Yogurt is a fermentation of milk with a benign bacterial culture to produce a flavoured, slightly sharp creamy product. It is a simple manufacture of yogurt that is described here because to enter into details would take up half this web-site in size.
Yogurts have developed considerably in the last 20 years. Touted for their probiotic content they have been at the forefront of cultured dairy foods and other supplements. They are now designed for health and especially so in both Europe and the USA. Variety of yogurt is also at the forefront with types such as low fat, no fat, creamy, Greek, drinking, bio-yogurt, organic, frozen, and so on. Manufacturers also find the yogurt drink market attractive due to the opportunities available for providing value-added ingredients in a beverage system (Douaud, 2007).
The health benefits of yogurt are discussed in a separate article because there is such a plethora of research that it would dominate the whole topic of yogurt manufacture. It is now one of the most recognizable health foods and firmly associated with the concept of ‘probiotic’.
A Brief History
Yogurt has fed civilisations since 5000 BCE. Food historians think it originated in Central Asia, almost surely by chance when ambient strains of Lactobacillus delbrueckii species vulgaris and Streptococcus thermophilus thickened and soured fresh milk. These bacteria will ferment any mammals milk – goat, sheep, cow, camel, yak, horse and mare, water Buffalo – so yogurt would have been a tremendous value too early pastoral peoples. Fresh milk, which spoils in hot climates in just a few hours could safely nourish tribes of people if encouraged to acidify milk into yogurt. By producing lactic acid and lowering the milks pH, the good bacteria in yogurt made life difficult for spoilage bacteria and pathogens. Drained and salted, yogurt last even longer, as the ancient tribespeople were able to discover. It is said that travelling herdsmen would hang curd or yogurt overnight to make it easier to carry while traveling. This thicker yogurt which might be like labneh was made palatable by mixing with nuts and sugar.
It is also fair to say that there are many variants of these fermented yogurt-like milk and dairy foods.
From Central Asia, yogurt likely passed over to Persia (now Iran), then west to Turkey and the Balkans, east to Afghanistan, and south to Pakistan and India. The line drawn from Belgrade to Baghdad to Bangalore means you can trace consumption through the middle and southern Asia into recent times. The food was mentioned in sacred Hindu texts and mythology. It is thought in the Bible that Abraham was serving curds and milk to guests. Pliny the Elder discussed the role of nomadic tribes being able to thicken milk to make a pleasant acidic curd.
Yogurt is discussed in the 11th century where it appears in one of the first dictionaries during the Seljuk reign in the Middle East. The English word ‘yogurt’ is Turkish in origin. This Turkish word derives from the root ‘yog’, meaning to condense or thicken. The Turks helped spread yoghurt to Western Europe. It was a Russian microbiologist, Ilya Mechnikov, who spread today’s widespread enthusiasm for yogurt.
Metchnikov who won the Nobel prize in 1908 was interested in the long lives of Bulgaria and thought their high yogurt consumption was responsible. During his years at the Pasteur institute in Paris, he developed a theory that lactic acid bacteria in the gut could help prolong life by combating toxic bacteria . The concept underlies probiotics research today. Metchnikov reasoned that we could, through diet, modified our intestinal flora, replacing the harmful microbes with beneficial ones.
Metchnikov’s work caught the attention of Isaac Carasso, a Greek who was moving to Spain in 1912. For Carasso was struck by how many Spanish children suffered from intestinal illnesses. He had read Mechnikov and noted how people in the Balkans ate yogurt for digestive issues. He started making yogurt in a small plant in Barcelona. The year was 1919 and yogurt was so little known in Spain and Carasso sold it through pharmacies as a medicine. He named the business Danone, after his son Daniel.
Daniel Carasso, who studied business, picked up where his father left off. He introduced Danone yogurt into dairy stores. Eventually the French embraced it, the then World War II intervened. Fleeing the Nazis, Carasso emmigrated to the United States in 1941, bringing his yogurt expertise with him. He soon found partners and began manufacturing in the Bronx in New York in 1942, packaging plain yoghurt in half-pint glass bottles. For his new customers, he mechanised the manufacture, rebranding the name. Daniel died in 2009 at the age of 103. Yogurt had moved into mainstream nutrition.
Definition of a Yogurt
The FDA Code of Federal Regulations defines yogurt as a food produced by culturing dairy ingredients such as cream, milk, partially skimmed milk, or skim milk alone or in combination with a bacterial culture that contains the lactic acid–producing bacteria Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus (Tamime & Robinson, 1989). This combination of bacteria is often described as a thermophilic homofermentative fermentation living symbiotically.
Yogurt has a titratable acidity (TA) of not less than 0.9%, expressed as lactic acid (21 CFR 131.200).
Common Microbes In Yogurt Fermentation
Lactobacillus bulgaricus and Streptococcus thermophilus. In addition, yogurt starter cultures may contain other microorganisms like Lactobacillus acidophilus, Lactobacillus casei, Lactobacillus helveticus, Bifidobacterium bifidus, Bifidobacterium infantis, and others.
Flavour And Texture of Yogurt
The sensory properties are frequently discussed and studied (Chandan et al., 2006). All the flavours etc. come from cultures of Streptococcus thermophilus and Lactobacillus bulgaricus.
The most dominant and pronounced flavour from all sensory studies is sourness. This is due to lactic acid produced by the starter culture and is synonymous with most fermented foods that rely on Lactobacillus bacteria. Most yogurts have a pH below 4.6, usually in the range of pH 4.2 to 4.5 and contain between 0.8% and 1.0% lactic acid, so it’s not surprising this flavour predominates.
The pH can drop below 4.2 but it starts to develop an overweaning acidity which makes it difficult to taste and is also not suited to the incorporation of other flavours.
The flavour of yogurt includes mainly key carbonyl compounds and saturated volatile free fatty acids (Cheng, 2010) as well as lactic acid.
Casein is the main protein in milk and of course yogurt. Enzymatic degradation produces 3-methylbutanol and 3-methylbutanoic acid. The 3-methylbutanol produces an ethereal, fresh green note whilst the equivalent carboxylic acid is cheesy, sweaty and rancid.
The fats are also enzymatically degraded alongside beta-oxidation generating hexanol (alcoholic, floral flavour), hexanal (green-cut grass aroma), nonanone (grassy-herbal, green-fruity and floral), and heptanone (fruity, spicy and cinnamon).
Fermentation also produces acetic acid (vinegar), acetaldehyde (ethereal, fresh, green & pungent), diacetyl (buttery, vanilla), acetoin (buttery), acetone (sweet, fruity), and 2-butanone which contribute most to the typical aroma and flavor of yogurt as well as lactic acid. The acetaldehyde content varies between 4 and 60 ppm, diacetyl varies from 0.1 to 0.3 ppm whilst acetic acid varies between 50 and 200 ppm.
Some microorganisms rapidly reduce diacetyl because they have an active diacetyl reductase. These bacteria compete well in the presence of rapid acid-generating or non-citrate fermenting Lactococci. They also produce relatively high concentrations of acetaldehyde which skews the ratio of diacetyl and acetaldehyde in favour of acetaldehyde. This skewing imparts an undesirable ‘green apple flavour’ to both cultured buttermilk and sour cream.
Some amino-acids such as threonine and methionine are precursors of acetaldehyde. Threonine in the presence o threonine aldolase produces glycine and acetaldehyde. It is also produced from glucose via acteyl CoA.
The Role of Fat in Texture
Fat has always played a significant role in all sorts of foods and none more so than in yogurt. Milk fat plays a role in gel strength and viscosity, in sensory perception of texture, consistency as well as flavour, and in syneresis as this food ages. In yogurt, the fat content varies from between 1 and 10% w/w depending on where it is a full-fat or low-fat style yogurt.
Fat reduction during processing produces texture and flavour defects if its is not managed properly.
Milk Base
Milk base composition plays a determinative role in the fermentation process affecting the microbial viability and metabolic activity of starter culture, fermentation time and thus impacting on the characteristics of the final product (Moreira et al., 2017).
Stabilisers in Low-Fat Yogurt
Low-fat yogurts must rely on stabilisers to generate the same creamy thickness. These stabilisers strengthen the 3-dimensional network structure formed by the casein micelles at an acidic pH.
Various nondairy ingredients such as starch, whey protein concentrates, gelatin, and hydrocolloids (pectin, gums) have been added to yogurt as fat replacers and to modify the rheological properties (Keogh & O’Kennedy, 1998; Guzmán-González et al., 1999). Other ingredients include milk powders (Mistry & Hassan, 1992).
Non-Dairy Sources
Sweet corn milk has been tried in Thailand using similar starter bacteria (Supavititpatana et al., 2010).
Low-Lactose Yogurts
Low lactose yogurts are generally to be expected given that fermentation will metabolize lactose.
The Manufacturing Process
Milk is modified before fermentation by reducing its fat content and increasing the total solids level. The fat is separated from the milk using a clarifier and separator such as a centrifuge. Another approach is to add fat back to skimmed milk to create a suitable fat content for a starter culture. If a low-fat yogurt is being produced, the stabilizers can be added at this time.
The modified milk is pasteurised and then homogenized. Pasteurization is 85ºC for 30 minutes or 95ºC for 10 minutes. A high heat treatment is required to denature the whey serum proteins. This pasteurization reduced the number of spoilage microorganisms including naturally-fermenting bacteria. It also provides a better environment for when the starter culture is added. Homogenization is needed between 2,000 and 2,500 psi to solubilize all the components and improve the consistency.
The milk is cooled to 42ºC to a temperature which is ideal for Lactobacillus growth. An inoculum is then added using a starter culture of milk-fermenting bacteria. For a fermented product to be labeled as yogurt it needs to contain two live bacterial strains of Lactobacillus bulgaricus and Streptococcus thermophiles. Ratios of 1:1 are commonly employed. Cultures are typically added at concentrations of about 2%.
The bacteria start converting milk sugar (lactose) into lactic acid. A value of at least 0.9% acidity and a pH of about 4.4 are the current minimum standards for yogurt manufacture in the United States before its considered ready.
The fermentation is maintained or held at 42ºC until its pH reaches 4.4- 4.5 and acidity is met. Such a process takes several hours. Lactic acid level or pH is used to determine when the yogurt is ready for cooling. A pH probe measure or an acidity titration are the most common methods used.
Chilling is often accomplished quickly by blast chilling to stop the fermentation process. It is still only the main manufacturing process to achieve this The yogurt is cooled to 4 – 7ºC to halt the fermentation process. It also means other ingredients such as fruits and flavours are added. The fruits are often added to the bottom of the yogurt product in a cup before yogurt is poured on top. The fruit can also be blended into the product and fermented along with the yogurt before it is packaged.
It is then packaged. Yogurt can be pumped into all sorts of packaging and then its is sealed.
Monitoring the End Point of a Fermentation
The monitoring of a milk fermentation such as yogurt is usually checked periodically using a pH probe but other methods have been tried to determine a suitable end point. The application of an electronic nose (EN), a near-infrared spectrometer (NIRS) and various bioreactor probes have all been tried (Cimander et al., 2002).
Defects
Manufactured yogurts can eventually become too acidic during low temperature storage because of the ability of the culture, to produce lactic acid. Post-fermentation acidification is one of the main reasons why consumers find perfectly good yogurt to spoil. Excessive acid concentration imparts a harsh, acrid flavour and masks the delicate dairy flavour notes such as diacetyl and its pervasive somewhat harsh butter-like flavour.
Syneresis results in the shrinkage of the curd with the expulsion of whey and other soluble components.
References
. (2007). Yogurt drinks are leading food and beverage product, ACNielsen. NutraIngredients-USA.com Consumer Trends. Chicago.
, . (1992). Manufacture of yogurt stabilized with milk powder. J. Dairy Sci. 75 pp. 947–57.
Moreira, T. C., da Silva, A. T., Fagundes, C., Ferreira, S. M. R., Cândido, L. M. B., Passos, M., & Krüger, C. C. H. (2017). Elaboration of yogurt with reduced level of lactose added of carob (Ceratonia siliqua L.). LWT-Food Science and Technology, 76, pp. 326-329.
Supavititpatana, P., Wirjantoro, T. I., & Raviyan, P. (2010). Characteristics and shelf-life of corn milk yogurt. Journal of Natural Science, 9(1), pp. 133-147.
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