Factors Influencing Food Spoilage and Preservation Techniques

Intrinsic factors of spoilage

Water activity (aw), pH, Redox potential, Nutrient content, Natural structures, Antimicrobials, Combinations

Extrinsic factors of spoilage

Temperature, Gaseous atmosphere, Relative humidity

Implicit factors of spoilage

Specific growth rate, Survivability, Biofilms, Microbial interactions (Neutralism, Commensalism, Mutualism, Interference (Amenalism, Competition, Parasitism))

cfu/ml in aseptic milk

102-103 cfu/ml

Microbes found in aseptic milk

Micrococci/streptococci

Infection that can increase cfu in milk

Mastitis

Temperature for storing raw milk

<5 °C

Effect of Pseudomonas on milk quality

Pseudomonas in high numbers produces heat-stable enzymes that break down proteins and fat, causing rancidity.

Bacteria that can survive pasteurization

Thermoduric bacteria (mainly gram-positives)

Main microbes causing milk spoilage

Psychrotrophs (Pseudomonas, Acinetobacter, and Psychrobacter), Bacillus spp.

How main spoilage microbes are introduced into milk

Post-pasteurization contaminants (filling or at home)

Part of Bacillus spp. that allows it to survive pasteurization

Spores

Water activity and nutrient characteristics of meat that make it prone to spoilage

High water activity and nutrient content

Type of microbial activity most common in meat spoilage

Most microbes are proteolytic (use carbs first, proteolysis later)

Glycogen breakdown post-mortem in meat tissue

Glucose then lactic acid

Relation of glycogen levels to meat pH

Lower glycogen results in high pH meat

Factors that cause meat to spoil sooner

Exercise, stress, cold

Are internal organs of a live animal free from microbes?

Yes

Conditions of aerobic storage that affect meat spoilage

High redox potential

Microbes that dominate in meat spoilage under aerobic storage

Psychrotropic aerobes (Gram-negative aerobic bacteria - Pseudomonas, Acinetobacter)

cfu/cm2 level that leads to off odours in meat

10^7 cfu/cm2

Metabolic change in bacteria that causes off odours in meat

Switch from glucose to amino acid use

cfu/cm2 level that leads to slime formation on meat

10^8 cfu/cm2

Compounds produced during meat spoilage

Ammonia and amines

Typical pH level in meat spoilage

pH > 8

Conditions of vacuum-packed/MAP that affect meat spoilage

High CO2 and no O2

Effect of high CO2 and no O2 on bacterial growth in meat

Restricts aerobic bacteria (Pseudomonas), selects for lactic acid bacteria (Lactobacillus, Carnobacterium)

Odour and flavour changes in vacuum-packed meat

Sour odour and flavours (not as much odour)

Maximum cfu/cm2 in vacuum-packed meat

10^7 cfu/cm2

Typical pH level in vacuum-packed meat

High pH (>6)

Bacteria that can grow in high pH vacuum-packed meat and compound it produces

Shewanella putrefaciens, produces H2S

Effect of water temperature on fish spoilage

Cooler waters spoil faster than warmer waters

Glycogen levels in fish post-mortem

Very low glycogen (reduced during death struggle)

Compounds that microbes immediately attack in fish

Amino acids

Microbes typically involved in fish spoilage

Psychrotrophic gram-negatives (Shewanella putrefaciens and Pseudomonas)

Compounds produced by microbes that cause off odours in fish

Thiols and amines

Bacteria that can still grow in vacuum and MAP fish packaging

Shewanella

Sources of spoilage microbes in plants

Pre- and post-harvest, some plant pathogens

Defenses plants have against spoilage

pH, barriers, phenolic antimicrobials

Post-harvest factors that affect plant spoilage

Temperature, aw, Atmosphere

Spoilage microbes found in cereals

Fungi (Field fungi - Cladosporium, pathogenic - Fusarium), Storage fungi (Penicillium, Aspergillus, Fusarium), Mycotoxins

Spoilage factors characteristic of fruits

High aw and low pH

Spoilage microbes common in fruits

Fungi (yeast and moulds, Penicillium in apples and citrus)

Control measures for fruit spoilage

Reduce temp and chemicals

pH of vegetables

Higher pH compared to fruit.

Spoilage microbes in vegetables

Bacteria (Pseudomonas), Fungi (Aspergillus, Fusarium).

Pathogen entry in vegetables

Cracks/wounds.

Softening in vegetables during spoilage

Pectin degradation.

Control measures for vegetable spoilage

Refrigeration and MAP.

Types of food preservation methods

Reduce microbe access, physically remove microbes, prevent microbe growth/spore germination, kill microbes/spores.

Impact of control methods on microbes

Less microbes initially, microbes in exponential growth phase, microbes are injured.

Resistance variations in microbes

Spores are more resistant than vegetative cells, Gram-positives are more resistant than gram-negatives.

Hygiene factors in food processing

Plant design, water quality, air quality, personnel training, equipment design, cleaning, sanitation, packaging.

Removing microbes from liquids

Centrifuge (dirt, cell clumps), Filtration (keeps heat-sensitive compounds).

Removing microbes from solids

Washing, Trimming visible growth.

Removal of microbes effectiveness

No, it reduces microbial load.

Difficult microbial structures to remove

Biofilms.

Appertisation

Commercial sterility, kills all vegetative cells, thermophiles won't grow at room temp.

LTLT pasteurization parameters

63C for 30 min.

HTST pasteurization parameters

72C for 15 sec.

Microorganisms killed by pasteurization

All pathogens.

Bacteria killed by pasteurization

90%.

Microbial forms surviving pasteurization

Spore formers.

Enzyme inactivation by pasteurization

No, some enzymes are not inactivated.

Heat treatment effects on bacteria

Heat shocked, sublethally injured, or dead (Loss of permeability, membrane, wall, DNA, RNA, enzyme damage).

Food-related factors affecting heat treatment

Composition, Higher aw or lower pH or antimicrobial agents.

Process-related factors affecting heat treatment

Large vs. small volume, Container composition.

Microbe-related factors affecting heat treatment

Previous heat exposure (Membrane structure, increase saturated fatty acids to reduce fluidity), Heat shock proteins (Large proteins created to protect smaller proteins and break down degraded proteins), Spore formation (Bacillus and Clostridium).

D-value (DT) in heat treatment

Time taken to reduce microbe count by 90%, 10-fold, or 1 log.

Z-value in heat treatment

Temperature change resulting in a 10-fold change in D.

F-value in heat treatment

Time taken to kill a specific number of cells at a specific temperature.

Aim of low-temperature preservation

Not to kill, but to prevent or reduce growth.

Effect of low temperature on microbial generation time

Increases it.

Microbes selected by low temperature preservation

Psychrotrophs.

Factors influencing low-temperature preservation

Rapid chilling, Small variations in storage temp.

Low temperature and microbial viability

May not cause a loss of microbial viability.

Cold shock in microorganisms

Back to fluid membrane, unsat fat, Cold shock proteins, improve protein synthesis.

Food property influencing freezing effectiveness

Cryopreservative function (Salmonella can live 7 years at -23C).

Freezing conditions effect on microbial death and food quality

Slower freezing is better for killing but reduces food quality.

Water activity level preventing microbial growth

Below aw of 0.6.

Effect of aw change from 0.955 to 0.950 on bacteria

50% reduction in water content.

Bacteria response to low aw environments

Uptake of compatible solutes, draws water back into bacterium.

Chemical preservatives

Both microbicidal and microbistatic.

Effectiveness factor of chemical preservatives

Crucial factor is not specified.

Low initial microbial numbers

Refers to the starting count of microorganisms present in food.

Cellular components targeted by chemical preservatives

Cell wall, membrane, enzyme, DNA, energy production

Regulation of chemical preservatives

Regulated by FDA

Origin of chemical preservatives in food

Added to food, already present, or formed during processing

Examples of unregulated chemical preservatives used as additives

NaCl, sugar, salt, oils, etc.

Examples of unregulated chemical preservatives formed during processing

Acetic and lactic acid, bacteriocins

Examples of regulated inorganic chemical preservatives

Nitrites, sulfur dioxide/sulfites, H2O2

Function of nitrites in food preservation

Inhibits bacteria and spore germination, used in cured meats, better at low pH, contributes to flavour and red colour

Nitrites and red colour of cured meats

Reacts with myoglobin when heated to form pink colour

Function of sulfur dioxide/sulfites in food preservation

Active against bacteria and fungi, used in dried fruits, fruit juices, wines, sausages, pickles

Compound formed when sulfur dioxide dissolves in water

Sulfurous acid (H2SO3)

Form of sulfurous acid that penetrates cells

Uncharged species

Effect of pH on sulfurous acid

Low pH produces more H2SO3 = higher activity

Examples of regulated organic chemical preservatives

Acids (acetic, lactic, benzoic (parabens), sorbic, citric, propionic), Bacteriocins - nisin

Most common group of organic chemical preservatives

Acids

Active form of organic acids

Undissociated acid