This summary was written in this academic year. It contains all lecture notes from this course, combined with the provided literature, quiz answers (incorporated in the text) and information from the preceding course Food Microbiology (FHM20306).
Topics covered in the summary:
- Notes from all ...
Introduction to food fermentation
In food microbiology we study the significance of microorganisms in food (chains).
• The good → fermentation
o Taste, shelf-life, health
• The bad → pathogens
o Bacteria, viruses, parasites, (myco)toxins
o Food safety
• The ugly → food quality
o Bacteria, yeasts, moulds
o Food quality
Food is fermented to improve nutritional aspects, prevent spoilage, increase microbial safety
(removing natural toxic compounds, preventing growth of disease-causing microbes), improve taste
and aroma, improve digestibility and improve textural properties.
= sustainable and natural processing technology.
Food fermentation is studied to:
1. Understand key process parameters in fermentations → better control (salt addition to
sauerkraut).
2. Modify existing food fermentations to make them more robust/reproducible (starter culture
vs spontaneous fermentations).
3. Improve existing fermentations by adding special properties: add Propionibacterium to
improve B12 content.
4. Design novel fermentations:
a. Fermentation of plant-based milk.
b. Valorisation of by-products.
Defining food fermentation
In the strict sense, fermentation refers to the anaerobic metabolism and generation of energy. In the
wider sense, fermentation is used to refer to biological processes resulting in desirable modifications
of ingredients.
• Historical: Louis Pasteur: “La fermentation c’est la vie sans l’air.” → based on studies of
alcoholic fermentation using yeast.
• Biochemical, bioenergetic: a biological process that derives metabolic energy from the
oxidation of organic compounds (mostly carbohydrates) and uses and endogenous electron
acceptor (usually also an organic compound).
Fermentation = a process driven by micro-organisms and their enzymes yielding a product with
desirable properties.
Fermentation should not be considered a ‘controlled spoilage’, because:
• in a well conducted fermentation process, desirable microorganisms (‘the good’) are
abundant/dominant.
• a spoiled/rotten food product is dominated by undesirable microorganisms (‘the bad’ and
the ugly’).
Process and principles
The desirable effect of microbial activity may be caused by its biochemical activity → microbial
enzymes break down food components, which improve digestibility and increase nutritional value
(e.g. B-vitamin secretion). As a result of their growth and metabolism, several microbial substances
are found in fermented food, affecting quality of the product, e.g.:
• the production of lactic and acetic acid by LAB has an inhibitory effect on spoilage.
• the production of ethanol and CO2 determine the acceptability of bread and beer.
In addition to enzymes and metabolites, the microbial growth causes increased amounts of microbial
cell mass, which may be of nutritional and aromatic interest (e.g. probiotics in yoghurt).
Requirements to run the bioprocess:
• Ingredients → food components, micro-organisms, water.
• Operations → physical (mixing), biochemical (adding enzymes), thermal (heating).
The workhorses of fermentation are lactic acid bacteria, yeasts and moulds.
Learning outcomes
• Describe the general characteristics of lactic acid bacteria (LAB).
• Explain why LAB are dominant in many food fermentations.
• Describe the metabolic properties and requirements of LAB.
• Explain the lifestyle of LAB.
General characteristics
Lactic acid bacteria (LAB) play an important role in several food fermentation processes. They are
Gram positive, oxidase negative, catalase negative, rods or cocci, and do not form spores. LAB are
anaerobic, but aerotolerant. They are obligate fermenters and produce mainly lactate by the
fermentation of carbohydrates. LAB consist of a heterogeneous group (morphology, pH,
temperature, salt tolerance). Mainly involved in fermentation are Streptococcus, Lactobacillus1,
Leuconostoc and Lactococcus.
Sources
LAB is generally found in nutrient-rich habitats →
carbohydrates as main energy source. Natural habitats include
milk, intact and decaying plant material, and intestinal tracts
and mucous membranes of humans and animals. They are an
economically important group of microbes:
• Application at industrial scale.
• Food fermentation and preservation → diverse range
of food products.
Dominance in foods
LAB become dominant in fermented foods because:
• LAB are ubiquitous in nature (and raw food materials).
• Food products are rich in nutrients (i.e. carbohydrates):
o quick acidification → inhibition other microbes
• Low water activity favours growth LAB:
o salt addition
o high sugar content
• LAB added as starter culture.
1
Reclassified based on whole genome sequence, physiology and ecology.
4
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