Food Microbiology
Course summary October 2022
Course: Food microbiology
Course code: FHM 20306 Rick Mannes
, Spoilage
Definition and application
Spoilage is defined as the process of decreasing quality in food products. A spoiled product thereby has a
perceived unacceptable quality. The shelf life of industrially processed foods is the time in which the food
product remains of acceptable quality to the consumer. The best before date is often used to indicate the
consumption date for which the manufacturer guarantees this acceptable quality.
There are different causes of spoilage:
Microbial (Acidification, odour formation)
Mechanical (Bruised, broken product)
Insect damage
Chemical and enzymatic spoilage (Browning, fat oxidation)
Physical spoilage (drying, fat seggregation)
Physiology of the product itself (overripe)
Microbial spoilage mostly occurs because the metabolic products that are produced by microorganisms
during growth. Sometimes, the microorganisms themselves can be the reason for spoilage. As a Rule of
thumb: spoilage becomes noticeable when at 10^7 cells per gram food/ml liquid food. The shelf life of a
food product depends on the initial contamination of a food product (N0) + the growth in time (μ).
Initial contamination (N0) is mostly originating primarily from animals: skin, intestines, or plants: from
soil, water of manure. Secondary contamination sources are:
Water: Process, sinsing, cooling, cleaning
Equipment: machines, tools, surfaces
Air: aerosols, dust
People: hands, hairs, coughing, sneezing
Vermin: rodents, birds, insects
Sources of contamination
Microorganisms (organisms on a micrometer scale (μm)(1 μm = 0.001 mm) are responsible for microbial
damage and can be categorized into four segments:
Bacteria
Fungi
Viruses
Parasites
Bacteria
The largest group is the group of bacteria. These
microorgamisms are differ in size and shape. Cocci
shaped bacteria mostly are around 1 μm. Rod shaped
bacteria are usually between 0.6 and 0.8 μm in width
and 1.5 μm in length. Bacteria can also have
endospores, flagella(tails) and have the a spiral shape
(vibrio, spirilli).
Figure 1, different shapes of bacterial species.
Food microbiology
,Fungi (yeasts)
Yeasts are a usually 5-10 times bigger than bacteria (10 μm) and are unicellular.
Fungi (moulds)
Moulds are multicellular organisms and a lot bigger than other microorganisms.
Viruses
Viruses are up to 40 times smaller than bacteria around 25-30 nm = 0.025 - 0.030 μm and are too small to
see with a normal light microscope. Viruses are strictly no micro-organisms and need a host to multiply
themselves. Viruses do not grow in foods. yet they can cause food-borne disease.
Microorganisms are everywhere. Vermin and air are important contamination sources on each location. But
the environment selects which organism is found where. A suitable environment (ecological niche) has
resources needed for growth (nutrients, oxygen or not) and physiochemical conditions that do not hinder the
growth (pH, aW). The typical flora of a certain environment is repeatedly found over time and does have
the properties that are best fitted for survival in that environment (competitive advantage).
Growth kinetics and factors influencing microbial growth
The microbial growth curve knows four stages known as the lag phase, in which bacteria need to adapt to
the environment, the exponential phase, in which the bacteria starts to multiply and grow, the stationary
phase, in which the growth equals the death rate, and the death phase, in which bacteria will die. The focus
is put ont he exponential part, since foods are spoiled before the stationary phase is reached. The main
relevance therebey is put on the lag and exponential phase.
When a bacterium is adapted to its environment,
and has sufficient nutrients, the exponential phase
starts. It then divides at approximately constant
intervals. The generation time is defined as the
time needed to a population to double. Exponential
growth is log-linear growth:
Figure 2, growth life cycle of microorganisms.
t = time in hours
n = number of generations
GT = generation time
N = number of cells (CFU)
N0 = initial contamination of the food
Growth rates are expressed on a logarithmic scale.
The natural logarith (ln) or regular logarithm (log)
can be used as interpretation to calculate growth
k = growth rate (log) k = specific growth rate
rates.
(ln)
Food microbiology
, Growth kinetics is defined as the study of increase of cell number in time (growth rate). A kinetic model
describes the growth kinetics mathematically. A worst case: exponential growth when lag time = 0. The
shelf life of a product can be calculated with the following expression:
The growth rate is influenced by four different major factors:
intrinsic factors (physico-chemical properties of food)
Nutrients, pH, Water activity, preservatives
Extrinsic factors (properties of food environment)
Temperature, relative humidity, gas composition
Implicit factors (properties and interactions of microorganisms)
Maximal specific growth rate, interactions, succession in time
Processing (changes food/environment/MO)
To preserve (pasteurization, irradiation)
To process to desired product (slicing, packing)
Microbial growth: Intrinsic factors
Mico-organisms are organisms, meaning that they have a metabolism (except). Catabolism are 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 are the metabolic routes that are involved
int he biosynthesis of polymeric compounds (DNA, RNA, Protein, lipids, cell wall constituents).
microorgamisms in foods are mostly heterotrophs: they use performed molecules from other organisms as
energy and carbon source. Enzymes are important in metabolism, they help to transform susbstrate (‘food’)
in products (‘metabolites’). These foods may contain:
Starch, glycogen, lactose, glucose
Protein, peptides, amino acids
Lipids, free fatty acids
Spore elements (iron, manganese)
Vitamins (vitamin B)
Water
Examples of natural antimicrobial barriers are physical barriers (nutshells), macromolecules (peel of fruit,
lining fat of meat). They do have a common aim: to hinder growth inside the product by creating a lack of
access to water or nutrients. Enzymes are present to help degrading these barriers. Examples are:
Table 1, Enzymes in foods are present to degrade natural barriers against microbial growth.
Type of enzyme Acts on Food Effect Organism
Pectolytic enzymes Pectine Fruit Degrades structure Pectobacterium
Releases nutrients serratia
Amylolytic enzymes Starch Rice Releases nutrients Aspergillus oryzae
Liptolytic enzymes Lipids Milk Releases fatty acids Pseudomonas
(leads to rancidity)
Proteolytic enzymes Protein Milk Coagulates protein BAcillus cereus
Releases nutrients
Food microbiology
