Fermented Milk Products

The primary function of fermenting milk was, originally, to extend its shelf life. With this came numerous advantages, such as an improved taste and enhanced digestibility of the milk, as well as the manufacture of a wide variety of products. Historically the fermentation of milk can be traced back to around 10 000 B.C. It is likely that fermentation initially arose spontaneously from indigenous microflora found in milk. Fortunately, the bacteria were lactococci and lactobacilli which typically suppress spoilage and pathogenic organisms effectively. The evolution of these products likely came as a result of the climate of the region in which they were produced: thermophilic lactic acid fermentation favours the heat of the sub-tropics; mesophilic lactic acid fermentation occurs at cooler temperatures. Today the fermentations are controlled with specific starter cultures and conditions. Some of the many fermented milk products are: acidophilus milk, crème fraîche, cultured buttermilk, kefir, koumiss, filmjölk, sour cream, and viili. Yogurt and cheese are also fermented milk products. More detail on yogurt and cheese can be found under their specific ingredient profiles.

Fermented milk products can be classified into 3 categories:

• viscous products
• beverage products
• carbonated products

Within these categories, the fermented milk products may be fresh, or have an extended shelf life. The fresh products contain live starter culture bacteria, including probiotics, while the extended shelf life products contain no live microorganisms.

*The shelf life may vary with manufacturers.

There are numerous factors which affect the outcome of the product including the chemical composition of the milk, additives and starter cultures used, as well as the processing of the product. They affect the ultimate flavour, texture, and consistency of the final product. It is not uncommon for the manufacturer to add stabilizers such as pectins and gums, in order to avoid the sedimentation of milks solids and the separation of whey in the package, while improving the mouthfeel of the product.

The general process by which fermented milk products are made begins with a preliminary treatment of milk which may include clarification, fat separation and standardization, and evaporation. Processing follows next, with de-aeration, homogenization, and pasteurization. The milk is then cooled to the appropriate fermentation temperature and starter cultures are added.

Starter cultures differ for each product. They consist of microorganisms added to the milk to provide specific characteristics in the finished fermented milk product in a controlled and predictable manner. The primary function of lactic acid starters is to ferment lactose into lactic acid, but they may also contribute to flavour, aroma and alcohol production, while inhibiting spoilage microorganisms. A single strain of bacteria may be added, or a mixture of several microorganisms may be introduced. The bacteria, yeasts and moulds work at different temperatures as well. Thermophilic lactic acid fermentation favour hot temperatures (40-45°C) while mesophilic lactic acid fermentation occurs at cooler temperatures (25 and 40°C).

As the starter cultures grow within the milk, fermentation takes place. Fermentation is the chemical conversion of carbohydrates into alcohols or acids. In fermented milk products both alcohol and lactic acid may be produced, like in kefir and koumiss, or just lactic acid, like in sour cream. The bacteria ingest the lactose (milk sugar), and release lactic acid as waste causing the acidity to increase. This rise in acidity causes the milk proteins to denature (unfold) and tangle themselves into masses (curds) while also inhibiting the growth of other organisms that are not acid tolerant. Following the completion of fermentation, flavourings can be added and the products are packaged, labeled and put into cold storage before being sent to stores.

• Kefir is a milk product traditionally fermented by "kefir grains". The grains are curds which act as a starter culture in each batch of kefir. These grains contain active microorganisms and when added to fresh milk, they produce kefir. Kefir grains have a complex microbial composition consisting of 83-90% lactic acid bacteria and 10-17% yeast, as well as acetic acid bacteria and possibly mould. Commercial starter cultures have been developed that allow production to be made more efficient and may also provide a longer shelf life for the product. The fermentation of the lactose by the microorganisms yields a sour, carbonated, slightly alcoholic beverage with the consistency of thin yogurt. It is white or yellowish in colour with a yeasty aroma. The taste is acidic, but refreshing with compounds including lactic acid, diacetyl, carbon dioxide and ethanol influencing its sensory properties. Kefir is sometimes commercially available without carbonation and alcohol (when yeast is not added to the starter culture), resulting in a product that is very similar to yogurt. Its composition and flavour is dependent on milk type and lactic acid content in the final product. Typical milks used for kefir include cow, goat, and sheep, with each eliciting varying nutritional and sensory qualities.
• Koumiss (koumiss, kumiss, kumis, kymis, kymmyz) is a fermented drink traditionally made from the milk of horses by people in Central Asia and from camel's milk in Mongolia . The word koumiss is thought to derive from the name of the Turkic Kumyks people. The capital of Kyrgyzstan, Bishkek, is named after the paddle used to churn the fermenting milk, showing the importance of the drink in the national culture. It would have been originally fermented in a horse hide bag which would have contained the microflora from the previous batch. Koumiss is similar to kefir, but is not produced using "grains", but using a liquid starter culture composed of lactobacilli and non-lactose-fermenting yeasts instead. As mare's milk has a higher sugar content than cow's and goat's milk, the resulting koumiss has a slightly higher alcohol content than kefir. Today, cow's milk is generally used for koumiss, with the addition of sugar to better approximate the composition of mare's milk. Koumiss is a milky white liquid with a grayish cast and is very light in body compared to most dairy beverages. It has a slightly sour flavour from lactic acid, and ethyl alcohol, and a fizziness from carbon dioxide.
• Filmjölk is Swedish fermented milk. Fermented milk products have a long history in the Scandinavian countries, dating back to the Vikings. Traditional fermented milks of the Nordic countries are fermented at lower temperatures by mesophilic bacteria (specifically ). Filmjölk is the modern variant of the traditional surmjölk. It is the most common fermented milk in Sweden and is frequently consumed for breakfast or lunch. Filmjölk is similar to yogurt or kefir, but it is fermented using different strains of bacteria, giving it a unique flavour. Filmjölk is a spoonable, semi-solid product made with standardized fat contents. It has a mild, slightly acidic flavour, with aromas from diacetyl and carbon dioxide.
• Viili is a traditional Finnish fermented milk made from unhomogenized milk. As the milk begins to ferment a layer of cream rises to the surface and a surface growing mould, Geotrichum candidum, forms a thin velvety surface. Viili is inoculated with a starter culture containing Lacobacillus lactis subsp. cremoris that creates its ropy character. It is advised that viili be eaten with a tablespoon so that it can be cut into portions. If it is mixed or eaten with a teaspoon the texture becomes ropy, making it difficult to consume. Viili has a mild acidic flavour and aroma with a thick consistency that maintains its shape without collapsing when placed on a plate.
• Acidophilus milk is typically a low fat or nonfat milk to which active cultures of Lactobacillus acidophilus have been added. The milk can be refrigerated to prevent further growth of the harmless bacteria producing sweet acidophilus milk. It can also be incubated at 38°C until a curd forms. Bifidobacterium bifidum may also be included.
• Cultured Buttermilk has been produced as long as butter has been made. Traditionally butter was made by churning milk or cream, but an improved method for fermenting milk became the preferred method for cultured buttermilk production. Cultured buttermilk is pasteurized skim milk fermented by a lactic acid bacteria culture (Lactobacillus lactis subsp. lactis, Lactobacillus lactis subsp. cremoris, and Lactobacillus lactis subsp lactis biovar. diacetylactis, and Leuconostoc mesenteroides subsp. cremoris) and by aroma bacteria. It possesses a mild acid flavour with a diacetyl overtone and a smooth texture. Cultured buttermilk has a soft white colour and may contain added butterflakes, fruit condiments, or flavourings.
• Sour Cream is an extremely viscous product that has been used for many years in a great number of countries. Traditionally, cream was left to sour, but today sour cream is made by lactic acid fermentation of cream using Streptococcus lactis, with or without the addition of rennet to create a thicker product. Stabilizers may be added to improve and maintain the consistency. Sour cream has a mild, subtle, tangy flavour and aroma which is similar to cultured buttermilk. It has a smooth, thick body and typically has a fat content of 10-14%. Lower fat varieties are also produced. Sour cream has a limited shelf life due to yeast and mould growth. The shelf life can be extended by a heat treatment after the fermentation has taken place.
• Crème fraîche is French for fresh cream. It is mild in taste and slightly acidic, with a smooth, rich, thick texture. It is made in the same manner as sour cream, and used for many of the same applications. It is higher in fat content (usually 30-40% fat) and as a result crème fraîche can be whisked into whipped cream. It also has a high enough fat content and low enough protein content that it can be cooked directly without curdling.

• Kefir is made most often from partially skimmed cow's milk. It can be packaged either as natural or plain kefir with no added fruit or flavours or as flavoured kefir. The final product contains live bacteria and yeasts that produce carbon dioxide gas. This gas production gives kefir a "sparkling" sensation on the tongue when eaten. Kefir has been referred to as the champagne of fermented dairy products.
• Koumiss: Mare's milk has higher sugar content than cow's and goat's milk, and as a result koumiss has a slightly higher alcohol content than kefir. Today, cow's milk is generally used for koumiss, with the addition of sugar to better approximate the composition of mare's milk.
• Cultured Buttermilk may contain added butter flakes, fruit condiments, or flavourings. It is also available with different fat contents.
• Viili comes in a wide range of varieties, including products of different fat content, lactose-reduced varieties and flavoured versions. Viili can be made from homogenized milk and without mould growing on the surface.
• Sour cream comes in full fat (minimum 14% fat), low fat and fat free varieties.
• Filmjölk has fruit flavoured variants and can have the addition of beneficial probiotic bacteria such as Bifidobacterium lactis and many species of lactobacilli.

Viili and filmjölk likely have compositions similar to kefir depending on the level of milk fat used to prepare them.

• Kefir and Koumiss can be used in smoothies, salad dressings, and sauces. They can be added to baked goods such as pancakes, waffles, and breads, or in soups and desserts as a replacement for other milk products such as yogurt or buttermilk. They are also delicious mixed with fresh fruit or cereal as a breakfast, lunch or snack. They all make refreshing beverages on their own or mixed with fruits, honey, maple syrup, iced coffees and teas as well as other sweeteners and flavours
• Filmjölk is eaten in the same way as yogurt, usually from a bowl using a spoon. It is sometimes drunk as a thick beverage. Many people add sugar, jam, applesauce, cinnamon or berries. Cereals, corn flakes or muesli are often added to filmjölk. In northern regions of Sweden, crushed crisp bread is sometimes put into it. It could be used in smoothies, salad dressings and sauces, as well as in baked goods.
• Cultured Buttermilk is a versatile ingredient in baking. It works very well in biscuits, breads, and desserts. Cultured buttermilk is often used in salad dressings and sauces, stirred into mashed potatoes and soups and it has even been used to make tangy buttermilk ice cream. It is also considered a refreshing beverage.
• Sour Cream has numerous applications. It is commonly used as a base for dips, salad dressings and sauces. It is eaten as a condiment on potatoes, chili, or with smoked salmon, as well as many other foods. Sour cream can be used in soups and works well in baked products like breads, cakes, pies and cookies. Sour cream has significantly less calories than mayonnaise and performs many similar functions. In Russian cuisine, sour cream is often added to borscht and other soups. In Tex-Mex cuisine, it is often added to tacos, nachos, burritos, taquitos or guacamole. Hungarian cooks use it as an ingredient in sauces and in recipes such as ham-filled crepes.
• Crème Fraîche has similar uses to sour cream, however it's sweeter flavour makes it particularly well suited to desserts, as a topping, or as a base for other flavours. Crème fraîche works well in dips, dressings and sauces or as an addition to soups.
• Viili is consumed fresh and chilled, in the same way as yogurt, and can be topped with fruits, nuts, or cereal, as well as other flavourings like spices, sugar and honey. Viili may be added to smoothies, or used in baked goods. It may also be flavoured like yogurt.
• Acidophilus milk is consumed as a beverage by the glass, or added to cereal. It has been used to make eggnog. Acidophilus milk also works well in sauces and desserts. 

Fermented milk products have numerous functional properties:

• Preservation: bacteria are inhibited from growing through pH reduction when lactic acid is formed, and shelf life is increased
• Flavour Enhancement: the sour characteristic of fermented milk products comes from fermentation products (lactic acid, diactyl, carbon dioxide, ethanol); these products act as excellent flavour carriers for herbs, spices and other flavourings
• Texture Enhancement: some fermented milk products (sour cream or crème fraîche) can add body and thickness to sauces, dips or vinaigrettes
• Reducing Caloric Content: many fermented milk products come in low fat or fat free varieties and can be used to substitute for higher fat ingredients
• Emulsification: milk proteins help stabilize fat emulsions in salad dressings, soups and cakes
• Foaming and Whipping: crème fraîche is capable of being whipped like whip cream
• Nutritional benefits: fermented milk products may contain probiotics (bacteria that are beneficial to health) as well as many vitamins and minerals.