W1 L1: Factors affecting growth and survival of microorganisms in food

Effects of food microflora:

• Food Spoilage (undesirable)

• Foodborne Illness (undesirable)

• Fermentation (desirable if purposeful)

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  Manifestations of microbial food spoilage:

4 Factors affecting microbial growth in food:

Milk spoilage

• Bacteria (Lactobacillus, Lactococcus) grow on milk sugars (lactose)

• Lactic acid is produced/build up → pH ↓ & sour taste

• Acid selects for Δ in the bacterial pop, pH ↓ further & more lactic acid produced until all sugars depleted

• Bacteria start to grow again at the higher pH & use proteins as their major nutrient (no sugars) & then 1º amines are produced

• In very acidic conditions, yeasts & moulds use lactic acid for growth & the pH ↑

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Effect of pH on microbial growth:

• pH→ expressing the H ion conc. [H+]

• [H+] is important for living things as the +ve charge alters the charge environ. of other mols. in sol.

• H+ ions can bond to nitrogen or oxygen containing molecules because nitrogen and oxygen have “non-bonding” pairs of electrons

• Acids → substances capable of donating an H+ to a base

• Bases → substances capable of accepting a H+ from an acid

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Microbial inhibition by weak organic acid

• Undissociated lipophilic acid mols. pass freely via the membrane from an external environment of low → high pH of cytoplasm.

• At higher pH, equilibrium shifts in favour of the dissociated molecule, so the acid ionises producing protons which will acidify the cytoplasm.

• The cell will try to maintain its internal pH but this slows growth.

• If external pH is low and the extracellular acid concentration is high, the cytoplasmic pH drops so that growth is not possible and cell dies

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The effect of redox potential (Eh) on microbial growth

• The transfer of H+ ions b/w chemical species determines pH; the transfer of electrons b/w chemical species determines the Eh
• **Redox potential (Eh)**→ tendency of a medium to accept or donate electrons, to oxidise or reduce.
• If it accepts e-s it has a +ve potential → [O] environment
• If it donates e-s it has a -ve potential → [R] environment
• Strong [R] agents have a high electron-transfer potential
• Strong [O] agents have low electron-transfer potential
• Obligate or strict aerobes need O2 and high Eh & will predominate at food surfaces exposed to air or where air is readily available
• Pseudomonas fluorescens, growing at Eh of +100 to +500 mV, & other oxidative Gram- rods produce slime and off-odours at meat surfaces.
• Bacillus subtilis (Eh−100 to +135 mV) produces rope in texture of bread
• Obligate anaerobes tend only to grow at low or negative Eh
• Obligate anaerobes, like clostridia, are of great importance in food microbiology.
  • Grown in anaerobic conditions (deep in meat tissues and stews, in vacuum packs and canned foods) causing spoilage or botulism by C. botulinum

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The effect of water activity (aw) on microbial growth

• Measure of the E status of water (how much is available/not bound)

• Water activity (aw) is the vapour pressure of water in equilibrium with the sample (p) ÷ by the vapour pressure of pure H2O at the same temperature (Po)

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• Pure distilled water → aw = 1

• Higher aw foods tend to support more microorganisms

• Microorganisms differ in their water activity requirements

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The effect of temperature on microbial growth

• How a particular microorganism responds to temperature is defined by 3 cardinal points (minimum, optimum, maximum temperatures for growth)
• Different microorganisms respond differently to temperature
• Microbial growth occurs from -8 o C to over 100o C, but:
  • some organisms have specific temperature requirements within this range
  • food is only stored at certain temperatures
  • Mesophiles and psychrotrophs are generally of greatest importance
• Mesophiles, w/ temp optima ~37 °C, - frequently of human or animal origin and inc. many more common foodborne pathogens: Salmonella, St. aureus & Cl. perfringens
  • Grow quicker at their optima than psychrotrophs ∴ spoilage of perishable products stored in the mesophilic growth range is more rapid than spoilage under chill conditions.
• Psychrotrophs → found in more diverse range of habitats and consequently are of greater importance in the spoilage of chilled foods.
• Thermophiles → far less important in food microbiology
  • thermophilic spore formers such as certain Bacillus and Clostridium species do pose problems in a restricted number of situations

Relative Humidity

• Relative humidity & aw are interrelated → rel. humidity is essentially a measure of the water activity of the gas phase
• Foods w/ a low aw will transfer from the gas phase to the food, when stored in an atmosphere of high relative humidity water
• Microorganisms start to grow they produce H2O as an end product of respiration→ ↑ aw of immediate environment so that micro-organisms requiring high aw can grow & spoil a food which was initially considered to be microbiologically stable
• Storage of fresh fruit & veg needs good control of rel. humidity
  • If it is too low then vegetables will lose water and become flaccid
  • If it is too high then condensation may occur - spoilage may start

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Gaseous Atmosphere

• Oxygen - most important gas in contact with food.
• Inhibitory effect of CO2 on microbial growth - applied in modified-atmosphere packing and at elevated pressures in carbonated mineral waters and soft drinks.
• Moulds & oxidative Gram -ve bacteria→most sensitive to CO2
• Gram +ve bacteria, particularly the lactobacilli→ most resistant
• Yeasts e.g. Brettanomyces spp. show tolerance of high CO2 levels & dominate the spoilage microflora of carbonated beverages
• Growth inhibition→ aerobic conditions > anaerobic and ↑ w/ temperature ↓ due to ↑ CO2 solubility at lower temps
• Some microbes are killed by prolonged exposure to CO2 but its effect is mainly bacteriostatic

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The effect of implicit factors on microbial growth

• Implicit factors→ inherent growth properties of the spoilage organisms themselves, how they respond to environment and interact with one another

Growth range

• temperature requirements (see previous table)

• pH requirements (e.g. acidophiles, etc)

• aw (halophiles, xerophiles)

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Microbial interaction - how microorganisms influence each other

• antagonism (antibiotics, acids)
• commensalism (one benefits the other doesn’t)
• symbiosis (both partners benefit)

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• Many moulds can grow perfectly well on fresh foods such as meat, but grow slower than bacteria ∴ out competed
• In foods where the faster growing bacteria are inhibited by factors e.g. reduced pH or aw, moulds assume an important role in spoilage
• Response of micro-organisms to Δ depends on the physiological state of the organism
• Exponential phase cells are almost always killed more easily by heat, low pH or antimicrobials than stationary phase cells
• Pre-adaptation will ↓ the damaging effect of adverse conditions
• Pre-exposure to a factor ↑ an organism’s subsequent resistance to it

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The effect of processing factors on microbial growth

• Processing factors→ the way a food is treated during production and how this influences the presence of microbes
• Slicing/Mincing→ higher exposure to O2 and spread of surface microorganisms to interior of food
• Heat treatments→ higher temperatures will destroy microbes
• Preservatives→ if present will kill or prevent specific microorganisms from growing
• Cooling→ Low temps retards growth

Chemical preservatives

• Preservatives→ substances capable of inhibiting, retarding or arresting the growth of micro-organisms or of any deterioration resulting from their presence or of masking the evidence of any such deterioration
  • Don’t include substances like antioxidants or phosphates
• Preservatives may be microbicidal & kill the target organisms or they may be microbiostatic in which case they simply prevent them growing
• Higher levels of an antimicrobial are lethal while lower concs that are generally permitted in foods tend to be microbiostatic
• Chem preservatives are useful only in controlling low levels of contamination & are not a substitute for good hygiene practices

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Food preservation by using chemicals:

Nitrite

• Usually in combination with curing: NaCl and nitrite/nitrates, action mediated by NO Nitrites are essential for colour and flavour development in refrigerated treated food Nitrosamines an issue (e.g. bacon and ham)

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Sulphites (SO2 , H2S3 , Na2S2O5 )

• Main use in sausages and wine (stops fermentation, antioxidant, preservative)
• Destroy thiamine but promote ascorbate retention
• Allergic reactions in the sensitive (asthma) a problem

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Antibiotics

• Must not be used in food if have therapeutic application→ Strict regulation
• Nisin
  • Polycyclic antibacterial peptide from Lactococcus lactis
  • Used to inhibit spore outgrowth in heat processed products (canned foods, cheeses)
• Natamycin (NatamaxTM)
  • From Streptomyces natalensis Antifungal
  • Used as a surface preservative for some cheeses and dried sausages

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Acids

• Benzoic acid, Sorbic acid, Parabens, Propionic acid

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Food preservation by the use of reduced temperature

• Below -10°C no microbial growth can occur

• Chilled Foods (0-5°C)

  • Foods stored above their freezing point
  • Changes flora to slow growing psychrotrophs that eventually spoil food

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• Freezing (-20°C)

  • Maintains sensory and nutritional factors
  • Does not sterilise food
  • Dependent on both temp and aw

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Food preservation by Modified Atmosphere Packaging

With specialised packing material and packing machines, the atmosphere around a food can be modified to inhibit microbial growth

1. Vacuum Packing→ product (meat) packed in vacuum
   • ↓ O2 , ↑ CO2, inhibits aerobes
   • creates temporary colour Δ (deoxyhemoglobin)
   • used for bulk packing and for some retail meats
   • concerns about Listeria monocytogenes, Clostrdium botulinum and Yersinia entercolitica
2. **Modified Atmosphere Packing (MAP)**→ tailored atmospheres that change via interaction with food:
   • CO2 inhibits aerobic bacteria + moulds Low O2, N2 used to replace O2 to inhibit aerobic spoilage organisms High O2 used to maintain fresh colour in meat, and respiration in fruit/veg
3. Controlled Atmosphere Packaging (CAP) → tailored atmospheres which are maintained (steady state)

• used for bulk storage/transport

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Hurdle technology

• Ensures that pathogens in food products can be eliminated or controlled (safe for consumption food)

• By combining more than one approach- pathogens have to overcome these "hurdles" to remain active in the food.

• When the E needed for biosynthesis is diverted into maintenance of homeostasis, cell growth is inhibited. When homeostatic E demands exceed the cell's energy-producing capacity→ cell dies

• Most foods preserved in this manner

  

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