Background
Yoghurt is a dairy product made by blending fermented milk with various ingredients that provide flavour and colour.
It is believed that yoghurt originated in Mesopotamia thousands of years ago. Evidence shows that these people had domesticated goats and sheep around 5000 B.C. The milk from these animals was stored in gourds and, in the warm climate, it naturally formed a curd. This curd was an early form of yoghurt. Eventually, a process for purposely producing yoghurt was developed.
While yoghurt has existed for many years, it is only in the last 30–40 years that it has become popular. This is due to many factors, including the introduction of fruit and other flavourings, the convenience of yoghurt as a ready-made breakfast food, and the image of yoghurt as a low-fat, healthy choice.
Manufacturers have responded to the growth in the yoghurt market by introducing many different types, including low-fat and no-fat, creamy, drinking, bio-yoghurt, organic, baby, and frozen. Traditional yoghurt is thick and creamy. It is sold plain and in a wide assortment of flavours—typically fruit such as strawberry or blueberry—however, newer, more unique flavours such as cream pie and chocolate have also been introduced.
Cereals and nuts are sometimes added to yoghurts. Yoghurt makers also sell products with varying levels of fat. Low-fat yoghurt, which contains between 0.5% and 4% fat, is currently the best-selling. Diet no-fat yoghurt contains no fat at all and contains artificial sweeteners that provide sweetness while still reducing calories. Creamy yoghurt is extra thick, made with whole milk and added cream. Drinking yoghurt is a thinner product with a lower solids level than typical yoghurt. Bio-yoghurt is made with a different type of fermentation culture and is said to aid digestion. Yoghurt made with milk from specially fed cows is called organic yoghurt; this type of yoghurt is claimed to be more nutritious than other yoghurts. Other types include pasteurised stirred yoghurt with extended shelf life, baby yoghurt made specifically for children, and frozen yoghurt.
Yoghurt has a generally aldehydic flavour, which is a result of the fermentation process. Since it is made from milk, yoghurt is rich in nutrients. It contains protein and vitamins and is a rich source of calcium. In fact, a small container of yoghurt contains as much calcium as a third of a pint of milk. In addition to these nutritional characteristics, yoghurt is also thought to have health benefits. One suggested benefit is that it acts as a digestive aid: yoghurt can encourage the growth of beneficial bacteria in the gut, helping to digest food more efficiently and protect against harmful organisms. Another potential benefit is for people who are lactose intolerant. These people have difficulty digesting milk products; however, they typically can tolerate yoghurt.
Raw Materials
In general, yoghurt is made with a variety of ingredients, including milk, sugars, stabilisers, fruits and flavours, and a bacterial culture.
When the milk arrives at the plant, its composition is modified before it is used to make yoghurt. This standardisation process typically involves reducing the fat content and increasing the total solids. Once modified, it is pasteurised to kill bacteria and homogenised to consistently disperse fat molecules.
The fermentation culture (Lactobacillus bulgaricus) interacts with the milk and converts it into curd. It also changes the flavour of the milk, giving the characteristic yoghurt flavour, of which acetaldehyde is one of the important contributors. The primary by-product of fermentation is lactic acid. The acid level is used to determine when fermentation is complete—usually three to four hours. Suppliers of yoghurt cultures offer various combinations to produce yoghurts with different flavours and textures.
To modify certain properties of the yoghurt, various ingredients may be added. To make yoghurt sweeter, sucrose (sugar) may be added at approximately 7%. For reduced-calorie yoghurts, artificial sweeteners are used. Cream may be added to provide a smoother texture. The consistency and shelf stability of yoghurt can be increased by stabilisers such as food starch, gelatine, locust bean gum, guar gum, and pectin. These materials are used because they do not significantly impact the final flavour. The use of stabilisers is not required, and some marketers choose not to use them to retain a more natural image.
To improve taste and provide variety, many kinds of fruits are added to yoghurt. Popular fruits include strawberries, blueberries, bananas, and peaches, but almost any fruit can be added. Beyond fruits, other flavourings—such as vanilla, chocolate, coffee, and even mint—are also used. Recently, manufacturers have become quite creative in the types of yoghurt they produce, using both natural and artificial flavourings.
The Manufacturing Process
The general process of making yoghurt includes modifying the composition of and pasteurising the milk; fermenting at warm temperatures; cooling; and adding fruit, sugar, and other materials.
Modifying milk composition
When the milk arrives at the plant, its composition is modified before it is used to make yoghurt. This standardisation typically involves reducing the fat content and increasing the total solids. The fat content is reduced using a standardiser and a separator (a device that relies on centrifugation to separate fat from milk). From the clarifier, the milk is placed in a storage tank and tested for fat and solids content. For yoghurt manufacture, the solids content of the milk is increased to 16%, with 1–5% being fat and 11–14% being solids-not-fat (SNF). This is accomplished either by evaporating some of the water or by adding concentrated milk or milk powder. Increasing the solids content improves the nutritional value of the yoghurt, makes it easier to produce a firmer product, and improves stability by reducing separation during storage.
(The milk is fermented until it becomes yoghurt. Fruits and flavourings are added to the yoghurt before packaging.)
Pasteurisation and homogenisation
After the solids content is adjusted, stabilisers are added and the milk is pasteurised. This step has many benefits. First, it destroys microorganisms in the milk that may interfere with controlled fermentation. Second, it denatures whey proteins, giving the final product better body and texture. Third, it does not greatly alter the flavour of the milk. Finally, it helps release compounds in milk that stimulate the growth of the starter culture. Pasteurisation can be a continuous or batch process; both involve heating the milk to a relatively high temperature and holding it there for a set time. One batch method is to heat a large stainless-steel vat of milk to 185°F (85°C) and hold it for at least 30 minutes.
While the milk is being heat-treated, it is also homogenised. Homogenisation breaks up fat globules into smaller, more consistently dispersed particles, producing a smoother, creamier end product. In commercial yoghurt making, homogenisation provides a uniform product that will not separate. Homogenisation is accomplished using a homogeniser (sometimes called a “viscoliser”). In this machine, the milk is forced through small openings at high pressure and fat globules are broken up by shearing forces.
Fermentation
When pasteurisation and homogenisation are complete, the milk is cooled to between 109.4–114.8°F (43–46°C) and the fermentation culture is added at about 2%. It is held at this temperature for three to four hours while incubation takes place. During this time, the bacteria metabolise certain compounds in the milk, producing the characteristic yoghurt flavour. An important by-product of this process is lactic acid.
Depending on the type of yoghurt, incubation is done either in a large tank or in the final individual containers. Stirred yoghurt is fermented in bulk and then poured into the selling containers. Set yoghurt (French style) is allowed to ferment in the container it is sold in. In both cases, lactic acid level is used to determine readiness. The acid level is found by taking a sample and titrating it with sodium hydroxide. A value of at least 0.9% acidity and a pH of about 4.4 is the current minimum standard for yoghurt manufacture in the United States. When the yoghurt reaches the desired acid level, it is cooled, modified as necessary, and dispensed into containers (if applicable).
Adding other ingredients
Fruits, flavours, and other additives can be added at various points, depending on the yoghurt type. Flavours in non-fruit yoghurts are added to the process milk before dispensing into cartons. Fruits and flavours can also be added to containers first, creating a bottom layer; the inoculated milk is then added on top, sealed, and incubated. If the fruit is pasteurised, it can be added as a purée to bulk yoghurt, which is then dispensed into containers. Finally, fruit can be provided in a separate package to be mixed with plain yoghurt upon consumption.
The finished yoghurt containers are placed in cardboard cases, stacked on pallets, and delivered to stores via refrigerated trucks.
Quality Control
Milk products such as yoghurt are subject to a variety of safety tests, including tests for microbial quality, degree of pasteurisation, and various contaminants. The microbial quality of incoming milk is determined using a dye reaction test to indicate the number of organisms present. If the microbial count is too high, the milk may not be used.
Since complete pasteurisation inactivates most organisms in milk, the degree of pasteurisation is determined by measuring the level of an enzyme called phosphatase. Regulations require this test to ensure pasteurisation is done properly. Beyond microbial contamination, raw milk may contain other contaminants such as antibiotics, pesticides, or even radioactivity. These are identified through safety testing and the milk is treated accordingly.
In addition to safety tests, the final yoghurt is evaluated to ensure it meets manufacturer specifications for pH, rheology, taste, colour, and odour. These factors are tested using laboratory equipment such as pH meters and viscometers, as well as human panellists.
The Future
The future of yoghurt manufacturing will focus on the development of new flavours and longer-lasting products. New flavours will be driven by consumer preferences and innovations from flavour manufacturers. Culture suppliers are researching approaches that hint at uniquely flavoured yoghurts. By varying the organisms in the cultures, yoghurt can be produced more quickly and last longer than conventional yoghurt.
Additionally, the nutritional aspects of yoghurt will be more thoroughly investigated. Some evidence suggests yoghurt consumption has a beneficial antibiotic-like effect. It has also been associated with reduced incidence of lactose intolerance and other gastrointestinal illnesses. Other purported benefits include reduced cholesterol, protection against certain cancers, and even immune-system support. While research is not yet conclusive, these factors will likely be important in the continued market growth of yoghurt.