A Closer Look at Yogurt: From Ingredient to Meal

This longer reference provides the background behind the topic. Read it alongside the recipe collection when you want to connect an ingredient or food idea with a dish to prepare.

In this story

Overview Safety Varieties and presentation Sweetened and flavored Straining Beverages Production

Overview

Yogurt (also spelled yoghurt, yogourt or yoghourt) is a food produced by bacterial fermentation of milk. The bacteria ferment the sugars in the milk and produce lactic acid, which acts on milk protein to give yogurt its texture and characteristic tart flavor. Yogurt is most commonly made from cow's milk, but milk from water buffalo, goats, ewes, mares, camels, and yaks can also be used.

The main bacteria used to produce yogurt are Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. Other lactobacilli and bifidobacteria are sometimes added during or after culturing yogurt. Some countries require yogurt to contain a specific amount of colony-forming units (CFU) of bacteria; for example, in China, the requirement for the number of lactobacillus bacteria is at least 1 million CFU per milliliter. Some countries also regulate which bacteria can be used: for example, in France, a product can only be labeled as yaourt or yoghourt if it has been fermented exclusively by Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, a requirement that aligns with the international definition of yogurt in the Codex Alimentarius on fermented milk (CXS 243-2003).

The bacterial culture is mixed in, and a warm temperature of 30 – 45 C is maintained for 4 to 12 hours to allow fermentation to occur, with the higher temperatures working faster but risking a lumpy texture or whey separation.

Safety

Yogurt made with raw milk can be contaminated with bacteria that can cause significant illness and even result in death, including Listeria, Cryptosporidium, Campylobacter, Brucella, Escherichia coli and Salmonella. Yogurts can also be contaminated with aflatoxin-producing Aspergillus flavus, Aspergillus parasiticus and Aspergillus nomius.

Contamination occurs in traditionally prepared yogurts more often than industrially processed ones, but may affect the latter as well if manufacturing and packaging practices are suboptimal.

When mold forms on yogurt, scraping it off does not eliminate it, as yogurt's consistency allows the mold to penetrate and spread deeply under the surface.

Varieties and presentation

Various techniques for fermenting milk into yogurts exist across countries and local regions.

Dahi is a yogurt from the Indian subcontinent, known for its characteristic taste and consistency. The word dahi seems to be derived from the Sanskrit word dadhi ("sour milk"), one of the five elixirs, or panchamrita, often used in Hindu ritual. Sweetened dahi (mishti doi or meethi dahi) is common in eastern parts of India, made by fermenting sweetened milk. While cow's milk is currently the primary ingredient for yogurt, goat and buffalo milk were widely used in the past, and valued for the fat content (see buffalo curd).

Bogurar doi is a traditional sweetened yogurt originating from Bogura, Bangladesh. It is a widely-consumed regional dairy product, forming part of Bangladeshi culinary heritage. It has formal recognition as a geographical indication product in Bangladesh.

Sweetened and flavored

To offset its natural sourness, yogurt is also sold sweetened, sweetened and flavored or in containers with fruit or fruit jam on the bottom. The two styles of yogurt commonly found in the grocery store are set-style yogurt and Swiss-style yogurt. Set-style yogurt is poured into individual containers to set, while Swiss-style yogurt is stirred prior to packaging. Either may have fruit added to increase sweetness.

Lassi is a common Indian beverage made from stirred liquified yogurt that is either salted or sweetened with sugar commonly, less commonly honey and combined with fruit pulp to create flavored lassi. Consistency can vary widely, with urban and commercial lassis having uniform texture through being processed, whereas rural and rustic lassi has discernible curds or fruit pulp.

Straining

Strained yogurt has been strained through a filter, traditionally made of muslin and more recently of paper or non-muslin cloth. This removes the whey, giving a much thicker consistency. Strained yogurt is made at home, especially if using skimmed milk which results in a thinner consistency. Yogurt that has been strained to filter or remove the whey is known as labneh in Middle Eastern countries. It has a consistency between that of yogurt and cheese. It may be used for sandwiches in Middle Eastern countries. Olive oil, cucumber slices, olives, and various green herbs may be added. It can be thickened further and rolled into balls, preserved in olive oil, and fermented for a few more weeks.

It is sometimes used with onions, meat, and nuts as a stuffing for a variety of pies or kibbeh balls.

Beverages

Ayran, doogh (دوغ; dawghe) or dhallë (Albanian) is a yogurt-based beverage in Middle Eastern cuisine made by mixing yogurt with water and sometimes salt.

Borhani (or burhani) is a spicy yogurt drink from Bangladesh. It is usually served with kacchi biryani at weddings and special feasts. Key ingredients are yogurt blended with mint leaves (mentha), mustard seeds and black rock salt (Kala Namak). Ground roasted cumin, ground white pepper, green chili pepper paste and sugar are often added.

Lassi is a yogurt-based beverage in South Asian cuisine that may be salty or sweet, and may be flavored with rosewater, mango or other fruit juice. Salty lassi is usually flavored with ground, roasted cumin and red chilies, and may be made with buttermilk.

Production

Yogurt is made by warming milk to a temperature (30 – 45 C) that will not kill the live microorganisms that turn the milk into yogurt, inoculating certain bacteria (starter culture), usually Streptococcus thermophilus and Lactobacillus bulgaricus, into the milk, and then keeping it warm for several hours (4–12 hours).

Milk with added dried milk produces a firmer yogurt. Yogurt production provides two significant barriers to pathogen growth, heat and acidity (low pH). Both are necessary to ensure a safe product. Acidity alone is insufficient, as shown by recent outbreaks of food poisoning by E. coli O157:H7 that is acid-tolerant. E. coli O157:H7 is easily destroyed by pasteurization (heating); the initial heating of the milk kills pathogens as well as denaturing proteins. The microorganisms that turn milk into yogurt can tolerate higher temperatures than most pathogens, so that a suitable temperature not only encourages the formation of yogurt, but inhibits pathogenic microorganisms. Once the yogurt has formed it can, if desired, be strained to reduce the whey content and thicken it.