Knowledge clip 1 – Introduction to spoilage
– Spoilage: the process of decreasing quality. When a product is spoiled, the quality is
perceived as unacceptable.
– Shelf life: the time food remains of acceptable quality to the consumer.
– Best-before date: the consumption date for which the manufacturer guarantees an
acceptable quality.
• Rule of thumb: spoilage becomes noticeable at 107 cells per gram food/ ml liquid
food.
– Initial contamination (N0) through either primary contamination or secondary
contamination
– primary contamination:
o Animals: skin, intestines.
o Plants: soil, manure, water.
– Secondary contamination:
o Water: process, rinsing, cooling, cleaning.
o Equipment: machines, tools, surfaces.
o Air: aerosols, dust.
o People: hands, hairs, coughing, sneezing.
o Vermin: rodents, birds, insects.
• Microbial spoilage of food = initial
contamination (N0) + growth (𝜇)
Figure (right); food is spoiled before the stationary
phase is reached. Lag phase = 𝜆.Exponential phase
= k.
Knowledge clip 2- Sources of contamination
– Bacteria:
o Cocci (1 𝜇𝑚)
o Rods (length: 1-5 𝜇𝑚)
o Other shapes: Vibrio, Spirilli
– Fungi: yeasts and moulds
o Yeasts are 5-10 times as big as bacteria (10 𝜇𝑚) and
are unicellular. They reproduce through budding.
o Moulds are multicellular and often have spores.
– Virus:
o A virus is 40x smaller than a bacterial coccus (25-30 nm). It is no
micro-organism, since it needs a host to multiply. Viruses cannot grow in food
(no host), but they are a major cause fo foodborne diseases.
, – Parasites:
o >5 𝜇𝑚
o A tick is a type of parasite that cannot be transferred by food.
• Microbes are everywhere, but vermin and air are the most important contamination
sources on each location.
• A suitable environment (ecological niche) has:
o Resources needed for the growth (nutrients).
o Physico-chemical conditions that do not hinder the organism (e.g. good pH).
• The environment selects organisms which can survive well in a certain habitat.
Knowledge clip 3 - Growth kinetics and factors influencing growth
• In the exponential phase, the bacterium is adapted to its environment and has access
to sufficient nutrients. It will then divide at approximately constant time intervals.
– Generation time (GT): the time needed for the population to double.
o At t = 0, generation = 0.
N(t) = N(0) * 2n = N(0) * 2t/GT
• n = number of generations.
𝐥𝐨𝐠 (𝟐)
Log(Nt) = log(N0) + *t
𝑮𝑻
• Exponential growth is log-linear
growth.
Log(Nt) = log(N0) + k * t
• Growth rate (log scale).
Ln(Nt) = ln(N0) + 𝝁 * t
• Specific growth rate (ln scale).
– Growth kinetics: the study of the
increase of cell number in time
(growth rate).
𝐥𝐧(𝑵𝒕 )−𝐥𝐧 (𝑵𝟎)
Shelf life t = 𝝁
• In order to improve the shelf life you should increase the nominator and decrease
the denominator.
The growth rate is influenced by:
– Intrinsic factors: physico-chemical properties of food; nutrients, pH, aw, preservatives.
– Extrinsic factors: properties of the food environment; temperature, RH, gas composition.
– Implicit factors: properties and interactions of microbes; 𝜇𝑚𝑎𝑥 , interactions, succession in
time.
, – Processing factors: changes in food/environment/microbes;
• To preserve: pasteurization, irradiation.
• To process to the desired product: slicing, packing.
Knowledge clip 4 - Nutrients and structure
– Catabolism: the metabolic routes that are involved in the degradation of a carbon- and
energy source to generate precursors for cell components and energy for cell
maintenance.
– Anabolism: the metabolic routes involved in the biosynthesis of polymeric cell
compounds (DNA, RNA, proteins, lipids, cell wall constituents).
• Microbes are mostly chemo-heterotrophs; they use preformed molecules from other
organisms as energy and carbon source. Enzymes help to transform the substrate
(food) into product (metabolites).
• Antimicrobial barriers aim to hinder the growth of microorganisms. They result in
lack of access to water, lack of access to nutrients. It provides no protection against
the environment (UV, desiccation).
o Physical barrier → shell of nuts.
o Macromolecules, resistant to degradation → peel of fruit or fatty lining of
meat.
Enzymes that degrade antimicrobial barriers:
– Pectolytic: acts on pectin (fruit).
– Amylolytic: acts on starch (rice).
– Lipolytic: acts on lipids (milk).
– Proteolytic: acts on protein (milk).
1. Increase of nutrients:
Fermented sausage: LAB should produce lactic acid quickly in order to protect against
pathogenic bacteria.
(-) LAB have a high requirement for manganese.
Solution: add extra nutrients to the sausage, e.g. spices (containing manganese).
2. Introducing new barriers:
Compartmentalization: separate cells from nutrients by making very small water droplets
(<10 𝜇m).
Knowledge clip 5 – pH preservation by acid
Microorganism pH range Optimal pH
• The pH describes the acidity of a substance.
Bacteria 4-9 6-8
pH = -log [H+] = log(1/[H+])
Yeasts 1.5 - 8 4.5 - 6
• The growth rate declines when the pH does
Fungi 0 - 11 3.5 - 4
not equal pHopt.
– Acidophiles: 0 < pHopt < 5.5 (filamentous fungi yeasts, LAB)
– Neutrophiles: 5.5 < pHopt < 8.0 (bacteria)
, – Alkalophiles: 8.0 < pHopt < 11.5 (Vibrio parahaemolyticus)
[𝑨− ]
pH = pKa + log ([𝑯𝑨]) → Henderson-Hasselbach
• [HA]: concentration undissociated acid. When the pKa is high, there’s more [HA].
Undissociated acids can pass the cell membranes easily.
• [A-]: concentration dissociated acid.
o If pH = pKa, then [A-] = [HA].
o If pH = pKa + 1, then [A-] = 10 * [Ha].
• Not only the pH is relevant, but also the type of acid (sorbic, lactic, acetic, citric, etc.).
– Fermentation: the use of LAB to produce weak acids that lower pH and inhibit growth of
pathogenic bacteria.
– Acidic preservative: addition of acid in order to lower the pH.
Knowledge clip 6 – Water activity and relative humidity (RH)
𝑷
– Water activity: the measure of ‘’free’’ water. aw = 𝑷 = 1/100 * ERH.
𝟎
o ERH: equilibrium relative humidity (%) at temperature (T).
o P: partial vapour pressure of the food at temperature (T).
o P0: vapour pressure of pure water at temperature (T).
– Xerophilic: grow on dry foods (aw,min = 0.60).
– Osmophilic: grow in high concentrations of unionized compounds, like sugar (aw,min =
0.62).
– Halotolerant: grow in salted products (they tolerate elevated levels of sodium chloride).
– Halophilic: requires high levels of salt (sodium chloride), usually above about 0.2M, go
grow.
• The water activity (aw) can be reduced by adding solutes (salt → NaCl), freezing and
drying (removal of water).
o Combination: drying and salting for cured sausages.
• No growth does not equal inactivation.
1. Preservation pitfall: local changes in aw:
This is often the case for bulk commodities (ships, silo’s) and packaged products.
o Sun shines → the temperature rises, water evaporates.
o Sun down (or moves) → temperature drops, water condensates on top of the
products.
In the area with a local higher aw, microbial growth is possible. This effect is accelerated by
micro-organisms, e.g. fungi producing extra moisture allowing other organisms to grow too.
2. Preservation pitfall: product reformulation:
Example (hazelnut conserve):
o Old process: 10 minutes heating 90oC + glucose syrup.
o New process: 10 minutes heating at 90oC + aspartame (healthier).
– Spoilage: the process of decreasing quality. When a product is spoiled, the quality is
perceived as unacceptable.
– Shelf life: the time food remains of acceptable quality to the consumer.
– Best-before date: the consumption date for which the manufacturer guarantees an
acceptable quality.
• Rule of thumb: spoilage becomes noticeable at 107 cells per gram food/ ml liquid
food.
– Initial contamination (N0) through either primary contamination or secondary
contamination
– primary contamination:
o Animals: skin, intestines.
o Plants: soil, manure, water.
– Secondary contamination:
o Water: process, rinsing, cooling, cleaning.
o Equipment: machines, tools, surfaces.
o Air: aerosols, dust.
o People: hands, hairs, coughing, sneezing.
o Vermin: rodents, birds, insects.
• Microbial spoilage of food = initial
contamination (N0) + growth (𝜇)
Figure (right); food is spoiled before the stationary
phase is reached. Lag phase = 𝜆.Exponential phase
= k.
Knowledge clip 2- Sources of contamination
– Bacteria:
o Cocci (1 𝜇𝑚)
o Rods (length: 1-5 𝜇𝑚)
o Other shapes: Vibrio, Spirilli
– Fungi: yeasts and moulds
o Yeasts are 5-10 times as big as bacteria (10 𝜇𝑚) and
are unicellular. They reproduce through budding.
o Moulds are multicellular and often have spores.
– Virus:
o A virus is 40x smaller than a bacterial coccus (25-30 nm). It is no
micro-organism, since it needs a host to multiply. Viruses cannot grow in food
(no host), but they are a major cause fo foodborne diseases.
, – Parasites:
o >5 𝜇𝑚
o A tick is a type of parasite that cannot be transferred by food.
• Microbes are everywhere, but vermin and air are the most important contamination
sources on each location.
• A suitable environment (ecological niche) has:
o Resources needed for the growth (nutrients).
o Physico-chemical conditions that do not hinder the organism (e.g. good pH).
• The environment selects organisms which can survive well in a certain habitat.
Knowledge clip 3 - Growth kinetics and factors influencing growth
• In the exponential phase, the bacterium is adapted to its environment and has access
to sufficient nutrients. It will then divide at approximately constant time intervals.
– Generation time (GT): the time needed for the population to double.
o At t = 0, generation = 0.
N(t) = N(0) * 2n = N(0) * 2t/GT
• n = number of generations.
𝐥𝐨𝐠 (𝟐)
Log(Nt) = log(N0) + *t
𝑮𝑻
• Exponential growth is log-linear
growth.
Log(Nt) = log(N0) + k * t
• Growth rate (log scale).
Ln(Nt) = ln(N0) + 𝝁 * t
• Specific growth rate (ln scale).
– Growth kinetics: the study of the
increase of cell number in time
(growth rate).
𝐥𝐧(𝑵𝒕 )−𝐥𝐧 (𝑵𝟎)
Shelf life t = 𝝁
• In order to improve the shelf life you should increase the nominator and decrease
the denominator.
The growth rate is influenced by:
– Intrinsic factors: physico-chemical properties of food; nutrients, pH, aw, preservatives.
– Extrinsic factors: properties of the food environment; temperature, RH, gas composition.
– Implicit factors: properties and interactions of microbes; 𝜇𝑚𝑎𝑥 , interactions, succession in
time.
, – Processing factors: changes in food/environment/microbes;
• To preserve: pasteurization, irradiation.
• To process to the desired product: slicing, packing.
Knowledge clip 4 - Nutrients and structure
– Catabolism: the metabolic routes that are involved in the degradation of a carbon- and
energy source to generate precursors for cell components and energy for cell
maintenance.
– Anabolism: the metabolic routes involved in the biosynthesis of polymeric cell
compounds (DNA, RNA, proteins, lipids, cell wall constituents).
• Microbes are mostly chemo-heterotrophs; they use preformed molecules from other
organisms as energy and carbon source. Enzymes help to transform the substrate
(food) into product (metabolites).
• Antimicrobial barriers aim to hinder the growth of microorganisms. They result in
lack of access to water, lack of access to nutrients. It provides no protection against
the environment (UV, desiccation).
o Physical barrier → shell of nuts.
o Macromolecules, resistant to degradation → peel of fruit or fatty lining of
meat.
Enzymes that degrade antimicrobial barriers:
– Pectolytic: acts on pectin (fruit).
– Amylolytic: acts on starch (rice).
– Lipolytic: acts on lipids (milk).
– Proteolytic: acts on protein (milk).
1. Increase of nutrients:
Fermented sausage: LAB should produce lactic acid quickly in order to protect against
pathogenic bacteria.
(-) LAB have a high requirement for manganese.
Solution: add extra nutrients to the sausage, e.g. spices (containing manganese).
2. Introducing new barriers:
Compartmentalization: separate cells from nutrients by making very small water droplets
(<10 𝜇m).
Knowledge clip 5 – pH preservation by acid
Microorganism pH range Optimal pH
• The pH describes the acidity of a substance.
Bacteria 4-9 6-8
pH = -log [H+] = log(1/[H+])
Yeasts 1.5 - 8 4.5 - 6
• The growth rate declines when the pH does
Fungi 0 - 11 3.5 - 4
not equal pHopt.
– Acidophiles: 0 < pHopt < 5.5 (filamentous fungi yeasts, LAB)
– Neutrophiles: 5.5 < pHopt < 8.0 (bacteria)
, – Alkalophiles: 8.0 < pHopt < 11.5 (Vibrio parahaemolyticus)
[𝑨− ]
pH = pKa + log ([𝑯𝑨]) → Henderson-Hasselbach
• [HA]: concentration undissociated acid. When the pKa is high, there’s more [HA].
Undissociated acids can pass the cell membranes easily.
• [A-]: concentration dissociated acid.
o If pH = pKa, then [A-] = [HA].
o If pH = pKa + 1, then [A-] = 10 * [Ha].
• Not only the pH is relevant, but also the type of acid (sorbic, lactic, acetic, citric, etc.).
– Fermentation: the use of LAB to produce weak acids that lower pH and inhibit growth of
pathogenic bacteria.
– Acidic preservative: addition of acid in order to lower the pH.
Knowledge clip 6 – Water activity and relative humidity (RH)
𝑷
– Water activity: the measure of ‘’free’’ water. aw = 𝑷 = 1/100 * ERH.
𝟎
o ERH: equilibrium relative humidity (%) at temperature (T).
o P: partial vapour pressure of the food at temperature (T).
o P0: vapour pressure of pure water at temperature (T).
– Xerophilic: grow on dry foods (aw,min = 0.60).
– Osmophilic: grow in high concentrations of unionized compounds, like sugar (aw,min =
0.62).
– Halotolerant: grow in salted products (they tolerate elevated levels of sodium chloride).
– Halophilic: requires high levels of salt (sodium chloride), usually above about 0.2M, go
grow.
• The water activity (aw) can be reduced by adding solutes (salt → NaCl), freezing and
drying (removal of water).
o Combination: drying and salting for cured sausages.
• No growth does not equal inactivation.
1. Preservation pitfall: local changes in aw:
This is often the case for bulk commodities (ships, silo’s) and packaged products.
o Sun shines → the temperature rises, water evaporates.
o Sun down (or moves) → temperature drops, water condensates on top of the
products.
In the area with a local higher aw, microbial growth is possible. This effect is accelerated by
micro-organisms, e.g. fungi producing extra moisture allowing other organisms to grow too.
2. Preservation pitfall: product reformulation:
Example (hazelnut conserve):
o Old process: 10 minutes heating 90oC + glucose syrup.
o New process: 10 minutes heating at 90oC + aspartame (healthier).
