MICROM 301 Lecture Notes - Lecture 5: Psychrophile, Saccharomyces Cerevisiae, Fermentation Starter
4/9 INDEPENDENT STUDY AND LECTURE NOTES HELEN KIM
FOOD MICROBIOLOGY:
INDEPENDENT STUDY –
-Fermented foods: foods that are intentionally altered during production
oBy controlling activity of bacteria, yeasts, or molds
oOnly use the metabolic activities that use PYRUVATE or another organic
compound as an electron acceptor
oResults in production of ALCOHOLS and ACIDS
FACTORS THAT INFLUENCE GROWTH OF MICROORGANISMS IN FOODS:
- Bacteria grow faster than yeasts, yeasts grow faster than molds/fungi
oBacteria > yeasts > molds/fungi
oBacteria: fresh meats, neutral pH, nutrient rich foods
oYeasts: usually outcompeted by bacteria
oMolds/fungi: when conditions lack moisture, high acidity (low pH), or anything
that limits bacterial growth
-INTRINSIC FACTORS: characteristics of the food itself
oWater availability (aw): water activity; amount of water available in foods
Pure water aw = 1.0
Most bacteria need aw > 0.90 for growth
Yeasts aw about 0.88
Fungi aw about 0.8
opH: some high pH foods can produce toxins
onutrients: an organism requiring a particular vitamin can’t grow in foods lacking
that vitamin
SOME however, can synthesize / make them themselves
obiological barriers (i.e. shells, peels, etc.): help protect foods from invasion by
microorganisms
oantimicrobial chemicals: egg whites have lysozyme; lysozyme susceptible
bacteria will be destroyed before causing spoilage
-EXTRINSIC FACTORS: environmental conditions under which a food is maintained
oTemperature: at low temps, many enzymatic reactions are very slow or non
existent
EXCEPT FOR PSYCOPHILES or PSYCHOTROPHS
oAtmosphere: presence or absence of O2
FOOD SPOILAGE: damage the quality of the food but are generally not harmful
- Undesirable change in tastes and odors
oEndproducts of metabolism
oSoftening
oSlime
oDegradation of flavor compounds – lactic acid producing bacteria
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4/9 INDEPENDENT STUDY AND LECTURE NOTES HELEN KIM
FOOD PRESERVATION (preventing/delaying spoilage)
-Killing organisms
oHeat
Canning (commercially sterile): destroys all spoilage and pathogenic
organisms that are capable of growing at normal storage temps
Low acidic foods are autoclaved at a high temp to destroy
ENDOSPORES of C. botulinum
High acidic foods don’t need to be autoclaved because C.
botulinum can’t grow in these conditions
Pasteurization: heating foods under controlled conditions at high temp
for a short period of time
Cooking: alter the characteristics of food
May cook unevenly – some organisms may still survive
oPressure: steaming under pressure (autoclaving)
oIrradiation: gamma radiation destroys microorganisms without specifically
altering flavors
-Inhibit growth of organisms
oRefrigerate: slows growth rate of microbes
oFreeze: stops microbial growth (water is unavailable b/c ice), some microbial
killing by ice crystals
oDry/reduce the aw: adding high concentrations of sugars / salts – decreases
available moisture
oLower the pH (more acidic): adding acids or encouraging fermentation by lactic
acid bacteria
oAdd antimicrobial chemicals
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4/9 INDEPENDENT STUDY AND LECTURE NOTES HELEN KIM
INDEPENDENT STUDY QUESTIONS:
1. What’s the difference between a fermented food and a spoiled food?
oFermented food: INTENTIONALLY altered during production by encouraging
activity of bacteria, yeasts, molds
oSpoiled food: UNDERSIRABLE unintentional biochemical changes in food
2. Which typically grows fastest – yeasts, bacteria, or molds?
Bacteria > yeasts > molds/fungi
3. What is the difference between food spoilage and food poisoning?
oFood spoilage: undesirable biochemical changes in food
oFood poisoning: illness from either a pathogen or its produced toxin being eaten
4. Describe 4 INTRINSIC FACTORS that influence the growth of microorganisms in food.
Water availability
pH
nutrients
biological barriers
antimicrobial chemicals
5. Describe 2 EXTRINSIC FACTORS that influence the growth of microorganisms in food.
Temperature
Atmosphere
6. Which would predominate on improperly stored ham – a Pseudomonas species or a
Staphylococcus species? Explain.
oStaphylococcus (0.86) because Psuedomonas (0.97) requires a higher aw while
ham has an aw of 0.91
7. How would storage temperature influence the rate of microbial growth?
oPathogens don’t grow well on frozen foods
oRefrigeration slows down the growth rate of microbes
8. How would the storage temperature influence the types of microorganisms that
would predominate on a food product?
Lower temperatures tend to slow down or stop many enzymatic reactions
Some can still grow though (ex. psychrophiles and psychotrophs)
9. What are some undesirable changes associated with spoilage?
Endproducts of metabolism
Softening
Slime
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Document Summary
Factors that influence growth of microorganisms in foods: Intrinsic factors: characteristics of the food itself: water availability (aw): water activity; amount of water available in foods. Most bacteria need aw > 0. 90 for growth. Fungi aw about 0. 8: ph: some high ph foods can produce toxins, nutrients: an organism requiring a particular vitamin can"t grow in foods lacking that vitamin. Some however, can synthesize / make them themselves: biological barriers (i. e. shells, peels, etc. ): help protect foods from invasion by microorganisms: antimicrobial chemicals: egg whites have lysozyme; lysozyme susceptible bacteria will be destroyed before causing spoilage. Extrinsic factors: environmental conditions under which a food is maintained: temperature: at low temps, many enzymatic reactions are very slow or non existent. Except for psycophiles or psychotrophs: atmosphere: presence or absence of o2. Food spoilage: damage the quality of the food but are generally not harmful.
