Microbiology of Food Preservation Notes

Food Preservation: Physical Methods

Thermal Processing

  • MOA (Mechanism of Action): Protein denaturation, cytoplasmic collapse, and cell lysis.
  • Preservation relies on microbial growth declining beyond their optimum temperature.
  • Heat can be applied as moist heat (more effective) or dry heat.
  • Conventional Thermal Processing:
    • Moist heat is more effective at penetrating microbial cells compared to dry heat.
    • Includes pasteurization and commercial sterilization.

Pasteurization

  • Mild heat treatment, often requires refrigeration.
  • Inactivates microorganisms and enzymes.
  • Eliminates the most heat-resistant pathogen of public health significance.
  • Heat-resistant microorganisms (including bacterial spores) survive.
  • Refrigeration retards the growth of surviving microorganisms.
  • Examples: refrigerated milk and juices.

Commercial Sterilization

  • Heat treatment for shelf-stability (not complete sterility).
  • Aims to render food free of microorganisms capable of reproducing under normal and refrigerated storage conditions.
  • Eliminates viable microorganisms, including spores of public health significance.
  • Commercially sterilized food may contain viable microorganisms that do not increase in number or cause changes in the food product.
  • May contain obligate thermophilic spore-forming bacterium as normal storage temperature is below the thermophilic range.
  • Acid-tolerant microorganisms may be present if food pH is too low for their growth.
  • Mesophilic or thermophilic spoilage spores may be recovered if coupled with other preservation techniques like lowering water activity.
  • Uses hermetically sealed containers (cans, jars, pouches) to prevent air passage.
  • Thermal processing often uses a retort (industrial-scale pressure cooker).
  • Heat exchangers are used for liquid food products.
  • Note: Composition of food affects the heat-resistance of microorganisms.
    • Low water activity increases heat-resistance.
    • Solutes that lower water activity increase heat-resistance.
    • Fats and proteins offer protection, increasing heat-resistance.
    • Highest heat-resistance is observed at pH values that provide optimum growth to the microorganism

Other Thermal Processing Methods

Aseptic Processing

  • Food product and packaging material are sterilized separately.
  • Applicable to liquid products like juices.
  • Juices are pumped through heat exchangers, heated, cooled, and placed in a pre-sterilized package, then sealed.
  • Other examples: UHT milk and sauces.
  • Canning involves pre-packing in a hermetically-sealed container before thermal processing (in contrast).

Ohmic Heating

  • Electricity generates heat.
  • Electricity passes through the food, energy is dissipated as heat, which results in rapid and uniform heating.
  • The electrical conductivity of the food affects Ohmic heating

Microwave Heating

  • Common in household settings.
  • Electromagnetic waves excite water molecules, causing intermolecular friction and heat.
  • Heat distribution is inherently uneven, potentially leading to uneven lethality.
  • Limited industrial application, more common in households.

Low Temperature Preservation

  • Involves removing heat to suppress microbial growth.
  • Includes freezing and chilling.

Freezing ($-18 °C$ or lower)

  • MOA: Inhibition of metabolic activity and mechanical injury of cells.
  • The freezing rate affects overall food quality and preservation efficiency.
Fast Freezing
  • Small ice crystals.
  • Produces better food quality than slow freezing.
  • Small ice crystals do not damage food cells.
  • Microbial growth inhibition is almost instantaneous.
  • Often uses cryogenic freezing (immersion in liquid nitrogen).
  • Example: cryogenic freezing of individual quick frozen (IQF) foods.
  • Cryogenic freezing is usually applied in individually quick frozen products (IQF) – products that are bulked in bulk but maintenance of individual product is important
  • Examples of IQF include frozen fish fillet, peas, etc.
Slow Freezing
  • Larger ice crystals.
  • Large ice crystals may damage both microbial and food cells.
  • Gradual ice crystal formation causes mechanical damage.
  • Concentration of food solutes in unfrozen water decreases water activity, leading to further damage.
  • Only cold-tolerant microorganisms (particularly fungi) can grow.
  • As freezing proceeds, solute precipitates, and residual water freezes.
  • Surviving microorganisms become metabolically inactive.
  • Household freezers employ slow freezing.
  • Example: home freezing.

Thawing

  • Greatly affects the microbial load of frozen foods.
  • Thawing results in:
    • Oxidative burst
    • Making the thawing process lethal to some of the surviving microbial cells
    • Survival of freezer-injured cells in the nutrient-rich liquid.
    • Release of nutrient-rich liquid from damaged food cells allows surviving microbiota to multiply.
  • Thawed products are vulnerable to rapid microbial spoilage or pathogen growth.
  • Thawing temperature greatly affects microorganism growth.
  • Recommended: thaw at refrigerated temperatures to slow growth.
  • Microwave thawing is faster but requires immediate cooking to prevent growth.
  • Room temperature thawing is not advisable due to higher risk of pathogen growth.

Chilling/Refrigeration ($-2$ to ~$16 °C$)

  • MOA: Inhibition of metabolic activity.
  • Suppresses growth and metabolic activity of mesophilic microorganisms, including many foodborne pathogens.
  • Concern: psychrophilic and psychrotrophic microorganisms.
  • They can still grow due to their ability to maintain membrane fluidity.
  • Examples: Pseudomonas, some Lactic Acid Bacteria, Listeria monocytogenes, Yersinia monolitica.
  • Usually coupled with other sublethal food processing techniques such as pasteurization.
  • Foods are held below ambient temperature but above freezing.

Modified Atmospheric Packaging (MAP)

  • Alteration of gaseous composition within food packaging.

Passive MAP

  • Relies on natural residual respiration of the food product and film permeability of the packaging material to attain the desired gas composition over time.
  • Residual respiration + firm permeability.
  • Example: fruits and vegetable packaging.

Active MAP

  • Rapid gas replacement by adding O2, CO2, N_2.
  • Widely used in preserving fresh produce like meat, seafood, fruits, and vegetables.
  • Vacuum packaging is considered as MAP.
  • Employed in packaging deli meats and cheeses.
  • Food safety concerns exist for ready-to-eat foods like fruits, leafy vegetables, deli meats, and cheeses.
  • Microbial load during post-harvest handling, processing, and distribution is critical

MAP Strategies

  • High O_2 MAP
    • Uses 20% oxygen, along with carbon dioxide and nitrogen.
    • High oxygen inhibits anaerobic respiration/fermentation.
    • Moderate CO_2 slows down aerobic microorganism growth.
    • Example: packaging of red meat; high oxygen maintains red color.
  • Low O_2 MAP
    • Barrier packaging materials provide decreased permeability to oxygen.
    • Examples: Low-Density Polyethylene (LDPE), Polyvinyl Chloride (PVC), Polypropylene (PP).
    • Low oxygen reduces respiration of fruits and vegetables.
    • Inhibits the growth of aerobic microorganisms.
    • CO_2 concentration is significantly increased.
    • Increased CO_2 provides growth inhibition.
    • Gram-negative bacteria are generally more sensitive to CO_2 than gram-positive.
    • High CO_2 inhibits Pseudomonas growth (common spoilage microorganism of meat).
    • Extremely low O_2 can allow growth of pathogenic anaerobes like Clostridium botulinum and Listeria monocytogenes.
    • Spoilage by Lactic Acid Bacteria is also high in low O2/high CO2 environments.
    • Nitrogen gas is generally used as filler gas to balance CO2 and O2 concentrations.

Controlled Atmosphere Storage (CAS)

  • Employs the same principle of reducing oxygen concentration and increasing carbon dioxide concentration to limit.
  • CAS is applied to prolong the shelf-life of fruits and vegetables in large scale compared to MAP.
  • Continuous monitoring and precise adjustment of gases.

Low Water Activity Preservation

  • Microbial growth is inhibited, leading to microbiostasis.
  • MOA: microbiostasis.

Drying

  • Lowers water activity by mobilizing water from inside the food matrix to the surface, then removing it by evaporation.
  • Employs convection ovens with hot air blown inside to evaporate surface water.
  • Heat from convection is transferred within the food via conduction.
  • Drying is less lethal than moist heat; preservation is achieved mostly by inhibiting metabolic activity.
  • Microorganisms on the surface may be killed through prolonged exposure to high temperatures.
  • Microorganisms inside may remain viable but static.
  • Examples: dried fruits and jerky (shelf-stable).
  • Increased relative humidity can lead to water reabsorption and increased water activity.
  • Common spoilage microorganisms: yeasts and molds (tolerate low water activity).

Freeze Drying

  • Freezing then sublimation.
  • Involves freezing first, then removing ice via sublimation.
  • Has minimal impact on the structure and flavor of food.
  • Higher quality compared to dried products.
  • More expensive, limiting its application to high-value products.

Osmotic Dehydration

  • MOA: microbiostasis/osmotic shock.
  • Involves hypertonic solutions (sugar, salts).
  • Microbial control relies on osmotic pressure: water is drawn out of microbial cells from low to high solute concentration.
  • Many microorganisms do not survive high osmotic conditions, except yeasts and molds.
  • Examples: Salted meat and fish, jams, and jellies.

Humectants and Intermediate Moisture Foods (IMF)

  • Humectants: Hygroscopic substances that attract and bind to water.
    • Used to lower water activity.
    • Absorb excess moisture in the air.
    • Prevent water activity increase due to high relative humidity.
    • Examples: sugar, salt, glycerin, sugar alcohols, syrups, egg components, molasses, and acids.
    • Important in producing intermediate moisture foods (IMF).
  • Intermediate Moisture Foods (IMF)
    • Moisture content: 15-50%.
    • Water activity: 0.60-0.85.
    • Considerations: S. aureus, yeast, and molds.
    • S. aureus is the only bacterium of public health importance that can grow at water activity of 0.86.
    • Water activity should be below 0.85.
    • Additional hurdles for yeasts and molds that still grow at low aw:
      • Lowering of pH.
      • Addition of chemical preservatives (sorbate and benzoate).
    • IMFs are shelf-stable at room temperature for varying periods.
    • Examples: dried foods, jams, and jellies, cakes, pastries, candies, fruit juice concentrates, sweetened condensed milk, and syrup.
    • IMFs are produced by withdrawing water through:
      • Adsorption: Food is first dried (often by freeze drying), then subjected to controlled rehumidification until achieving desired water activity.
      • Desorption: Food is placed in a solution with higher osmotic pressure (e.g. syrup, salt water) until reaching equilibrium and desired water activity.

Food Irradiation

  • Food exposure to controlled amounts of electromagnetic radiation.
  • Electromagnetic spectrum: lower energy (longer wavelength, non-ionizing) and higher energy (shorter wavelength, ionizing).
  • The choice of the spectrum depends on the food product.

Ultraviolet (UV) Light

  • Longer wavelength; lower energy (non-ionizing radiation).
  • Only useful for treating the surface of food due to limited penetration.
  • Penetration is affected by turbidity in liquid foods.
  • Effectivity diminishes as liquid becomes turbid.
  • Clear liquid food disinfection is done by passing through a chamber with a UV lamp.
  • Effective for clear juices (e.g., apple juice) to reduce pathogens like E. coli O157:H7.
  • MOA: Formation of thymine dimers.
  • Poor penetrating power.
  • Surface and clear liquid disinfection.

Ionizing Radiation

  • Shorter wavelength; higher energy (X-rays, Gamma Rays).
  • Preferred for treating bulky food products (e.g., a sac of spices).
  • Radiation source is isolated and secured by walls and shields.
  • Foods are loaded on conveyors passing through a chamber for exposure.
  • Dosage is controlled by conveyor speed and package thickness.
  • Eukaryotic microorganisms like yeasts and molds are more susceptible than bacteria due to larger genome size.
  • Among bacteria, gram-negative are more susceptible than gram-positive.
  • Among gram-positive bacteria, spore-formers are more resistant than non-spore-formers.
  • MOA: DNA damage by free radical formation.
  • Treatment of fresh produce (with Radura symbol).
  • Irradiated foods are labelled with the international symbol called radura.

High Pressure Processing (HPP)

  • Relatively new method of food preservation.
  • Application of isostatic pressure (50-1000 MPa).
  • Pressure is applied uniformly and simultaneously in all directions.
  • MOA: Protein synthesis inhibition, protein denaturation, cell rupture.
  • Produces high-quality foods that are microbiologically safe and have an extended shelf-life.

Food Preservation: Chemical and Biological

Chemical Antimicrobials

  • Agents used to ensure food remains safe and unspoiled during its shelf-life.
  • Exert -static (inhibit growth) or -cidal (kill) effect.
  • Target specific parts of the cell (cell wall, cell membrane, enzymes, nucleus, etc.).
  • Will not preserve food indefinitely; used in combination for synergy.
  • Generally Recognized as Safe (GRAS)
    • Should not cause negative side effects to humans at used concentrations.
    • GRAS status indicates substances are safe under intended use conditions.
    • Pre-market reviews are done to these substances and they are eventually approved by regulatory agencies before they are added to food
    • Should be heat-resistant (stable during processing).
    • Should not be destroyed by reactions in food.
    • Should not be inactivated by microbial metabolic end products.
    • Should not stimulate resistance.

Factors Affecting Chemical Antimicrobials

  • Food preservation is successful when antimicrobial type, concentration, storage time, temperature, pH, buffering capacity, and the presence of other components are known.

Microbial Factors

  • Inherent resistance (susceptibility varies among strains).
  • Initial population (higher numbers are harder to eliminate).
  • Growth phase (slower growth rates have higher survival rates).
  • Interaction with other microorganisms (antagonism). For example disruption of the virulence factors of the S. aureus by Bacillus spores
  • Cellular composition/structure (cell wall composition affects entry).
  • Protective structures (capsules, biofilms).

Intrinsic Factors

  • pH (most important factor).
  • Water activity.
  • Nutrients.
  • Electric potential.
  • Other inhibitory substances.

Extrinsic Factors

  • Temperature (most important factor).
  • Relative humidity.
  • Gases.

Organic Acids

  • Most widely used antimicrobials.
  • Reduce pH and improve flavor.
  • Present in three ways:
    • Naturally (e.g., citric acid in citrus, benzoic acid in berries).
    • Produced during fermentation (e.g., lactic acid, propionic acid, acetic acid).
    • Added intentionally (regulated by governing agencies).
    • Acetic Acid, Benzoic Acid, Lactic Acid, Propionic Acid, Sorbic Acid

Acetic Acid

  • pKa 4.8.
  • At pH values higher than 5, the undissociated fractions of acetic acid are quite low.
  • Inhibits yeasts and bacteria.
  • 0.2% concentration: -static.
  • \geq 0.3% concentration: -cidal.
  • More effective against Gram-negatives (especially bacterial pathogens).
  • Mild effect on molds; acid-tolerant bacteria are resistant.
  • Application: bakery products (inhibits Bacillus subtilis), cheeses condiments and relishes, dairy product substitutes, sauces, meat products.

Benzoic Acid

  • pKa 4.2.
  • Most effective at pH range of 2.5 to 4.5.
  • Added at 500-1000 ppm.
  • Inhibits yeasts and molds > bacteria.
  • Used in high acid foods.
  • Can inhibit aflatoxin production of Aspergillus flavus.
  • Some yeasts and molds are resistant (e.g., Byssochalmys nivea, Pichia membranifaciens, Talaromyces flavus, Zygosaccharomcyes bailii).
  • Application: high acid foods (soft drinks, tomato catsup, jams, fruit juices, pickles, salad dressings).
  • Present naturally in cranberries, plums, prunes, cinnamon, clove.

Lactic Acid

  • pKa 3.80.
  • Like other organic acids, it is more effective in low pH.
  • Among other organic acids, lactic acid is the least effective due to its relatively low pKa value
  • Naturally produced by LAB during lactic acid fermentation.
  • Effective against pathogenic and spoilage bacteria.
  • Ineffective against yeasts and molds.
  • Application: 1-2.5% antimicrobial agent, pH control agent, flavor enhancer, carcass sanitizer (0.2-2.5% sprays/dips).

Propionic Acid

  • pKa 4.87.
  • effective against bacteria and molds
  • ineffective against yeasts.
  • Directly added to bread (no effect on baker’s yeast).
  • Inhibits rope formation in bread caused by bacteria and molds.
  • Used in concentrations of 1000-2000 ppm.
  • Applied in bakery products and bread.
  • Application: bread and bakery products, certain cheese, jams and jellies, tomato puree
  • Formed by certain plants and Propionibacterium.
  • Produced in the fermentation of Swiss cheese by the heterofermentative, gram-positive Propionibacterium.

Sorbic Acid

  • pKa 4.76.
  • 50 to 200 ppm – antifungal and antibacterial (antilisterial and anticlostridial).
  • Application: bakery products, fruit juices, salad dressings, pickles, jams and jellies, dairy products, condiments; spray, drip, coating in packaging materials

Parabenzoic Acid (Parabens)

  • Increase pKa (8.50)
  • Increase chain length, increase antimicrobial activity
  • Decrease solubility
  • Used in combination (2:1 or 3:1 methyl and propyl parabens).
  • Inhibits enzymatic functions, interfere with membrane functions
  • Application: baked goods, beverages, fruit products, syrups, dressings, wines, and fillings

Nitrites

  • Sodium and potassium salts.
  • Inhibits Clostridium botulinum in cured meat.
  • Membrane permeability interference; enzyme inactivation.
  • Example: pyruvate-ferrodoxium oxidoreductase.
  • Enhanced activity at low pH and reducing conditions.
  • Synergy with ascorbate and erythorbate.
  • Improves color of meat.

Sulfites

  • Salts of sulfur dioxide.
  • Inhibits bacteria and fungi.
  • Reactive molecule – disrupt metabolism and break disulfide linkages/bonds; interfere with redox potential
  • Application: wine (inhibit malolactic fermentation); chilled and frozen seafoods, fresh and dried fruits

Biological Preservation

Natural Antimicrobials

  • Present in herbs, spices, vegetables, milk, and egg.
  • Concentration is low to inhibit microbial growth.
  • Spices – inhibitory concentrations above tolerable taste thresholds.

Beneficial Bacteria

  • Lactic Acid Bacteria.
  • In situ acidification in bacon.
  • Production of bacteriocin and antimicrobial peptides.

Bacteriocins and Antimicrobial Peptides (AMPs)

  • Proteinaceous or peptidixic compounds.
  • Inhibition through membrane interference.
  • Nisin (Lactococcus lactis) (anticlostridial and antilisterial).
  • Pediocin (Pediococcus) (antilisterial).

Bacteriophages

  • Application: Biopreservatives and Disinfection of food-contact surfaces.