FMLec M4 | Microbiota of Food Products

3 factors influencing the microbiota of food products

• Intrinsic food properties

  • Natural characteristics of food itself that affect microbial growth, including pmonab

    • pH

    • Moisture content

    • Oxidation-reduction potential

    • Nutrient

    • Antimicrobial constituents

    • Biological structures

• Extrinsic process variables

  • Environmental conditions surrounding the food that impact microbial survival, including srg

    • Storage temperature

    • Relative humidity

    • Gases

• Implicit microbial characteristics

  • Biological traits of microbes themselves that influence their ability to dominate a food environment

  • Microbial interactions and their beneficial/adverse effects, including cmmca

    • Competition

    • Metabiotic

    • Mutualism

    • Commensalism

    • Antagonism

T/F: Fruits with covering will spoil faster than those without

FALSE

Fruits without covering will spoil faster than those with covering (intrinsic parameter)

Which parameter?

• Inherent part of the tissues

• Some offer natural preservation

• Allows prediction of the general type of microorganisms likely to grow

• Contributes to the overall stability of food

Intrinsic parameters

Enumerate 5 intrinsic parameters/food properties

pmonab

• pH

• Moisture content

• Oxidation-reduction potential

• Nutrient

• Antimicrobial constituents

• Biological structures

_ is the optimum pH for most microorganisms

Neutral (6.5 - 7.5)

• From most acidic to most basic

  • HCl > Lemon juice > Vinegar > Tomato > Banana > Potato > Water > Eggs > Baking soda > Bleach

• Acidic foods: pH < 6.5 (e.g., fruits, fermented dairy)

  Neutral foods: pH 6.5–7.5 (e.g., fresh meat, milk)

  Alkaline foods: pH > 7.5 (e.g., egg whites, some seafood)

Rank from most acidic to most basic

Tomato, bleach, baking soda, lemon juice, vinegar, potato, HCl, eggs, water, banana

HCl > Lemon juice > Vinegar > Tomato > Banana > Potato > H2O > Eggs > Baking soda > Bleach

Food with pH around _ range would be more exposed to a higher diversity of microorganisms and thus would spoil faster

neutral (6.5 - 7.5)

Rank the ff. from those w/ highest acid tolerance to least

• LAB

• Molds

• S. aureus

• Vibrio

• Yeast

Molds > Yeast > LAB > S. au > Vibrio mylsv

The most fastidious microbes in terms of pH (i.e., can only tolerate a narrow pH range) are _

pathogenic bacteria

• e.g., Salmonella spp., L. monocytogenes, Yersinia, C. botulinum, B. cereus, Campylobacter spp., Shigella spp., V. cholerae, C. perfringens

T/F: Acidic food products (pH < 4) are generally safer from bacterial contamination, except for a few species like Alicyclobacillus spp. and Acetobacter spp.

TRUE

Most bacterial pathogens struggle below pH 4, but acid-tolerant bacteria like Alicyclobacillus spp. and Acetobacter spp. can survive

T/F: Both V. parahaemolyticus and molds can survive in highly basic environments (pH = 11)

TRUE

_ are very resilient and can growth within pH 0.5 (acidic) to 11 (basic)

Molds

T/F: Bacteria are more fastidious than fungi in terms of pH requirement

TRUE

T/F: Bacteria would have a lower pH minima than fungi

FALSE

Fungi are less fastidious than bacteria in terms of pH range and can thus survive much lower pH than bacteria

Fruits are generally _ in terms of pH and would most likely be spoiled by fungi

acidic

Vegetables are generally _ in terms of pH and thus would most likely be spoiled by bacteria

near neutral

T/F: pH minima and maxima of microbes are fixed and cannot be influenced by other parameters

FALSE

pH minima and maxima of microbes are dependent on other growth parameters, e.g., moisture content—if a microbe is in a high moisture environment and thrives in that kind of environment, then they most likely can tolerate lower or higher pH than the typical range that they can tolerate (TLDR: increase in favorable growth conditions widen their pH tolerance range)

T/F: LAB is more acid tolerant in acetic acid than in HCl

FALSE

LAB is more acid tolerant in HCl than in acetic acid

Explain 2 situations where pH minima & maxima of microbes change depending on certain growth parameters

• Alcaligenes faecalis when grown in

  • A = 1% peptone (moderate pH range)

  • B = 0.2 M NaCl (wider pH range)

  • C = 1% peptone + 0.2 M Na citrate (narrower pH range)

• LAB is more acid tolerant in HCl than in acetic acid

Fruits are generally acidic in terms of pH and thus would most likely be spoiled by _

fungi

Dairy products, meat & poultry, and fish & shellfish dmf have pH ranging from _ and thus would most likely allow growth of and be spoiled by either bacteria or fungi

acidic to neutral

Vegetables are generally near neutral in terms of pH and thus would most likely be spoiled by _

bacteria

T/F: Vegetable soft rotting is more likely caused by fungi than bacteria

FALSE

Vegetable soft rotting is more likely caused by bacteria than fungi
*Soft rotting = Bacteria

dmf Dairy products, meat & poultry, and fish & shellfish have pH ranging from acidic to neutral, thus allowing the growth of and may be spoiled by _

either bacteria or fungi (yeast/hyphal)

Which will spoil first: meat from a well-rested animal or fatigued animal? (pH intrinsic)

Meat from fatigued animal will spoil first due to a direct consequence of the final pH attained upon completion of rigor mortis

• Meat from well-rested animal would have sufficient glycogen stored before slaughter, such that after slaughter, this produces sufficient lactic acid that lowers the pH and acidifies the environment, thus inhibiting microbial growth

• Meanwhile, meat from fatigued animal would have depleted glycogen levels before slaughter, leading to less lactic acid produced after slaughter and thus ultimately resulting in a higher, more neutral pH that creates a more favorable environment for microbial growth

Give examples of food with biological activity (pH intrinsic)

• Acidity or pH is due to the actions of certain microorganisms

  • Fermented milk, sauerkraut, pickles

(pH intrinsic)

_ are foods that tend to resist changes in pH due to the buffering capacity of various proteins, resulting in low succession of microorganisms

Buffered food

T/F: Buffered foods are foods that do not undergo drastic changes in pH due to the buffering capacity of the proteins it contains

TRUE

T/F: Higher diversity of microorganisms is typically observed in spoilage of buffered foods

FALSE

Lower diversity of microorganisms is typically observed in spoilage of buffered foods because fewer changes in pH = fewer changes in microbial communities that can grow in it

T/F: There would most likely be higher succession of microorganisms in meat than in vegetables

FALSE

Meat would have lower succession due to it being a buffered food by virtue of the buffering capacity its proteins offer; vegetables, on the other hand, would likely have higher succession due to more fluctuating pH changes

T/F: Meats would have higher buffering capacity than veggies

TRUE

How does pH affect microorganisms?

• Optimal pH is required for proper enzyme function and nutrient transport

• Longer lag phase is observed when pH is nonoptimal

• Morphology, e.g., Penicilium chrysogenum’s hyphal length is shorter when grown in pH > 6

Enumerate and explain microbial adaptations to extreme pH

• Acidophiles mpah

  • Maintain cytoplasmic pH values nearer to neutrality

  • Pump protons out of their cells at a relatively high rate

  • Amino acid decarboxylase overexpression to consume cytoplasmic protons thru catalytic reactions

  • Highly impermeable cell membrane to restrict H+ influx

• Alkaliphiles mt

  • Maintain cytoplasmic pH values nearer to neutrality

  • Transport reactions and flagellum rotation driven by sodium motive force instead of proton motive force (B. firmus)

_ refers to the ratio of the vapor pressure of food to vapor pressure of pure water at the same temperature

Water activity (A_w)

• _ is an intrinsic parameter that pertains to water requirement of microorganisms

• Pure water = _

• Most fresh food = _

• Water activity

• Pure water A_w = 1

• Most fresh foods A_w = 0.99

T/F: Even though acidophiles/alkaliphiles thrive in environments that are acidic/basic, their internal pH would still likely be similar to those of neutral range

TRUE

_ are enzymes that use the protons that enter the cell to catalyze biochemical reactions; used by acidophiles to adapt to extreme pH

Amino acid decarboxylases

T/F: Water activity is affected by concentration of salt solutions

TRUE

Water activity is inversely proportional to salt solution concentration; higher salt concentration = lower water activity & vice versa

T/F: Most spoilage bacteria, yeasts, and molds would require A_w close to 1

TRUE

• Spoilage bacteria = 0.9

• Yeast = 0.88

• Molds = 0.8

T/F: Halophilic bacteria require lower A_w than xerophilic molds and osmophilic yeasts

FALSE

• Halophilic b = 0.75

• Xerophilic molds = 0.61

• Osmophilic yeasts = 0.61

T/F: Osmophilic yeasts would survive lower A_w than xerophilic molds

FALSE

• OY = 0.61

• XM = 0.61

T/F: Generally, bacteria would require higher A_w than fungi; yeast & molds would survive a wider A_w range; and Gram-negative would require higher A_w than Gram-positive

TRUE

T/F: Generally, Gram-positive would require higher Aw than Gram-negative

FALSE

Gram-negative A_w > Gram-positive A_w

T/F: Yeast & molds require higher Aw than bacteria

FALSE

Bacteria A_w > Fungi A_w

T/F: Yeast and molds have wider pH and A_w range

TRUE

Enumerate and explain factors related to or affecting A_w

tnp

• Low A_w = reduced ability to grow any temperature

• Wider range of A_w = optimum temperature & nutrient

• Higher minimum A_w requirement = nonoptimal pH

T/F: Being in a low A_w environment can lead to increased ability to grow any temperature

FALSE

Being in a low A_w environment can lead to decreased ability to grow any temperature

T/F: A nonoptimal temperature and nutrients can lead microbes to have a wider A_w range

FALSE

Optimal temperature and nutrients can lead microbes to have a wider A_w range

T/F: A nonoptimal pH can result in higher minimum A_w requirement in microbes

TRUE

Enumerate and explain effects of low A_w on microorganisms

olgpa

• High osmotic stress

• Longer lag phase

• Decreased growth rate

• Decreased population size

• Adverse effect on all metabolic activities

Enumerate and explain adaptations of microbes to low A_w environments

• Keep their cell membrane fluid

• Ability to concentrate salts, polyols, amino acids spa

  • Prevents the cells from losing water

  • Allows the cell to extract water from water-depressed external environments

T/F: Some microbial adaptations to low A_w include keeping cell membrane rigid and concentrating salts, polyols, & amino acids to not only prevent cells from losing water but also allow cells to extract water from water-depressed external environments

Some microbial adaptations to low Aw include keeping cell membrane fluid and concentrating salts, polyols, & amino acids to not only prevent cells from losing water but also allow cells to extract water from water-depressed external environments

T/F: O/R (Eh) is a consideration when dealing with aerobes

FALSE

O/R (Eh) is a consideration when dealing with anaerobes because lower Eh would allow anaerobes to survive

_ is the intrinsic parameter that pertains to the ease with which a substrate loses or gains electrons

Oxidation-reduction potential (Eh)

T/F: A compound that loses electrons is oxidized and a compound that gains electrons is reduced

TRUE

Oxidation = lost; reduction = gained OILRIG

Explain oxidation-reduction potential as an intrinsic parameter

• Ease with which substrate loses or gains electrons

• Expressed by the symbol Eh

  • + Eh values (oxidized) → aerobes

  • - Eh values (reduced) → anaerobes

  • Zero Eh values → [oxidant] = [reductant]

T/F: Pre-rigor meat would have lower Eh, while post-rigor meat would have higher Eh

FALSE

Pre-rigor meat would have higher Eh (higher O2 supply), while post-rigor meat would have lower Eh (lower O₂ supply)

T/F: Highly oxidized (+Eh) food would favor anaerobes, highly reduced (-Eh) food would favor aerobes

FALSE

Highly oxidized (+Eh) food would favor aerobes, highly reduced (-Eh) food would favor anaerobes

Explain oxidation-reduction potentials (Eh) relative to growth of certain microorganisms

-Eh = reducing (anaerobic)

• -350 rumen contents → min. for methanogenic rumen bacteria

• -200 cooked meat medium

• -100 solid meat cuts & some cheeses → min. for Desulfovibrio
  

  [oxidant] = [reductant]

• 0
  

+Eh = oxidizing (aerobic)

Favors yeasts & molds

• +200 comminuted meats (mechanically processed into smaller pieces, such as ground, chopped, or emulsified meat, e.g., ground beef, sausages, hot dogs, patties, etc.)

• +400 fruit juices

T/F: Comminuted meats spoil faster than solid meat cuts because their higher Eh makes them more prone to oxidation and aerobic microbial growth

TRUE

T/F: Desulfovibrio spp. are more likely to grow in fruit juices than in solid meat cuts

FALSE

Desulfovibrio spp. require an Eh of around -100 mV (solid meat cuts & some cheeses), while fruit juices have a highly oxidizing environment (+400 mV), making it unsuitable for their growth

T/F: Methanogenic bacteria can survive in comminuted meats because of their relatively low Eh

FALSE

Methanogens require a highly reducing environment (< -350 mV), whereas comminuted meats have an Eh of +200 mV, which is far too oxidizing for them

T/F: Yeasts and molds are more likely to grow in fruit juices than in cooked meat medium

TRUE

Yeasts and molds thrive at higher Eh values (>+200 mV), which matches the oxidized environment of fruit juices (+400 mV), whereas cooked meat medium is more reducing (~-200 mV)

T/F: The oxidation-reduction potential of a food product can shift over time due to microbial activity, making it more favorable for different microorganisms at different spoilage stages

TRUE

Foods rich in -SH groups (sulfhydryl / thiol) in meats and ascorbic acids would create a _ environment, favorable for growth of _

reducing = anaerobes

T/F: Reducing sugars in fruits and vegetables are best suited for growth of anaerobes

TRUE

Enumerate and explain effects of microbes on Eh (O/R potential)

• Aerobes can lower Eh of their environment

  • O2 is depleted in the medium

  • The medium becomes poorer in oxidizing and richer in reducing substances

• Metabolic byproducts, e.g., H2S, can lower Eh

T/F: Aerobes can lower Eh of their environment by depleting O2 in the medium, resulting in medium becoming poorer in reducing and richer in oxidizing substances

FALSE

Aerobes can lower Eh of their environment by depleting O2 in the medium, resulting in medium becoming poorer in oxidizing and richer in reducing substances

Metabolic byproducts, e.g., _, can lower Eh

Hydrogen sulfide (H2S)

T/F: Eh can influence what type of microorganisms grow in the environment, at the same time, microbes can change Eh and influence what type of microbes grow in the environment

TRUE

_ is the intrinsic parameter that concerns itself with source of energy, carbon, nitrogen, vitamins & minerals

Nutrients

T/F: Almost all natural foods have an abundant quantity of vitamins and minerals for those unable to synthesize their own essential requirements, e.g., Gram (+)

TRUE

Rank ff. in terms of nutrient requirements

• Yeast

• Molds

• Gram (+)

• Gram (-)

Gram (+) > Yeast > Gram (-) > Molds

T/F: Gram (+) bacteria and molds can synthesize their own vitamins and minerals

FALSE

Gram (-) bacteria and molds can synthesize their own vitamins and minerals; Gram (+) needs to be supplied with vitamins and minerals

Enumerate 3 sources of energy and carbon

saacf

• Sugars, alcohols, amino acids

• Complex carbohydrates

• Fats

Enumerate 3 primary sources of nitrogen

nap

• Nucleotides

• Amino acids

• Peptides and proteins

• _ refers to the type of nutrient that includes growth factors required in low quantities

• Almost all natural foods have an abundant quantity of these for those unable to synthesize their own essential requirements

• Need to be supplied to Gram (+)

• Can be synthesized by Gram (-) and molds

Vitamins and minerals

_ is the intrinsic parameter that refers to naturally occurring substances that possess and express antimicrobial activity and thus contribute to food stability

Antimicrobial constituents

Give 4 examples of antimicrobial constituents (intrinsic)

elml

• Essential oils, e.g., allicin in garlic

• Lactoferrin in cow’s milk (iron-binding glycoprotein)

• Milk casein

• Lysozyme in eggs and milk

_ is the intrinsic parameter that provides excellent protection against the entry and subsequent damage by spoilage organisms; usually refers to natural covering of the food

Biological structures

Give 5 examples of biological structures (intrinsic)

tosh s

• Testa of seeds

• Outer covering of fruits and nuts

• Shell of eggs

• Hides of animals

• Scales of fish

_ refers to properties of the storage environment affecting both the foods and their microorganisms

Extrinsic process variables

Enumerate and explain 3 extrinsic process variables

• Storage temperature

  • Most important extrinsic parameter that affects spoilage of highly perishable foods

  • Dependent on temperature growth range of

    • Foodborne pathogens

    • Spoilage microorganisms

  • Maintenance of food quality must also be considered

• Relative humidity of storage environment

  • Affects the Aw within food

  • Affects ability of microbes to grow at food surfaces

  • Aw food is directly proportional to RH storage but inversely related to temperature storage

    • Low Aw food = Low RH storage = High temp storage

    • High Aw food = High RH storage = Low temp storage

• Gases

  • Influences growth of microorganisms

    • CO2 controls microorganisms in foods

    • O3 has antimicrobial properties

• Presence & activities of other microorganisms

  • Some foodborne pathogens produce substances that are either inhibitory or lethal to others

  • e.g., abho antibiotics, bacteriocins, hydrogen peroxide, organic acids

_ is the most important extrinsic parameter affecting spoilage of highly perishable foods

Storage temperature

Storage temperature is dependent on the _

temperature growth range of foodborne pathogens and spoilage microorganisms; maintenance of food quality must also be considered

_ affects Aw within food and the ability of microbes to grow at food surfaces; directly related to Aw but inversely related to temperature storage

Relative humidity of storage environment

_ influence growth of microorganisms, such that CO2 may control microorganisms in foods, while O3 may have antimicrobial properties

Gases in the storage environment

_ is an extrinsic parameter that considers some substances produced by foodborne pathogens, including antibiotics, bacteriocins, hydrogen peroxide, organic acids

Presence & activities of other microorganisms

_ refers to microbial interactions, including cmmca competition, metabiotic, mutualism, commensalism, and antagonism, as well as beneficial and adverse effects of such interactions

Implicit microbial characteristics

• Microbial interaction where 2 or more microorganisms help one another

• Metabolic byproducts as growth requirement

• e.g., Lactobacillus arabinosus gets Phe, which it cannot synthesize, from Enterococcus faecalis, while E. faecalis gets folic acid from L. ara

Mutualism

• Microbial interaction where there is limited energy and nutrient resources

• Outcomes depend on growth rate of competitors, such that

• Fast-growing outcompetes slow-growing microorganisms

• Can lead to competitive exclusion (i.e., complete inhibition of slow-growing)

Competition

• Microbial interactions where one benefits from the other, while the other is neither harmed nor benefitted

• e.g., Vinegar production

  • S. cerevisiae produces ethanol (alcohol) from sugar

  • Acetobacter aceti uses ethanol to produce acetic acid

Commensalism

• Microbial interactions where changes in food is due to metabolic activity of predominant microorganisms

• Succession of microorganisms icc

  • Initial optimal growth of 1 or 2 types

  • Creates an environment not optimal for their growth

  • Creates changes in envi favorable for other types of microorganisms

Metabiotic

Give examples of metabiotic microbial interactions

• Ground meat in an air-tight bag

  • Aerobes grow first

  • Colonization of anaerobes follows

• Manufacturing > Ripening > Cheese variety

  • les > dg > aps

  • Manufacturing (Pasteurization, Cutting, Whey drainage, Salting)

    • LAB, Enterobacteriaceae, Staphyloccus

  • Ripening (Temperature, Time, Humidity)

    • Debaryomyces, Geotrichum

  • Cheese variety

    • Fresh

    • Soft

    • Hard

    • Smear-ripened

    • Blue-veined

• Microbial interaction where microorganisms adversely affect each other in growth environment

• One may kill the other

• Production of antimicrobial components

• e.g., Nicin of L. lactis killing other Gram (+) and inhibiting other Gram (-)

Antagonism