CS代写 Food Microbiology and Safety

Food Microbiology and Safety
AProf Helen Billman-Jacobe

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Food Biotechnology
Intended Learning Outcomes
• Explain microbial fermentation and its use in food production

Food biotechnology
Food biotechnology can be broadly divided into traditional and modern technologies.
Fermentation of foods started as a method to preserve raw ingredients when they are abundant and to develop flavours and improve digestibility of food materials.
Fermentation is a very important part of traditional cultures and cuisines Some modern approaches to food biotechnology include development of
• new foods using fermentation
• microbial conversion of substrates to products
• biofortification of foods to change their nutritional value or digestibility
In this lecture we shall explore some examples of fermented food and microbial food products

Fermented foods and starter cultures
• Fermentation means that raw materials are converted by microbes to fermented food
• It requires the growth and metabolism of the microbes which may be bacteria or fungi (yeast and molds)
• nutrients in food are used to generate energy for microbial cell growth
• all kinds of raw materials are fermented: milk, meat, fish, vegetables
• many processes are of ancient origin, but we now know much more about the microbes, their activity and the biochemical processes

1. Be Fermented food: examples
Food Groups
cheeses, yoghurt, buttermilk, sour cream, dahi, kumiss, kefir
salami, chorizo, pickled meat, nahm
breads, dosa, miso
Fruits/vegetables
olives, sauerkraut, kimchi, aachar, pickled fruits, pickled vegetables
tofu, tempeh, soy sauce, mizo, natto
fish sauce, Hákarl, bagoong, tarama, paak, izushi
beer, wine, distilled spirits, coffee, cocoa
Starch crop
fermented potato, sweet potato, cassava, bananas
miscellaneous
vinegar, fermented eggs,

Fermented food: examples
Sake Tempeh Kombucha Shrimp paste Miso Coffee Chocolate Kimchi Bread Sourdough bread Yoghurt
Fish sauce Mirin
Soy sauce Sauerkraut Stout Olives Vegemite Miso Salami Kefir

Microorganisms involved
• There many different types of fermented foods and there are diverse bacteria, yeasts those foods and moulds are associated with such foods
• A fermented food may be made with a single species or mixture of different bacteria, or combination of bacteria/yeasts/molds
• if a mixture is involved , then should they should be able to work together (synergistic, not antagonistic) or in sequence
• some cases, the growth of different species is sequential, not simultaneous during fermentation (e.g. fish sauce, yoghurt)

Sources of the microorganisms involved in fermentation
Natural fermentation
• using flora of raw ingredients, often variable outcomes Example: Sauerkraut
• “back-slopping” using some of last batch to inoculate next batch Example Sourdough or yoghurt
Controlled fermentation
• pure cultures added. These are often commercially produced for industry
• Raw ingredients may be heated first to kill off natural flora
• high levels, e.g. > 106 microbes/ml added to speed up processing and overwhelm any residual flora
• specific growth conditions used to optimise growth
• large scale, consistent, less chance of failure

Kimchi: Lactic acid fermentation
Example of natural fermentation which relies on natural flora of plants and ingredients
Shredded radish Chilli
Green onion sauce or oyster sauce

Sourdough: Lactic acid fermentation
Two ways of adding cultures: back slopping vs starter culture inoculation

Yoghurt: example of a dairy fermentation
• Yoghurt fermentation (plain yoghurt)
• a semisolid mass, from coagulation of milk by starter-culture bacteria
• sharp, acidic taste, smooth texture
• flavour– acetaldehyde (aroma), lactate (tart taste), diacetyl and acetate
• 90% of taste is due to the acetaldehyde
• many variants of plain yoghurt

Batch process of yoghurt production:
Mix ingredients, standardize (can +/- water, milk solids, etc.)
Heat by pasteurization (mandatory in Australia)
a. kills off pathogens, and reduces microorganisms that would compete with starter culture
b. destroys enzymes and immunoglobulins (may interfere)
c. also improves gel formation/texture (denatures whey proteins)
d. releases nutrients favourable to starter culture bacteria
homogenise – break fat globules down to smaller, more consistent size > smoother/creamier product
Starter culture added (44-46 oC)
6 hrs incubation leads to ~pH 4.8 (lactic acid levels measured) Rapidly cooled to 29 oC to stop further fermentation Packaged and cooled to 4.4 oC and stored 24 hr ( to pH 4.3)
4. 5. 6. 7.

Starter cultures for yoghurt production:
Streptococcus thermophilus & Lab. delbrueckii ssp. bulgaricus Initial ratio of 1:1
excellent for yoghurt fermentation
other bacteria may be added, such as “probiotic” species e.g. Lab. acidophilus, Bifidobacterium spp.
-but don’t compete well or survive well during fermentation -so often added after fermentation

Yoghurt culture
Streptococcus produces formate which is used by Lactobacillus which in turn produces amino acids used by Streptococcus

Protocooperation: Growth of starter cultures in yoghurt production
Two strains show synergy or ‘proto-cooperation’
Streptococcus
• grows rapidly in oxygen, produces formic acid and CO2
• removes oxygen, and under anaerobic conditions formic acid & CO2 • stimulate growth of as good protease activity
• digests casein to amino acids (outside cells).
• Some of these (glycine, valine, leucine) are needed by Streptococcus, so
stimulates its growth
pH drops to 5.5, then Streptococcus grows poorly while Lactobacillus grows further until pH is at target (pH 4.8)

Inoculation of yoghurt
Starter culture.
In this case all of the ingredients for the yoghurt are in the sachet. This is for home use

Live bacteria in yoghurt
Gram stain, HBJ

Summary: Fermented food
• There are many types of fermented food
• Bacteria, yeasts and moulds used to ferment food
might be part of the natural flora of animals or plants
• Starter cultures are pure cultures of characterised organisms which are safe to use to make food
• Lactic acid bacteria play a major role in food fermentation
• Examples: Kimchi, Sourdough bread, Yoghurt

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