Molecular processes initiated during fermentation of dairy milk by lactic acid bacteria:
- Proteolysis of caseins into peptides and amino acids
- Utilisation of carbohydrates for form lactic acid and exopolysaccharides
Fermented milk products
https://dairyprocessinghandbook.tetrapak.com/chapter/fermented-milk-products
- Milk is inoculated with starter culture that converts (part of) the lactose into lactic acid. Depending on type of lactic acid bacteria (LAB), in addition CO2, acetic acid, diacetyl, acetaldehyate, ethanol and other substances can also be formed ⇒ gives the final product its distinct taste.
- For production of fermented milk products, the best possible growth conditions must be created for the starter culture:
- Heat treatment to destroy competing microorganisms
- Hold milk at optimum temperature for the relevant starter cutlure.
- Cool product quickly once best flavour has been achieved.
- In today’s industrial production, milk solid (milk solid no fat - MSNF) content is usually adjusted.
Yogurt
- Depending on region, can be highly viscous liquid, soft gel, or drink.
- Set-type: Incubated and cooled in package.
- Stirred type: Incubated in tanks, cooled, packaged.
- Drinking type: Similar to stirred type, but coagulum is broken down to liquid before packing.
- Concentrated/strained: Incubated in tanks, concentrated (i.e. whey removed), packaged.
- Volatile aromatic substances include acetic acid and acetaldehyde.
- May have a fat content of 0-10%, most typical is 0.5-3.5%.
Milk standardization
Dry matter (DM) content is standardized by:
- Evaporation
- Addition of skim milk or protein powder
- Addition of milk concentration
- Ultrafiltration or reverse osmosis
De-aeration
Air content should be as low as possible (LAB are facultative anaerobic). In addition de-aeration improves stability, removes off flavours and reduces risk of fouling during heat treatment.
Homogenization
Generally done at 20-25 MPA and 65-70C. Improves viscosity of final product and prevents creaming.
Heat treatment
Before inoculation, usually 90-95C for 5 min. Denatures about 70-80% of the whey proteins (mainly beta-lactoglobulin), which then interacts with the kappa-casein to form a stable body.
Choice of culture
Different mouth thickness, gel firmness, acidity, fermentation time, post acidification.
In industrial dairy yogurt production, fermentation time is 4-5 hours at 42-43C, and pH will reach 4.2-4.5 in the end. Then cooling to below 20C to stop fermentation.
Recipes
Kishk
https://www.196flavors.com/kishk/
Makes 400g:
- 1 cup coarse bulgur (khichin)
- 2½ cups thick yogurt (labné)
- 2 teaspoons salt
- In a bowl, mix the yogurt and the bulgur.
- Cover the bowl with a cloth and let the mixture sit in a cool place for 8 hours, by which time the bulgur should have absorbed most of the moisture from the yogurt.
- While adding the salt, knead the mixture then fold it and turn it several times with the hands.
- If the dough feels too dry and is able to absorb some more moisture, add more yogurt and knead again.
- Cover with a cloth and let ferment at room temperature for 10 days. (During these 10 days, mix the kishk daily).
- After 24 hours, fold and turn over the mixture several times.
- Continue to mix the mixture every day for 9 days to unify the surface and protect the kishk from mold growth.
- At the end of these 10 days, dry the kishk by spreading it out on a baking sheet, and place it in a sunny place, in a dehydrator, under a fan, or in a hot oven at 100 F (40°C). As it dries, crumble it into small pieces.
- Once the kishk is completely dry, use a mortar and pestle or food processor to powder it.
- Store at room temperature, in a dry place, in a tightly closed glass jar.