BIO2041: Lecture 9

what is food spoilage?

1) food loses acceptance qualities 2) Unacceptable to a consumer because of smell, taste, appearance, texture, presence of foreign bodies 3) Economic loss 4) Loss of foods for consumption

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what are the 2 categories for food spoilage?

1) non-microbial 2) microbial

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what are the 3 reasons for non-microbial spoilage?

1) Physical – dehydration causing wilting 2) Chemical – oxidation, browning, autolytic degradation (pectinases) 3) Infestation – insects, rodents

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what are 2 reasons for microbial spoilage?

1) direct effect of growth 2) effect of microbial enzyme on the food

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what concentrations of cells is associated with spoilage?

10^6 - 10^8 cells/g ,/ml, /cm^2 - varies with type of spoilage and micro-organism

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which organisms (moulds, yeasts, bacteria) have the highest growth rate?

bacteria>>yeasts>>moulds

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which type of organism is a problem in foods stored for long periods of time?

Moulds- conditions are unfavourable to bacteria and yeasts

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what characteristic makes some organisms more prominent in food spoilage?

spoilage dominated by those with most rapid growth under those conditions

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what are psychotrophs?

organisms that can grow below 5C

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what are 2 aerobes prominent in food spoilage?

1) Pseudonomas genus 2) Flavobacterium genus

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are Pseudonomas G- or G+?

G- rods

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are Pseudonomas motile and do they have spores?

motile but have no spores- most produce yellow-green water soluble pigments, can use many organic compounds as C source

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are Flavobacterium G- or G+, are they motile and do they produce spores?

colonies often yellow in colour, limited C and energy sources – glucose preferred- G- rods, non-motile, no spores

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what are 3 examples of facultative anaerobes?

1) Lactobacillus 2) Leuconostoc 3) Enterococcus

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Are lactobacillus G+ or G-, are they motile and do they have spores?

G+ rods, mostly non-motile, no spores- homo- (mostly) or hetero-fermentative, grow at pH as low as 4

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which lactobacillus species is particularly a problem in food spoilage?

Lactobacillus viridescens

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Are Leuconostoc species G+ or G-, are they motile and do they have spores?

G+ cocci in pairs or chains’ non-motile, no spores, heterofermentative, gas production associated with spoilage

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Are Enterococcus G+ or G-, are they motile and do they have spores?

G+ non-motile cocci in pairs or chains,

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are Enterobacter G- or G+?

G- motile rods

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are serratia G+ or G-?

G- motile rods, white, pink or red colonies

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are Hafnia G- or G+?

G- motile rods

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are proteus G- or G+?

G- very motile rods

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where do Enterococcus, Enterobacter, Serratia, Hafnia and Proteus inhabit?

intestine

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what do facultative anaerobes and anaerobes have in common?

thermoduric pyschotrophs

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spores of Bacillus sp (facultative anaerobes) give rise to what kind of bacterium?

G+ rods, motile or non-motile- spores highly heat resistant

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what does the species Bac. coagulans play a role in?

important in spoilage of canned foods

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spores of Clostridium sp give rise to what kind of bacterium?

G+ rods, motile or non-motile. Spores are heat resistant. Cl. saccharolyticum important in food spoilage

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which 2 genus are thermophilic?

Bacillus and Clostridium sp

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describe Pediococcus acidilactici?

G+ non-motile cocci in pairs or tetrads, homolactic, can survive pasteurisation, some grow at 50C

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describe streptococcus thermopiles

G+ non-motile, occur in pairs or chains, some grow at 50 C

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name 4 common food spoilage yeasts?

Saccharomyces sp, Rhodotorula sp, Torulopsis sp, Candida sp

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describe saccharomyces sp

round, oval, or elongated- produce alcohol and CO2

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which of these yeats causes discolouration of meat?

Rhodotorula

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which of these yeats can spoil milk?

Torulopsis sp: can ferment lactose so may spoil milk

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which of these yeats can grow in high acid, salt and sugar conditions and can cause lypolysis?

candida sp

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what are 4 common food spoilage moulds?

1) Aspergillus 2) Mucor 3) Penicillium 4) Rhizopus

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describe Aspergillus sp

septate hyphae, black spores on conidia, many grow at low Aw, may produce aflatoxin

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describe Mucor sp

non-septate hyphae, produce cottony colonies, spoil vegetables in particular

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describe Penicillium

septate hyphae, blue-green conidia, some may produce mycotoxin

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describe rhizopus

non-septate hyphae, black conidia

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what is microbial succession?

1) Initial growth of a spoilage organism can alter the environmental conditions 2) The new conditions can favour microbes previously dormant 3) May get several stages of succession over a long period of time as spoilage occurs

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which nutrient compositions of milk allow microbial growth?

pH ~6.5, protein 32 g/kg, carbohydrate 48 g/kg, lipid 39 g/kg

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what about milk limits the type of organisms can live in it?

Lactose as major source of carbohydrate limits organisms which can grow in milk

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where does contamination of milk initially happen?

from within udder, exterior of teat, milking equipment

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for raw milk, if maintained at ambient temperatures which kind of bacteria dominate?

mesophilic lactic acid producing bacteria dominate spoilage- produce lactic acid from lactose- Gives sour taste and eventually coagulates the casein

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If maintained at refrigeration temperature, which bacteria dominate?

psychrotrophs dominate particularly Pseudomonas sp.-Degrade amino acids and later proteins and lipid to give off odour and flavour

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which human pathogens can potentially spoil milk?

Staph. aureus, E. coli, Salmonella sp., Listeria monocytogenes, Mycobacterium tuberculosis, Coxiella burnetii, Brucella abortis, Campylobacter sp.

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what are the remaining organisms in pasteurised milk?

G+ thermoduric species Micrococcus, Enterococcus and Lactobacillus; spores from Bacillus and Clostridium, Psychrotrophic G- rods contaminating post-pasteurisation: Pseudomonas, Alcaligenes.

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Contamination with psychrotrophs after pasteurisation gives similar effect to what?

to that seen with raw milk if milk held at low temperature after pasteurisation

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germination of spores of which psychotrophs can lead to curdling due to coagulation of casein as a result of peptidase action?

Bacillus circulans, B. sphaericus

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what else do these organisms produce?

lecithinase- hydrolyses phospholipids and releases fat globules

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BIO2041: Lecture 9

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