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Food Fermentations 
 
 
Principles of fermentations

 

Types of fermentations 

Solid state cultures:

microorganisms grow on a moist solid with little or no free water (sausage, salami, mould-ripened cheeses, beaver feet and tails)

Submerged fermentations

use dissolved substrates or solid substrates suspended in a large amount of water (e.g., wine, yoghurt)

 

Aerobic processes (requiring oxygen)

e.g., submerged culture citric acid production

Anaerobic processes

Solid state, e.g., Fermented meat products

Submerged, e.g., yoghurt

 

Single species fermentations

May use sterile substrate

Most food fermentations are non-sterile

Require participation of several microbial species, acting simultaneously or sequentially

Modifications of culture environment (salt content, pH, water environment) allow selection of desired microorganisms

Controlled starter cultures (single strain or multi strain versus “wild fermentations”)

 

Sources of starter bacteria 

Present in raw material in sufficient quantity (traditional vegetable and sausage fermentations)

Present in equipment used to ferment or prepare the foods

Materials from previous fermentation is used to inoculate a new batch

Additives empirically known to contain starter bacteria are added

Cultures that contain a complex mixture of strains are added

Cultures that contain one or more defined strains are added

 

Submerged fermentations 

Batch

may take days to months

Fed-batch

sterile culture medium is added continuously or periodically to fermentation batch

Continuous culture fermentation

uncommon in food fermentations, but used in some large scale beer brewing processes

 

 Microbial growth during fermentations

 

Lag phase:

amount of culture used

growth history of inoculum

media composition

short lag phase is desirable

Exponential growth

negligible death rate

Growth rate depends on concentration of growth limiting substrate, although high concentrations may also limit growth rate (substrate or product inhibition, effect on water activity)

 

Aeration and oxygen demand

aeration may be required for certain fermentations

Heat generation

fermenters may need to be cooled

 

Solid state fermentations 

Substrate characteristics

Water activity

Particle size

Substrate pH

Aeration and agitation

Heat transfer 

 

Controlling fermentations in various foods 

Acid: For examples, natural milk fermentation by Lactococcus lactis and subsequently by Lactobacillus decreases pH until the respective LABs are inhibited

Alcohol: Yeasts cannot tolerate their own alcohol and fermentation products beyond certain levels (12-15% alcohol)

Temperature: different microorganisms dominate a mixed fermentation depending on the fermentation temperature (e.g., in sauerkraut fermentation temperatures of about 21 C are used to favor growth of Leuconostoc mesenteroides in the initial stages

Oxygen: some organisms may have different requirements with respect to oxygen for growth and for fermentation activity (e.g., Bakers yeast)

Salt: LABs used for fermenting olives, pickles, sauerkraut, and certain meat sausages tolerate 10-18% salt; many spoilage organisms do not tolerate >2.5% salt

Starter Cultures: Pure cultures used to ensure controlled fermentation; food may be heated prior to addition to inactivate detrimental types of contaminating organisms

Non-starter LAB sometimes play important role in fermentation 

 

Substrates and media for fermentations

Medium requires presence of bioavailable nutrients and absence of inhibitory or toxic constituents

Carbon, nitrogen, inorganic ions, growth factors

Carbon sources

Almost all food microrganisms are chemoorganotrophs, i.e., they obtain their energy and carbon by metabolizing organic substrates

Carbon biopolymeres, mono-, di, tri-, and oligosacharides, alcohols, organic acids, peptides and polypeptides etc.

 

Nitrogen sources

Required for biosynthesis of structural proteins, functional enzymes, and nucleic acids

Food microorganisms require supply of organic or inorganic nitrogen sources (e.g., gaseous ammonia, ammonium salts, nitrate, urea)

Inorganic ions

Macronutrients: nitrogen, phosphorus, sulphur, potassium, and magnesium

Micronutrients (trace elements): sodium, calcium, chlorine, iron, cobalt, zinc, molybdenum

 

Growth factors

organic compounds, which are required in low concentration

perform specific catalytic and structural roles in microbial physiology

Examples includes vitamines, purines, amino acids, fatty acids etc.

Auxotrophs: organisms unable to synthesize one or more essential growth factors

 

Principle constituents of cheese whey 

Carbon source: lactose

Nitrogen source: globulin, albumin, amino and urea nitrogen compounds

Minerals: phosphorous, potassium, magnesium, sulphur

Vitamins: Range present

Trace elements: Range present

Other components: lipids, NaCl, Lactic and citric acid

 

Design and preparation of Food Fermentation Media 

Media design:

Media supply: consistency; cost effectivness etc.

Media type: liquid or solid; complex, synthetic/defined, or semisynthetic

Media properties: e.g., buffering capacity, heat-labile components

Required media treatment: pasteurization, sterilization, pre-treatment etc.

Complex, inexpensive, readily available agricultural derived media are often very variable

 

Control of fermentation conditions 

Control of fermentations aids in goals of producing the desired food/bioproduct in an efficient, fast, reliable, and simple manner

Environmental conditions which can be monitored include nutrient concentration, temperature, pH, dissolved oxygen tension etc.

 

Air flow

In situ sterilization/CIP

Sensors: pH, temperature, pressure, foam

Sampling valve

Sight glasses

Valve for introduction of compounds/liquids (e.g., anti-foam)

Harvest valve

Mechanical agitator

 

Downstream processing/product recovery 

Separation of desired products from fermentation broth and purification can be expensive and technically challenging

Possible recovery techniques/steps

Filtration

Centrifugation

Solvent extraction

Adsorption and elution

Precipitation

Ion exchange and gel filtration

Membrane separation (microfiltration, ultrafiltration)

Extraction of intracellular products

 

Microorganisms of importance 

Lactic acid bacteria

Gram-positive, non-sporeforming (Sporolactobacilli?), nonrespiring, catalase-negative cocci or rods that form lactic acid as a sole or main end product from carbohydrate metabolism

Term was initially used synonymously with “milk-souring organisms”

Important for many food fermentations (cheese, yoghurt, sauerkraut, fermented sausages, wine, sour dough)

Other bacteria

Coliforms, Clostridium spp. etc. (often producing product spoilage)

Propionibacterium: production of eyes in Swiss cheese

Brevibacterium linens: Pigmented cheese surface smears, flavor

 

Yeasts:

Alcoholic fermentations, e.g., beer, wine, Kefir and Koumiss

Surface fermentations of sausages and cheeses (e.g., penicillium species)

Molds:

e.g.,Rhizopus, Mucor, Aspergillus

used in many oriental and indigenous fermentations

Surface fermentations of sausages and cheeses (e.g., penicillium species) 

 

The End