Greek Yogurt: The different from regular yogurt
Who does not know yogurt? Yogurt is one of the oldest fermented milk products, tremendously popular all over the world. Yogurt is an acidified, coagulated product obtained from milk by fermentation with lactic acid producing bacteria and contains a rich source of protein and calcium. There are many types of yogurt that commonly known and available in the retail markets such as natural yogurt, Greek yogurt, stirred yogurt and drinking yogurt. However, we will specifically talk about Greek yogurt and how it differs from natural yogurt.
Greek yogurt or also known as strained yogurt is originating from Middle East or Central Asia. Greek yogurt is consumed in a variety of occasions such as a snack, as a meal, even as a condiment. However, different country has its different style to forms Greek yogurt or strained yogurt. For example, in Turkey, a type of strained yogurt is called suzme. It is obtained by further draining cacik (a local yogurt) by using white cotton kitchen towel to obtain meze (kind of dry yogurt).
The straining process of the cacik by using white cotton kitchen towel
However, in Europe specifically Albania, strained yogurt is called salcë kos where it is drained in a cloth sack from few hours to overnight. The water released from this process is called ‘hir’ and can be used to preserve cheese or as a drink while the strained yogurt is used variedly in Albanian cuisine.
The straining process of the salcë kos in a cloth sack
Flow diagram for the production of Greek Yogurt
Greek yogurt is basically made from cow milk and needs to undergo few processes to produce Greek yogurt and the process does not much differ from natural yogurt. The raw milk will be homogenised first to form a stable emulsion before being heat treated (pasteurization) at temperature 72 °C for 15 sec to kill the pathogenic bacteria. The milk will be cooled down to 40-45 °C to give suitable temperature before the addition of starter culture to start the fermentation process. The fermentation process occurs in batch tank and the yogurt needs to be incubated until the pH drop below 5. The starter culture that normally being used in the production of Greek yogurt is the probiotic bacteria which are Lactobacillus bulgaricus, Streptococcus thermophilus and Bifidobacterium. Some other good bacteria might also be used such as Lactobacillus acidophilus and Lactobacillus casei.
The function of starter culture is to generate lactic acid by the fermentation of the major sugar in milk (lactose). The fermentation process will cause the yogurt to drop below pH 5. What happens at this low pH value? When the pH drops, micelles of caseins which are the amphiphilic proteins will losses its tertiary structure due to the protonation of the amino acid residues. Casein is the major protein that can be found in milk and mostly it is in form of alpha-casein. The denatured of casein proteins will reassemble by interacting with other casein proteins and these intermolecular interactions will result in a network of molecules that provides the semisolid texture of yogurt.
Besides that, this fermentation process is also essential for the tangy flavour of Greek yogurt. Mainly, the production of lactic acid is by Lactobacillus bulgaricus which will give the sour acidic and refreshing taste. It will produce relatively high concentrations of acetaldehyde, which skews the ratio of diacetyl and acetaldehyde. The various carbonyl compounds like acetone, diacetyl and acetaldehyde become major contributors to the tarty yogurt flavour. Yogurt has a high concentration of acetaldehyde due to the low utilization of this metabolite by yogurt bacteria. During fermentation, acetaldehyde is produced directly from lactose metabolism as a result of pyruvate decarboxylation. Acetaldehyde content varies from 4-60 ppm in yogurt while diacetyl varies from 0.1-0.3 ppm.
Pathways to acetaldehyde production (from scienceandfooducla.wordpress.com)
Then, the yogurt will then undergo cooling process. The main purpose of cooling the fermented yogurt is to restrict the growth of yogurt culture and its enzyme activity as quickly as possible and maintain the desired pH, body, and texture. Cooling with agitation at pH 4.7 or above can result in a grainy body and undesirable texture in the finished yogurt. The rate of cooling should be steady but not too fast. Cooling too rapidly can bring unfavourable changes in the structure of the coagulum contributing to whey separation in the finished yogurt.
The process continues with the straining process to form Greek yogurt. This step involves the removing of lactoserum or whey, which consists primarily of water and lactose. As a result, the yogurt acquires a thicker texture because it is more concentrated than natural yogurt.
Finally, the yogurt will be packed in the suitable container immediately within 20 min to avoid microbial contamination or microbial growth and must be handled smoothly with the lowest level of mechanical shear through the pumps to ensure the end product reaches consumer expectations in term of viscosity, flavour and appearance. The packed Greek yogurt will be stored at chilled temperature around 8-10 °C before being distributed to the marketers.
The composition of natural yogurt compared to Greek yogurt
Composition
|
Natural Yogurt (per 100g)
|
Greek Yogurt (per 100g)
|
Calories (kJ)
|
363
|
255
|
Carbohydrate (g)
|
3.3
|
4.7
|
Protein (g)
|
7.3
|
3.5
|
Fat (g)
|
5.3
|
3.3
|
Vitamin A (IU)
|
133
|
99
|
Calcium (mg)
|
66.7
|
121.0
|
Cholesterol (mg)
|
13.3
|
13.0
|
From nutritiondata.com
It can be compared that Greek yogurt has a lower amount of calories than natural yogurt. Greek yogurt is suitable for those who want to reduce their weight as it contains lower calories. However, Greek yogurt contains a higher amount of carbohydrate and calcium content but lower in protein and fat content compared to natural yogurt. This is due to the straining process that occurs in the production of the Greek yogurt that causes a lot of whey protein and fat content to be removed from the yogurt.
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