Food Microbiology: Preservation Methods

BIOL 3203 Food Microbiology: Physical and Chemical Preservation Methods

• Food preservation prevents deterioration and spoilage of food products
• It extends the product's shelf-life
• Food preservation assures consumers of a product free of pathogenic microorganisms without altering sensory attributes or the nutritional composition.
• Food preservation protocols kill microbes (major stresses) or inhibit the growth of microbes (minor stresses)
• Food preservation is divided into physical and chemical methods

Microorganisms Control: Mode of Action

• Destruction: A -cidal effect kills microorganisms
• Inhibition: A -static effect slows down or stops the growth of microorganisms
• Separation: Removes microorganisms by filtration
• Separation: Applies to liquid or gas

Action on Microbial Cells

• Damages cell wall and/or plasma membrane
• Damages protein and/or nucleic acid

Factors Affecting Antimicrobial Effectiveness

• Number of microorganisms: The larger the initial population, the longer elimination takes
• Exposure time: Longer exposure to control methods increases microbe elimination
• Chemical antimicrobials often need extended exposure for resistant microbes or endospores

Microbial Characteristics

• Resistant Microbe: More resistant microbes are more difficult to kill
  • Endospores vs non-endospores
  • Gram-positive vs Gram-negative
  • Enveloped virus vs non-enveloped virus

Physical Methods for Preservation

• Includes Heat Treatment
• Includes Dehydration / Drying
• Includes Freezing
• Includes Changing Atmosphere
• Includes Radiation

Heat Treatments

• Heat is a widely used antimicrobial method that is relatively inexpensive and effective
• Cell Multiplication: When the temperature of a medium exceeds optimum for a microorganism, cell multiplication slows, eventually ceasing
• Temperature Increase: A temperature increase detrimentally affects membranes, proteins, ribosomes, and microbial cells, leading to injury or death
• Temperature Range: Mildly high temperatures (55-90°C) damage cell components of psychrotrophic and mesophilic microorganisms
• Process Optimization: Processors optimize temperature and time to protect customers from pathogens of concern and confirm acceptable product quality
• Pathogens of Concern: Examples include enterohemorrhagic Escherichia coli in fruit juices, Salmonella enterica serovar Enteritidis in liquid eggs, and Listeria monocytogenes in milk and ready-to-eat meat
• Heat Sterilization: Measuring the effectiveness involves different Time and temperature combinations
  • High temperature, long time - practically not necessary
  • High temperature, short time - requires adjustment
  • Low temperature, short time - doesn't kill all microbes
  • Low temperature, long time

Heat Treatments - D-Value

• Decimal reduction time (D-value): Time (minutes) at a given temperature for viability to decrease by a factor of 10
• Heat Resistance: The D-value indicates a microbe's heat resistance at a single temperature
• Suspending Medium: D-value changes with the strains and characteristics of the suspending medium
• PH: A low pH sensitizes microorganisms to heat

D-Value and Temperature

• Processing Temperature: It decreases D-value as processing temperature increases

Heat Treatments - Z-Value

• Thermal resistance constant (Z-value): Degree required to change the D-value by a factor of 10
• Lethality: It describes how lethality changes with temperature
• Microorganism Resistance: A high Z-value indicates resistance to heating temperature variations

Heating Effectiveness

• D-Value and Z-Value: If the D-value is 5 mins at 120°C, and the D-value at 130°C lowers by 1 log, then the Z-value is 10°C
• Temperature Impact: 10°C temperature increase reduces by 1 log, while a decrease raises it by 1 log
• Heating Effectiveness: An example shows of 120°C for 5 mins, 130°C for 0.5 mins, and 110°C for 50 mins are the same

Heat Treatments - F-Value

• Thermal Death Time (F value): The time necessary to kill a given population of organisms (targeted reduction) at a set temperature
• Heat Resistance: It shows how heat resistance changes at different temperatures
• Microbial Population Reduction: A process designed to reduce a microbial population from 102 to 10-3 CFU/g will use the F value to find holding time to achieve 5-log reduction
• Commercial Sterility: In the canning industry, C. botulinum spore population must be reduced by 12 log, and its spores have a D121°C is 0.21 min, which equals 2.52 min

Heat Treatments - Moist Heat (Pasteurization)

• Pasteurization: A mild treatment inactivates spoilage-causing enzymes and many spoilage microorganisms that aren't spore forming
• Pasteurization Temperatures: Foods can be treated at
  • 62°C for 30 mins (LTLT - low temperature long time)
  • 72°C for 15-17s (HTST - high temperature short time)
  • 120-138°C for 2-4s (UHT - ultra-high temperature)
• Rapid Cooling: Rapid cooling follows to prevent microorganisms from growing

Heat Treatments - Moist Heat (Sterilization)

• Commercial Sterilization: All canned foods are sterilized in a retort, confirming no viable organisms are present.
• Pathogen Destruction: Foods with a pH>4.6, like meat and vegetables, are heated under pressure at 121°C to destroy pathogenic and spoilage microorganisms
• Spore Destruction: Conditions are applied to destroy Clostridium botulinum spores since they produce botulinum toxin under anaerobic conditions
• Detection: The organism produces no gas or taste; it remains undetected

Heat Treatments - Microwave Sterilization

• Microwave heating: Radio frequency (1-500 MHz) or microwave (500 MHz - 10 GHz) generates heat with rapid oscillation of water molecules (electric dipoles, 𝛅- on O and 𝛅+ on H)
• Reorientation: They reorient with each change in the field direction increasing intermolecular friction
• Heat Production: It produces heat directly in food

Microwave Sterilization - Compared to Conventional Heating

• Location: Microwaves transfers heat throughout the material, faster heating rates and shorter times
• Color and Texture: The food colour and texture are often better
• Inherent Unevenness: Energy distribution is uneven, leading to thermal nonuniformity and uneven microorganism lethality.

Dehydration / Drying

• Preservation: Drying is an one of the old food preservation methods
• Process: As foods dry, hot air evaporates surface water, leading to concentrated foods with many retained nutrients.
• Microbial Growth: The drying temperature and reduced water activity, aw, delays or prevents bacterial growth

Freezing

• Temperature Reduction: Freezing lowers the temperature to <-18°C, stopping the metabolic activity of most foodborne ones
• Microorganism Impact: Enzymes are inactivated, proteins denatured, and metabolic processes prevented, hindering product deterioration
• Microbes - Osmotic Shock: Freezing produces osmotic shock in microbes
• Crystal Formation: Intercellular ice crystal formation causes mechanical injury
• Cell Membranes: The cell membranes suffer major damage
• The storage life of frozen food ranges from months to more than a year
• Microbes - Reversibility: The microbial injury can be reversible or irreversible
• Microbes - Variables: The injury, repair, death, and survival vary with the freezing, frozen storage, and thawing conditions
• Thawing: Thawing releases nutrient-rich liquid from damaged food cells, promoting multiplication of surviving microbiota

Changing the Atmosphere - Cool Storage

• Cool Storage: Chilled storage refers to storing food between -2 to 16°C
• Storage Time: It may range between days to weeks
• Microorganism Metabolism: Low temperatures reduce metabolic activity and microbial growth/metabolism

Changing the Atmosphere - Controlled-Atmosphere Storage (CAS)

• Use: It is widely used for certain fruits and vegetables
• Levels: The O₂ is reduced from 21% to 0.5-2.5%, and the CO₂ content is increased from 0.045% to 8-10%
• Microbial Activity: High CO₂ reduces microbial growth by inhibiting respiration and reducing pH

Changing the Atmosphere: Modified-Atmosphere Packaging (MAP)

• Use: It alters a food's gaseous composition by controlling mixtures of O2, CO2, N2, and others
• Microorganisms: It's inhibitory to certain microbes and improves food quality

Radiation - UV Radiation

• UV-radiation: Non-ionizing radiation at 240-280nm is destructive
• DNA Damage: Damages nucleic acids by cross-linking thymine dimers in DNA, preventing repair and reproduction
• Bacterium Impact: Gram-negative bacteria are easily killed compared to resistant bacterial endospores and molds
• Viruses: Viruses are more UV resistant

Radiation - Use of UV Irradiation

• Limitation: Food processing is has been limited due to penetration ability
• Application: Applied more frequently to the disinfection of surfaces

Radiation: Ionizing Radiation

• Properties: radiation (e.g. X-ray, gamma-ray) has strong penetration power to kill microbes by damaging DNA without affecting the quality of food
• Susceptibility: Gram-negative bacteria, including spoilage and pathogenic species, are more sensitive than vegetative gram-positive bacteria

Novel Physical Methods

• Includes Dense-Phase Carbon Dioxide
• Includes Cold Plasma

Dense-Phase Carbon Dioxide (DPCD)

• Processing Type: Non-thermal processing technology for food preservation
• Supercritical State: CO₂ held under high pressure
• Safety: CO₂ is thermodynamically stable, with a lack of flammability and toxicity
• Processing: Liquid food is often heated to 30-50°C and exposed to pressure above the critical point, with CO₂ injected and released

DPCD Use

• pH: added CO2 decreases liquid food medium pH and denatures microbial enzymes
• Chemical Reactions: Formation of bicarbonate complexes or metal ion precipitation
• Mechanism of Action: Expansion of CO₂ in cells causes cell lysis when released during depressurization
• Cell Viability: Expansion may promote release of vital biomolecules, affecting cell viability

Cold Plasma

• State: It is a gas with ionized molecules and atoms, categorized as the fourth state of matter
• Gas Type: generated from nitrogen, oxygen mixtures, noble gases
• Temperature: Cold plasma refers to plasma generated near room temperature
• High Enery: High energy (UV radiation) causes collisions and inactivates microorganisms through degradation of cell membranes and essential biomolecules

Physical and Chemical Preservation Methods: Food Antimicrobials

• Food antimicrobials are chemicals either added to or found in foods that slow the growth of—or kill—microorganisms
• Action: Mostly bacteriostatic or fungistatic at the used concentrations, which means antimicrobials do not indefinitely preserve food
• Legal Definition: Legally defined as "preservatives”, often in combination with other methods

Factors Affecting Antimicrobial Activity

• Antimicrobial Activity: It is affected by microbiological, intrinsic, extrinsic, and process factors in complex food systems

Antimicrobial Classes

• Naturally Occurring Compounds: Extracted from natural sources with "friendly" labels
• Chemical Antimicrobials: Made by synthetic or natural means, used for years, and approved by many countries

Naturally Occurring Antimicrobial

• Iron: It stimulates growth in many genera, e.g. Clostridium, Escherichia, Listeria, Pseudomonas, Salmonella, Staphylococcus
• Iron-binding proteins: It is found in Milk and eggs, lactoferrin (milk), transferrin (low levels in milk), ovotransferrin (egg albumin)
• Microbe Benefit: Iron sequestration limits availability of essential iron
• Susceptibility: Gram-positive bacteria (Bacillus and Micrococcus) are generally more sensitive

Naturally Occurring Antimicrobial - Avidin

• Avidin: An egg albumin protein (0.05% of total albumin protein)
• Stability: Stable to heat and a wide pH range
• Competes with Biton: Binds biotin to inhibit bacteria and yeasts
• Transport Protens: Interfere transport proteins of E. coli.

Naturally Occurring Antimicrobial - Spices

• Spices & Oils: Flavoring agents for foods
• Antimicrobial nature: Mainly through interfering the microbes membrane function