To make commercial yogurt, which microorganism should be used for acid production?

Dried yoghurt

Dried yoghurt or powder yoghurt production is practiced in many countries. The main objective for the production of dried yoghurt is to extend shelf life and to facilitate easy handling and transport. Dried yoghurt can be produced by sun drying, air drying, spray drying, freeze drying, or microwave drying. Spray-dried instant yoghurt powder and skim-milk yoghurt powder are prepared commercially. Drying of yoghurt causes low water contents and creates unfavorable conditions for the growth of spoilage-causing microorganisms, especially yeasts and molds. Kurut in Turkey is an example of dried yoghurt. However, the nutritive value of yoghurt is degraded during the concentration process and the drying causes a loss of volatile flavor compounds. Powder yoghurt can be used as an ingredient for the manufacture of confectioneries, bakery foods, soup bases, dips, and sauces.

Introduction

Yogurt is a fermented milk product and as such is a means of preserving the nutrients in milk. A wide variety of yogurts are now available around the world, ranging from very-low-fat fruit yogurts to Greek-style yogurt with a fat content around 8 g per 100 g. Yogurt can be made from cows', ewes', goats', or buffalo's milk. This article reviews the nutritional composition of a range of yogurts, provides data on yogurt consumption around the world and discusses the importance of yogurt for different population groups. (See BUFFALO | Milk; MILK | Dietary Importance; SHEEP | Milk.)

Since yogurt is derived from milk, it provides protein, calcium, and other minerals, and a range of vitamins (Table 1). Levels of some vitamins, such as vitamin B1 and pantothenic acid, are reduced as they are utilized by the bacterial culture used to produce the yogurt. However, folic acid levels are typically higher than in milk since folic acid is produced by the bacteria. Refer to individual nutrients.

Table 1. Nutritional value of varieties of yogurt

na, data not available.

Source: Holland B, Unwin ID and Buss DH (1989) Milk Products and Eggs. Fourth supplement to McCane and Widdowson's The Composition of Foods, 4th edn. London: Royal Society of Chemistry and Ministry of Agriculture, Fisheries and Food, with permission.

Some people have speculated that the bioavailability of some minerals in yogurt is enhanced, but research to support this claim is inconclusive. (See BIOAVAILABILITY OF NUTRIENTS.)

Flavored yogurts

Flavored yogurts are prepared by adding flavorings to plain yogurt. Sundae-style yogurt is prepared by layering 15–18% of total weight of yogurt with fruit purée or syrup on the bottom of the containers and then filling the containers with warm inoculated mix, followed by sealing the containers and incubation. The fruit in the product may be mixed with the yogurt gel by consumers before eating.

Swiss or stirred-style yogurts are prepared by blending fruit purée, sucrose, or glucose into bulk prepared fresh plain yogurt. Since the coagulum is broken during blending, plain yogurt is usually prepared with a higher level of stabilizer (0.7%) than normal (0.3%). The product after mixing with fruit is chilled to 4 °C.

Concentrated Yogurt

Concentrated yogurt, strained yogurt, labneh, or Greek yogurt is yogurt that has been strained in a cloth or paper bag or filter to remove the whey. It has a consistency between that of yogurt and cheese, and yogurt's distinctive sour taste. It is a traditional food in the Levant, Eastern Mediterranean, Near East, and South Asia, where it often is used in cooking preparations, as it is high enough in fat not to curdle at higher temperatures.

Like many yogurts, strained yogurt often is made from milk or enriched milk by the addition of powdered milk or butterfat. In the traditional method, cold and unsweetened natural or plain yogurt is strained through a cloth bag, animal skin, or earthenware vessel. In addition, two different systems of ultrafiltration (UF) have been used for the production of concentrated yogurt: (1) the fermentation of UF retentate that has the solids content desired in the final product, and (2) UF of the yogurt at 40 °C to produce a concentrate at about 24 g.100 g−1 of total solids.

Because of the straining process to remove excess whey, even nonfat varieties are rich in texture and creamy. In Western Europe and the United States, strained yogurt has become increasingly popular because it is richer in texture than unstrained yogurt, but it is low in fat. Because straining removes whey, strained yogurt is higher in protein and lower in sugar and carbohydrates than unstrained yogurt.

Yogurt

Yogurt, a medium-acid fermented milk, is fermented by symbiotic cultures of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus characterized by a smooth, viscous gel with a delicate walnutty flavor. The two yogurt bacteria reside either naturally in the milk (in the regions where yogurt is traditionally made) or are added as starter cultures at 2–5% inoculum, preferably at a 1:1 ratio. Incubation is at 42–45 °C for 3–6 h until pH 4.4 and 0.9–1.2% titratable acidity is reached.

The delicate flavor of plain yogurt is achieved through a protocooperation relationship between rods and cocci, which is influenced by factors such as incubation temperature and acid concentration. Lactobacillus delbrueckii subsp. bulgaricus produces amino acids and peptides required by Sc. thermophilus as growth factors, while folate is produced by Sc. thermophilus to support the growth of Lb. delbrueckii subsp. bulgaricus.

Volatile compounds include small amounts of acetic acid, diacetyl, and acetaldehyde produced by Lb. delbrueckii subsp. bulgaricus, which contributes much to the unique flavor of yogurt. Heat treatment of yogurt after fermentation is permitted in most countries. Alternative nutritive sweeteners such as aspartame are also permitted. Based on the method of production, there are two types of yogurt, namely, set and stirred yogurt.

Yogurt and yogurt-like products are made widely in the Mediterranean area, Asia, Africa, and central Europe. Synonyms for yogurt or related fermented milks throughout various countries are shown in Table 2. ‘Zabady’ or Egyptian yogurt is traditionally made from sheep’s milk. ‘Dahi’ and ‘dadih’ are Indian and Indonesian yogurts, respectively. Buffalo milk is often used in the manufacture of these products, sometimes in combination with bovine milk. See FERMENTED MILKS | Asian Fermented Milks; FERMENTED MILKS | Middle Eastern Fermented Milks; FERMENTED MILKS | Yogurt: Role of Starter Culture; and FERMENTED MILKS | Yogurt: Types and Manufacture.

Table 2. Yogurt-related fermented milks in various countries

Abstract

Yogurt is a popular fermented milk product containing both Lactobacillus bulgaricus and Streptococcus thermophilus cultures. Most yogurts contain several nutrients that are lacking in the American diet, such as calcium, vitamin D, and potassium. Both intervention and observational studies have associated yogurt consumption with several benefits to human health, including increased digestibility of lactose and assimilation of certain nutrients, enhancement of bone mineral acquisition, weight management, heart health, metabolic health, and digestive and immune health. Additional and special beneficial cultures can also be added to conventional yogurt and provide additional health benefits. This chapter is primarily centered on the benefits of regular yogurt consumption without added cultures. It describes some of the most relevant studies on yogurt and discusses the plausible mechanisms of action associated both with interactions at the gastrointestinal epithelium and with the gut microbiota.

29.5.1.1 Yogurt

Yogurt or the yogurt-manufacturing technique is an ancient one, dating back to thousands of years. Symbiotic thermophilic starter cultures, namely Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus, were the principal starter strains used in the yogurt preparation. The symbiotic blend of yogurt starter cultures helps in fermentation. Ultimately, the typical yogurt texture and flavor are due to the starter culture combination and acetaldehyde is the principal flavor compound in yogurt (Vinderola et al., 2002). Yogurt is available in several physical forms namely set, stirred or fluid (drinking yogurt), frozen and dried yogurt. In some countries, namely USA and European countries, the presence of live yogurt culture (>106 cfu/mL) is made essential in yogurt preparations (Yildiz, 2010). The addition of probiotic organisms as adjunct cultures to improve the therapeutic value of yogurt has been a recent trend. Yoghurt is prepared from both cow and buffalo milks. The general method of its preparation involves standardization of milk to meet the minimum legal requirements as laid down by local food regulatory authorities. Standardized milk is pasteurized (90°C–95°C for 10–20 min) to kill all the pathogenic and almost all the spoilage-causing organisms. After heat treatment, milk is quickly cooled to 42°C followed by the addition of yogurt starter culture at 2% level. After inoculation, milk is incubated at 42°C for 4–6 h or up to a period until the pH reaches 4.6, followed by cooling to stop the fermentation. Yogurt obtained in this process can also be added with various fruits and flavors to enhance its aesthetic appeal.

Introduction

Yogurt is a semisolid fermented milk product obtained by the action of specific lactic acid bacteria (Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus) when added to heat-treated milk. In some countries, the term yogurt is restricted to products made exclusively by the two cultures, while other countries also allow adjunct probiotic cultures to be labeled as yogurt. According to the National Yogurt Association's criteria for ‘live and active culture yogurt,’ the finished product must contain live bacteria in amounts of ≥ 108 organisms per gram at the time of manufacture, and the cultures must remain active at the end of the stated shelf life.

Regulatory authorities and scientific institutions in most countries acknowledge the importance of yogurt as part of a balanced and healthy diet. Yogurt is often included in the lists of healthy foods due to its high nutritive value. It is an excellent source of proteins, vitamins, and minerals. Yogurt can be prepared specifically to meet particular nutritional requirements of people whose normal processes of metabolism are altered or those who want a particular effect by controlled intake of foods or certain nutrients. Dietary strategies include lowering of calorie content by reducing fat and lactose content and fortification of minerals, vitamins, bioactive substances, or beneficial organisms.

Yogurts

Yogurt is traditionally produced from milk and/or cream through the action of two particular strains of LAB – Lactobacillus bulgaricus and Streptococcus thermophilus. Codex currently defines yogurt as a milk product obtained by fermentation using these two specific strains with or without optional additions (such as milk powder, other LAB, and sugar). In many countries (e.g., Sweden, France, Belgium, Portugal, Spain, USA) legislation exists allowing only L. bulgaricus and S. thermophilus to be used to produce yogurt. In the UK yogurt can be made using both of these strains or just one of them. In other countries (e.g., Switzerland) strains such as L. acidophilus and bifidobacteria may be used in addition to the traditional yogurt strains. It is likely that the new Codex standard will build on the draft definition currently used for mild yogurt (a product made from S. thermophilus and lactobacilli other than L. bulgaricus). In Japan no separate legislation exists for yogurt and so it is not obligatory that the two traditional strains are used, either collectively or separately. Similarly, in Finland there are also no compositional regulations for any milk products.

Conclusion

Yogurt is an ancient traditional fermented food that has known, since half a century, a tremendous industrialization of its manufacturing conditions. In more recent years, an intense innovative diversification of recipes and products occurred, which partly explain the increase of their consumption. Although artisan production schemes persist, including homemade yogurt manufacture, the industrial manufacture of yogurt is now well established. Breakthrough innovations are limited and the improvements in process productivity relate mainly to the design and the management of the manufacturing units. Nevertheless, the interesting nutritional properties of yogurts and more generally of fermented milks, their diversity, and the opening and increasing of new markets offer important perspectives for their development at world scale.