually >48 hours)
Rapid methods of microbe testing
- Dye reduction test: based on assumption that microbes present will metabolise carbohydrates in milk to reducing substance that reduces dyes (methylene blue) to colourless compound
- test freshness in milk (colorless=MO present; acid generated and pH will change) - Electrical impedance measurements
- ATP photometry
- All living organism have ATP which can be measured using the luciferin/luciferase reaction
- the presence of ATP can be measure using a photomer as the chemical reaction produces light proportional to the amount of ATP present
- ELISA
- DNA technology (PCR reactions)
-Microscopy:
Direct epifluorescent filter technique uses fluorescent dyes to attach to live cells, but can produce false positive reactions
-Labelled bacteriophage uses colourometric and light based testing
Discuss electrical impedance measurements
Food samples are placed in cell with two electrodes, if MO are present, they will produce metabolites that increases conductance
- Temperature has a significant effect on capacitance and conductance, so, automated systems utilising this technology use a solid block heating system to ensure a stable baseline and to eliminate false positives due to temperature shifts
- in these systems, hydrogen ions are seven times better than sodium ions as a conductor, so buffer selection is important
Limitations of Electrical Impedance
limitations
- large capital cost to set up and high cost of specific media
- method may no detect “shocked organism” (dead but still can cause harm)
- not good to check for yeast and mold presence
discuss the meaning of viable but not culturable
- MO that are partially damaged (lost part of cell wall) still alive but cant reproduce
- to force them to reproduce, need to add nutrients to ‘repair’
- when dealing w a large number o these MO, wont get a food result with impedance as they cant grow and hydrolyse the substrate
why is EI not suitable for yeast and mould
they are different from bacteria
- dont degrade the food sample into gas/hydrolysed substrate
- produce intermediate components which are poor conductors namely alcohol, lactic acid
what is impedance
resistance to flow of an alternating current as it passes through a conducting material
