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6252 YOGURT/The Product and its Manufacture Bhaduri S, Turner-Jones C, Taylor MM and Lachica VR (1990) Simple assay of calcium dependency for virulent plasmid-bearing clones of Yersinia enterocolitica. Applied and Environmental Microbiology 28: 798–800. Bottone EJ (1999) Yersinia enterocolitica: overview and epidemiological correlates. Microbes and Infection 1: 323–333. Cleary TG (2000) Yersinia. In: Behrman RE, Kliegman RM and Jenson HB (eds) Nelson Textbook of Pediatrics, 16th edn, pp. 857–859. London: W.B. Saunders, Harcourt Brace Jovanovich. Cloak OM, Duffy G, Sheridan JJ, Blair IS and McDowell DA (1999) Isolation and detection of Listeria spp, Salmonella spp and Yersinia spp using a simultaneous enrichment step followed by a surface adhesion immunofluorescent technique. Journal of Microbiological Methods 39: 33–43. Feng P (1992) Identification of invasive Yersinia species using oligonucleotide probes. Molecular and Cellular Probes 6: 291–297. Gray JT, WaKabongo M, Campos FE et al. (2001) Recognition of Yersinia enterocolitca multiple strain infection in twin infants using PCR-based DNA fingerprinting. Journal of Applied Microbiology 90: 358–364. Kampfer P (1999) Yersinia. Introduction. In: Robinson RK, Batt CA and Patel PD (eds) Encyclopedia of Food Microbiology, vol. III, pp. 2342–2350. London: Academic Press. Kapperaud G, Varadund T, Skjerve E, Hornes E and Michaelsen TE (1993) Detection of pathogenic Yersinia enterocolitica in foods and water by immunomagnetic separation, nested PCR and colorimetric detection of amplified DNA. Applied and Environmental Microbiology 59: 2938–2944. Khare SS, Kamat AS, Doctor TR and Nair PM (1996) Incidence of Yersinia enterocolitica and related species in some fish, meat and meat products in India. Journal of Science Food and Agriculture 72: 187–195. Manafi M and Holzhammer E (1994) Comparison of the vitek, API 20E and Gene-trak systems for the identification of Yersinia enterocolitica. Letters in Applied Microbiology 18: 90–92. Nagaraju NR, Isloor S, Rao MS and Rajasekhar M (2001) Prevalence of yersinia antibodies in serodiagnosis of bovine brucellosis. Journal of Veterinary Research 5: 197–204. Schiemann DA and Toma S (1978) Isolation of Yersinia enterocolitica from raw milk. Applied and Environmental Microbiology 35: 54–58. Stock I and Wiedemann B (1999) An in vitro study of the antimicrobial susceptibilities of Yersinia enterocolitica and the definition of a database. Journal of Antimicrobial and Chemotherapy 43: 37–45. Vishnubhatla A, Fung DYC, Oberst RD et al. (2000) Rapid 50 nuclease (TaqMan) assay for detection of virulent strains of Yersinia enterocolitica. Applied and Environmental Microbiology 66: 4131–4135. Wauters G (1981) Antigens of Yersinia enterocolitica. In: Bottone EJ (ed.) Yersinia enterocolitica, pp. 41–53. Boca Raton, FL: CRC Press. YOGURT Contents The Product and its Manufacture Yogurt-based Products Dietary Importance The Product and its Manufacture N Shah, Victoria University of Technology, Victoria, Australia Copyright 2003, Elsevier Science Ltd. All Rights Reserved. Introduction 0001 At the beginning of this century, Nobel Laureate, Elie Metchnikoff, at the Pasteur Institute, linked health and longevity to the ingestion of bacteria present in fermented foods such as yogurt. This observation provided a major boost to the manufacture and consumption of yogurt and other fermented milk products. Although no records are available to trace the origin of yogurt, it is believed that fermentation was the first technique employed by humans to preserve foods. The word ‘yogurt’ was derived from the Turkish word ‘Jugurt.’ Yogurt is defined as ‘a product resulting from milk by fermentation with a mixed starter culture consisting of Streptococcus 0002 YOGURT/The Product and its Manufacture 0003 0004 0005 0006 tbl0001 thermophilus and Lactobacillus delbrueckii ssp. bulgaricus.’ However, in some countries, including Australia, other suitable lactic acid bacteria are permitted for use as starter cultures. As a result, some yogurt manufacturers use Lactobacillus helveticus and Lactobacillus jugurti for yogurt manufacture. The first US Federal Standards of Identity for yogurt were published in 1981. The standard does not permit the use of culture organisms other than Lb. delbrueckii ssp. bulgaricus and Sc. thermophilus for making yogurt, and the titratable acidity must be at least 0.9%, expressed as lactic acid. Yogurts differ according to their chemical composition, method of production, flavour used and the nature of postincubation processing. Based on the fat content, there are three main types of yogurt: full-fat yogurt, reduced-fat yogurt, and low-fat yogurt (Table 1). On the basis of the method of production and the physical structure of the coagulum, yogurts are classified as either set or stirred. Set yogurt is the product formed when the fermentation of milk is carried out in a retail container, and the yogurt produced is in a continuous semisolid mass. In contrast, stirred yogurt results when the coagulum is produced from milk, and the gel structure is broken before cooling and packaging. Fluid yogurt can be considered as stirred yogurt of low viscosity. On the basis of flavorings, yogurts are divided into three categories. Plain or natural yogurt is the traditional product, which has a typical sharp ‘nutty’ flavor. Fruit yogurts are made by addition of fruits, usually in the form of fruit preserves, puree or jam. Flavored yogurts are prepared from plain or natural yogurt by adding sugar and/or other sweetening agents, flavorings and colorings. The method of yogurt production has changed very little over the years. However, some newer varieties have been added including frozen-, concentrated-, dried-, and pasteurized yogurt. Postincubation processing of yogurt may lead to pasteurized/UHT yogurt, concentrated yogurt, frozen yogurt, and dried yogurt. Pasteurized/UHT yogurt is prepared by heat- treating yogurt after incubation. Heat treatment destroys yogurt starter bacteria and reduces the levels Table 1 Compositional standards for yogurt Fat (%) Milk solids not fat (%) Titratable acidity (%) pH Full-fat yogurt Reduced-fat yogurt Low-fat yogurt 3.0% 8.25 0.9 4.5 0.5–2 8.25 >0.9 4.5 0.5 8.25 >0.9 4.5 6253 of volatile compounds associated with the flavor of yogurt. In some countries, such as Australia, heat treatment after production to inactivate the starter culture is not permitted. In other countries, such as the USA and several countries in Europe, heat treatment is permitted; however, auxiliary labeling ‘heat-treated after culturing’ is required if yogurts are heat-treated after culturing. Frozen yogurt resembles ice cream, and the chemical composition and manufacturing details up to the freezing stage are similar to those of yogurt. Frozen yogurt can be either soft or hard. Dried yogurt can be produced by sun-drying, spray-drying or freeze-drying of yogurt. The drying process transforms the junket into powder. Drying also causes loss of some flavor compounds and destruction of the starter culture. Another type of yogurt, which may find favor among diet-conscious consumers is low-calorie yogurt, in which intense sweeteners are used, and the viscosity of the product is improved by the addition of stabilizers and thickening agents such as carrageenan and/ or gelatin. In the Middle East, concentrated yogurt is prepared by adding wheat flour or parboiled wheat, and the yogurt–wheat mixture is shaped into rolls and sundried. The product is popularly known as ‘kishk.’ In some countries, such as India, Nepal, and Bangladesh, yogurt is made in earthenware pots, and because of evaporation of moisture through the pores of the earthenware pots, the product becomes concentrated. 0007 Processing and Manufacture of Yogurt Preparation of Mix A flow diagram for making yogurt is shown in Figure 1. Traditionally, yogurt is manufactured from milk, which has been concentrated by boiling to increase the viscosity of the product. Nowadays, milk is fortified with nonfat dry milk for the same purpose. The level of fortification varies from as little as 1% to as much as 6%. However, the generally recommended level of fortification is around 3–4%. This increases the total solids level in yogurt mix to 15–17%. Ultrafiltration and reverse osmosis may also be used to achieve a higher solids content in order to improve the viscosity of the product. Milk concentrated by ultrafiltration to 18–20% total solids has been reported to produce good-quality yogurt. The viscosity of yogurt is almost wholly dependent on the protein content of milk. Hence, a high protein concentration is essential for the production of viscous yogurt. Casein is the major contributor of viscosity 0008 0009 6254 YOGURT/The Product and its Manufacture Preparation of mix (standardization, fortification with skim milk powder) Homogenization (65 ⬚C,1.8 × 104 kPa) Heat treatment (85 ⬚C/30 min) Cool to incubation temperature (42 ⬚C) Inoculation with starter (1% each of Sc. thermophilus and Lb. delbrueckii ssp. bulgaricus) Pack in retail container (or inoculate in bulk for fruit yogurt) Incubate (42 ⬚C for ~4 h) Cool (4 ⬚C) (for fruit yogurt, mix with fruit when the product is cooled to 4 ⬚C) fig0001 0010 0011 Figure 1 Flow diagram for yogurt manufacture. followed by fat and whey proteins. Stabilizers can also influence the consistency of yogurt. In practice, gelatin, starch, vegetable gums, carrageenan, and pectin are used most widely as stabilizers for yogurt. The best yogurt texture is achieved by using gelatin at 0.3–0.8%. Good yogurt can be made without the use of added stabilizers, but yogurt without a stabilizer is more vulnerable to a number of stress factors than one that has not been stabilized. When properly chosen and used, stabilizers play an important role in improving the body, texture, mouth feel, and appearance of yogurt. The fat content may vary from 0.5 to 3.5%. Milk fat also tends to ‘mask’ the acid flavor of yogurt. Obviously, when milk fat is incorporated into the mix formulation, homogenization becomes important for the overall texture quality of yogurt. Sweeteners are added to plain yogurts, generally with fruits. Addition of sugar is a method of cutting the sharpness of yogurt flavor. An appropriate level of sugar should be used to mask the full degree of acidity, but enough sugar should remain in the product after fermentation for a desirable acid–sugar blend. Up to 10% sucrose is usually used when fruit is added to yogurt. According to Ravula and Shah, a mix containing 4% or more sucrose, may reduce acid production and lower cell counts of both microbial species when incorporated in the mix prior to fermentation. Nonnutritive or intense sweeteners such as aspartame (NutraSweet) may find favor among diet conscious consumers. Since aspartame is approximately 200 times sweeter than sucrose, only a small amount is required for desirable sweetness. Because of this, yogurts containing intense sweeteners require a bulking agent such as polydextrose. As yogurt mix is prepared, particular attention should be given to blending, homogenization, and heat treatment of the mix. The blending and homogenization steps are important to the uniformity of ingredient distribution. Homogenization is usually carried out before heat treatment, but in some cases, it may take place after the heat treatment. Homogenization Homogenization is the typical industrial process used to effect stabilization of the lipid phase against separation by gravity. The specific gravity of milk fat is 0.9, whereas that of skim milk is 1.036. Fat, being lighter, tends to separate from the serum or skim milk if left undisturbed. The diameter of the fat globules in milk ranges from 1 to 15 mm, with an average of 3–4 mm. Homogenization reduces the average diameter of fat globules to 1 or 2 mm. As a result, the fat globules do not cream during the incubation of yogurt. Because of the size reduction, there is usually a four-to sixfold increase in the surface area. The fat-globule membrane protects the fat globules from lipase. Upon homogenization, the fat-globule membrane is destroyed, and the fat globule is vulnerable to attack by lipase, which is naturally present in milk. Because of this, the milk is pasteurized to inactivate lipase before homogenization. If the milk is to be homogenized before pasteurization, the milk must be pasteurized immediately to prevent lipolysis. Since the efficiency of homogenization is much better at higher temperatures, the mix is warmed to around 65 C to liquefy fat and then forced at high pressure (1.8 104 kPa) through a small orifice to reduce the size of the fat globules. 0012 0013 0014 Heat Treatment Yogurt mixes are heated to a high temperature, typically 85 C for 30 min. Such a high heat treatment has many objectives: (1) to destroy all pathogenic bacteria, (2) to inactivate all the enzymes that may be 0015 YOGURT/The Product and its Manufacture 0016 present in milk, including lipase, (3) to destroy most of the spoilage-causing bacteria, including thermodurics, and, most importantly, (4) to denature whey proteins, b-lactoglobulin and a-lactalbumin. This heat treatment denatures more than 90% of the b-lactoglobulin compared with 60% of the a-lactoglobulin. A complex is formed between k-casein and denatured b-lactoglobulin, which increases the hydrophilic properties of the casein, reduces the propensity of the gel to syneresis, and facilitates the formation of a stable coagulum. The hydration effect of the protein is maximal when milk is heated at 85 C but decreases as the temperature is raised above 85 C. A higher heat treatment decreases the hydrophilic properties of the b-lactoglobulin–kcasein complex. As a result, syneresis occurs, and the structure of the yogurt gel becomes weak and fragile. After heat treatment and complex formation between b-lactoglobulin and k-casein, the heated milk forms a smooth gel-like structure when the pH drops during fermentation to 4.6, which is the isoelectric point of casein (the pH at which casein proteins carry no electrical charge). Inoculation and Incubation 0017 0018 0019 After pasteurization, the mix is cooled to 45 C and inoculated at a level that varies from 0.5 to 5%. The former level (0.5–1%) may be susceptible to growth conditions and a longer incubation time. The maximum amount recommended is 5%. This level will cause very rapid acid production, but leads to defects in aroma, and a large amount of culture must be prepared. The optimum level is 2%, 1% each of Lb. delbrueckii ssp. bulgaricus and Sc. thermophilus. Lb. delbrueckii ssp. bulgaricus hydrolyzes milk proteins, the caseins, thus releasing essential amino acids, including valine, which stimulate the growth of Sc. thermophilus. Initially, Sc. thermophilus grows rapidly, reducing the pH to around 5.4, which stimulates the growth of Lb. delbrueckii ssp. bulgaricus, which is acid-tolerant and produces large amounts of lactic acid, which reduces the pH. Sc. thermophilus uses oxygen during its growth, which makes oxidation–reduction potential more favorable for Lb. delbrueckii ssp. bulgaricus; it also produces formic acid, which stimulates the growth of the lactobacillus. The starter bacterial cells are distributed in the inoculated mix by stirring for 10–15 minutes after the addition of the cultures. This is followed by dispensing the mixes into consumer-sized containers and incubating at 42 C for approximately 4 h until the pH decreases to 4.5. For the manufacture of stirred 6255 yogurt, the inoculated mix is incubated in bulk, and when the pH reaches the desired value, the yogurt is stirred, cooled, mixed with various flavorings, if required, and filled into retail containers. The incubation temperature of 42 C is optimal for the associated growth; the optimum temperature of the growth of Sc. thermophilus alone is 37 C, and that for Lb. delbrueckii ssp. bulgaricus is 45 C. Incubation temperatures above 42 C will promote the growth of the Lactobacillus, whereas an incubation temperature of less than 42 C will favor the growth of Sc. thermophilus, and lack of flavor results due to the poor growth of Lb. delbrueckii ssp. bulgaricus. Deviation in either direction will lead to a disturbance in the ratio of lactobacilli to streptococci. The ratio of lactobacilli and streptococci in natural yogurt for optimum flavor should be 1:1. By using monocultures, this ratio of 1:1 can be maintained easily by adjusting the inoculum level of both Lb. delbrueckii ssp. bulgaricus and Sc. thermophilus. When a 2% inoculum is used at an incubation temperature of 42 C, the milk will coagulate, and a firm gel will form in 3.5–4 h, and the pH will decrease to 4.4–4.5. When frozen concentrated cultures are used, an incubation period of 5–8 h may be required. Similarly, freeze-dried cultures may require longer times than fresh and active cultures. A new trend in yogurt making is to incubate the yogurt mix at a significantly lower temperature than normal for a longer period. One of the advantages is that products require less starter cultures and shorter cooling times. An incubation temperature of 30 C with 0.25% starter culture may take 12–14 h. If the incubation temperature is too low, the growth of yogurt starter may be affected adversely. The final acidity of yogurt is 0.9–1%. The US standard requires a titratable acidity of 0.9% or higher, whereas the Australian standard requires a pH of 4.5 or lower. According to IDF, the minimum acidity of yogurt should be 0.7%. The yogurt cultures produce acetaldehyde, which gives natural yogurt its typical flavor that resembles that of green apples. Other volatile compounds include acetic acid and diacetyl. 0020 0021 0022 0023 0024 Cooling Once the desired acidity has been reached, the product is cooled to <10 C as quickly as possible. In the case of stirred yogurt, in one-phase cooling, the product is cooled from the incubation temperature to <10 C prior to the addition of flavoring material and filling. In two-phase cooling, the temperature of the product is reduced to 15–20 C during the first phase cooling before addition of flavoring materials and filling of containers followed by the second stage 0025 6256 YOGURT/The Product and its Manufacture 0026 0027 cooling to <10 C in a cold store. Yogurt quality may be improved by packaging at 24 C followed by final cooling of the product in the container. To maximize the effect, the second stage of cooling should be carried out as slowly as possible over a 12-h period. The viscosity of yogurt improves during storage for 1–2 days. During the first 24–48 h of cold storage, an improvement in the physical characteristics of the coagulum is observed, mainly hydration and/or stabilization of casein micelles. Proper hydration is required to avoid syneresis. It is therefore important to delay the sale or distribution of yogurt for 24–48 h. During cold storage, it is important to minimize rough mechanical handling of the packaged yogurt, and to maintain the temperature of <5 C. During transport, especially in summer, shaking of yogurt can lead to a reduction in viscosity and syneresis. Flavored yogurts 0028 0029 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. Frozen yogurt 0030 Frozen yogurt is a fermented milk or yogurt mix containing a typical yogurt flavor and has been subjected to fermentation by the two yogurt bacteria to a pH of 4.5 or lower or a titratable acidity of 0.9% or higher, expressed as lactic acid. Frozen yogurts, frozen flavored yogurt desserts, yogurt frozen on a stick, or frozen fermented dairy desserts are relatively new products that were developed in the 1960s. Frozen yogurts are prepared in a similar fashion as icecream, and therefore are made most conveniently in icecream factories. Low-lactose Yogurt 0031 Lactose malabsorbers do not produce sufficient lactase (b-d-galactosidase) and thus cannot hydrolyze the ingested lactose completely. In addition to the gastrointestinal discomfort, such as stomach upset and diarrhea brought about by ingestion of milk by these individuals, a general impairment in the normal digestion process has been observed. Low-lactose or lactose-free milk for lactose sensitive individuals can be prepared either by the physical removal of lactose by ultrafiltration or by hydrolysis of lactose into the corresponding monosaccharides, glucose, and galactose. Lactose can be hydrolyzed using strong mineral acid or enzymes. The enzyme b-galactosidase (bd-galactoside galactohydrolase, EC 3.2.1.23), commonly called lactase, catalyzes the hydrolysis of the b-1 ! 4-galactosidic linkage present in lactose. Lactase has been isolated commercially from the fungi, Aspergillus niger, A. oryzae, and A. flavus, or the yeast Saccharomyces lactis or from the bacterium Escherichia coli. Yogurt bacteria (Lb. delbrueckii ssp. bulgaricus and Sc. thermophilus) possess the highest amount of b-galactosidase activity among the lactic acid bacteria. b-Galactosidase from Saccharomyces (known as neutral lactase) has an optimum activity at pH 6.8– 7.0, is stable in the pH range of 6.0–8.5, and works best at 35 C. It is suitable for treating milk (pH 6.6) or sweet whey (pH 6.2), but the lack of stability below pH 6.0 precludes its use for treating acid whey (pH 4.5). A. niger lactase (also known as acid lactase), with a pH optimum of 4.0–4.5, good stability over a wide range of pH (3.0–7.0), and an optimum temperature of 55 C, is suitable for the modification of acid whey. Low-lactose yogurt is then produced by the use of low-lactose milk during processing. 0032 0033 0034 0035 Heated or Pasteurized Yogurt Even though natural yogurt is held at 4 C, the titratable acidity increases due to residual activity of the starter bacteria. This process is referred in the industry as ‘post-acidification,’ as a result of which the product becomes too tart. Without heat treatment, the shelf-life of yogurt is 4–5 weeks at 4 C. Heating destroys most of the starter bacteria and yeast and molds, and if post-processing contamination is avoided, the shelf-life of the product can be extended to 8 weeks. No postacidification occurs in heated yogurt. Heated or pasteurized yogurt can be prepared by heat-treating yogurt in the package at about 55 C for 30 min, followed by cooling. The main problems associated with pasteurized yogurt are loss of flavor and syneresis. Stabilizers can be used to overcome the latter problem. Controversies still exist regarding the heat treatment of yogurt after fermentation as heat-treated yogurt contains very few or no viable yogurt bacteria. It also raises questions about the definition of yogurt, 0036 0037 YOGURT/The Product and its Manufacture which must contain an abundant viable count of Sc. thermophilus and Lb. delbrueckii ssp. bulgaricus. Yogurt is considered to be an excellent nutritional food, and consumers ingest several million live cells through a typical serving of yogurt. Prebiotics, Probiotics, and Synbiotics 0038 0039 0040 The term ‘probiotic’ is derived from two Greek words meaning ‘for life.’ Probiotic organisms and substances produced by these organisms contribute to the microbial balance in the intestines. The generally accepted definition of probiotics is that ‘they are live microbial food or feed supplements that provide a beneficial effect on hosts (human or animal) by improving the microbial balance in the intestine.’ The intake of these bacteria is reported to help restore the balance in the intestinal microflora, which may have been lost due to stress, antibiotic use, or illness. The major strains of bacteria used in probiotics, Lb. acidophilus and Bifidobacterium spp., are dominant organisms of human large intestines. These microorganisms are claimed to inhibit the growth of pathogenic organisms through the production of organic acids and bacteriocins. Other benefits include reduction in lactose malabsorption, suppression of potentially harmful enzymes, increased immune response due to increased production of secretory immunoglobulins A, reduction in serum cholesterol, and antimutagenic effects. The beneficial effects of the presence of bifidobacteria in the gastrointestinal tract are dependent on their viability and metabolic activity. The growth of bifidobacteria is dependent on the presence of complex carbohydrates such as oligosaccharides and other substrates such as N-acetyl glucosamine and lactulose. These carbohydrates that stimulate the growth of bifidobacteria are known as ‘bifidogenic factors.’ Some oligosaccharides, due to their chemical structure, are resistant to digestive enzymes and therefore pass into the large intestine, where they become available for fermentation by bifidobacteria. Compounds that are either partially degraded or not degraded by the host and are preferentially utilized by bifidobacteria as carbon and energy sources are defined as ‘prebiotics.’ Some of the bifidogenic factors that are of commercial significance include fructo-oligosaccharides, lactose derivatives (such as lactulose, galacto-oligosaccharides), isomalto-oligosaccharides, xylo-oligosaccharides, gluco-oligosaccharides and soybean oligosaccharides. Resistant starch and nonstarch polysaccharides are classified as colonic foods but not as prebiotics because they are not metabolized by certain beneficial bacteria. Oligosaccharides have been recognized for their 6257 health benefits in Japan, and many products containing oligosaccharides were developed during the 1990s. Products that contain both prebiotics and probiotics are referred to as ‘synbiotics.’ An example of synbiotic includes SymBalance yogurt, which contains inulin as prebiotic and Lb. reuteri, Lb. acidophilus, and Lb. casei as probiotics. It is not clear how probiotics work. Acidification of the gut is claimed to be one of the mechanisms. Breast-fed infants have a much higher percentage of Bifidobacterium bifidum than formula-fed infants. The intestinal microflora of the latter group is more like that of adults; it is a mixed microflora, including coliform bacteria. Bifidobacteria produce acetic acid, butyric acid, lactic acid, and pyruvic acid. The lactic acid and acetic acid account for more than 90% of organic acids produced. It is widely accepted that because of acid production by Lb. acidophilus and bifidobacteria, the enteropathogenic bacteria are unable to grow. The growth of clostridia and E. coli, when cocultured with bifidobacteria, has been found to be inhibited, even at a neutral pH, suggesting that acid production may not be solely responsible for inhibition. Metabolites produced by bifidobacteria may be partly responsible for the inhibition of pathogens. 0041 Acidophilus and Bifidus Yogurt (AB Yogurt) Lb. acidophilus and bifidobacteria are normal inhabitants of the intestine of many animals including man. Lb. acidophilus is Gram positive and rod-shaped, while bifidobacteria are Gram-positive rods of variable morphology that show branching and pleomorphism. Bifidobacteria were first isolated by Tissier at the Pasteur Institute, Paris, France, and predominate the gut flora in breast-fed infants. Yogurt containing Lb. acidophilus and bifidobacteria has gained popularity in many countries, including Japan, France, Germany, Canada, Australia, and USA, and more than 70 products containing Lb. acidophilus and bifidobacteria, including sour cream, buttermilk, yogurt, milk powder, and frozen desserts, are produced world-wide. It is estimated that about 11% of all yogurt sold in France now contains Lb. acidophilus and Bifidobacterium spp. In 1978, the Yakult Co. launched a Bifidus fluid yogurt named MilmilTM, which contains Bifidobacterium breve, B. bifidum and Lb. acidophilus. More than 54 different types of milk products containing Lb. acidophilus and Bifidobacterium bifidum are marketed in Japan. Lb. acidophilus and Bifidobacterium spp. are difficult to propagate because of their specific nutritional requirements. Bifidobacteria are not as acidtolerant as Lb. acidophilus, and the growth of 0042 0043 0044 6258 YOGURT/The Product and its Manufacture Bifidobacterium species is significantly retarded below pH 4.0. Lb. acidophilus and Bifidobacterium spp. are slow acid producers; the slow growth rate of these organisms can be compensated by adding a higher level of inoculum, such as 5 or 10%. Yogurt bacteria are usually added to carry out fermentation. If pure cultures of Lb. acidophilus and/or Bifidobacterium spp. are used, the time required to reduce the pH of milk to 4.5 could be as long as 18–24 h at 37 C. When yogurt bacteria (Lb. delbruecckii ssp. bulgaricus and Sc. thermophilus) and AB (Lb. acidophilus and Bifidobacterium spp.) cultures are used, the incubation time is about 4 h. Recent Advances in Probiotic Yogurt 0045 0046 0047 0048 The most commonly used species in commercial probiotic products are Lb. acidophilus, Lb. casei, Lactobacillus GG (a close relative of Lb. casei subgroup rhamnosus, ATCC 53103), B. bifidum, B. longum, B. breve, and B. infantis. Additional blends are also being investigated, such as Lb. reuteri, Lb. plantarum, and Lb. casei. In Australia and Europe, yogurt containing Lb. acidophilus and Bifidobacterium spp. is referred to as AB yogurt. The recent trend is to incorporate Lb. casei in addition to Lb. acidophilus and Bifidobacterium spp, and such products are known as ‘ABC yogurt.’ A wide variety of probiotic cultured products are now available world-wide such as Nu-Trish a/B, USA (acidophilus þ bifidus), AB-yogurt, Denmark (acidophilus þ bifidobacteria þ yogurt culture), Miru-Miru, Japan (Lb. acidophilus þ Lb. casei þ B. breve), Bifighurt (B. longum þ Sc. thermophilus). Products that contain both prebiotics and probiotics include SymBalance yogurt (Lb. reuteri þ Lb. acidophilus þ Lb. casei þ bifidobacteria, inulin) produced in Switzerland and Fysiq (Lb. acidophilus þ Raftiline brand prebiotic). Several probiotic products contain both yogurt bacteria and one or more types of probiotic bacteria. Because of sensitivity to acid, Lb. acidophilus and Bifidobacterium spp. in yogurt begin to die within a few days after manufacture because of acid produced during manufacture and storage. In order to provide health benefits, the suggested level for probiotic bacteria is 106 cfu per gram of a product. However, studies have shown a low viability of probiotics in market preparations. Many yogurt manufacturers use a starter culture devoid of Lb. delbrueckii ssp. bulgaricus but a combination of Lb. acidophilus, bifidobacteria and Sc. thermophilus (known as ‘ABT’) as starter cultures to overcome the postacidification problem. However, the use of ABT starter culture increases incubation time significantly as Sc. thermophilus is the main organism responsible for fermentation in ABT starter cultures, and this organism is less proteolytic than Lb. delbrueckii ssp. bulgaricus. Bifidobacteria are anaerobic and are fastidious organisms, requiring specific growth factors and prefering a low oxidation–reduction potential for growth. Cysteine is usually incorporated in the media used for selective enumeration of bifidobacteria. Cysteine is an essential growth factor for bifidobacteria and also reduces the oxidation–reduction potential for optimum growth of anaerobic bifidobacteria. Dave and Shah have shown a better viability of bifidobacteria, in yogurt made from ABT starter cultures, with an additional source of nitrogen, such as acid casein hydrolysate, than with lowering the oxidation– reduction potential with cysteine. These authors have also shown that incorporation of cysteine (at > 250 mg l1) to lower the redox potential affected the growth of Sc. thermophilus, as this organism is considered aerobic. Since Sc. thermophilus is the sole fermenting organism in ABT starter cultures, the incubation time increased significantly. The viability of bifidobacteria in yogurt made with starter cultures containing both yogurt bacteria was not affected as much, as Lb. delbrueckii ssp. bulgaricus is proteolytic and provides peptides and amino acids for the growth of bifidobacteria. See also: Bifidobacteria in Foods; Casein and Caseinates: Uses in the Food Industry; Heat Treatment: Chemical and Microbiological Changes; Homogenization; Lactic Acid Bacteria; Pasteurization: Principles; Prebiotics; Probiotics; Starter Cultures; Sweeteners: Intensive; Others; Yogurt: Yogurt-based Products; Dietary Importance Further Reading Augustin MA, Cheng LJ and Clarke PT (1999) Effects of preheat treatment of milk powder on the properties of reconstituted set skim milk yogurts. International Dairy Journal 9: 415–416. Dave RI and Shah NP (1997a) Viability of yoghurt and probiotic bacteria in yoghurts made from commercial starter cultures. International Dairy Journal 7: 31–41. Dave RI and Shah NP (1997b) Effect of cysteine on the viability of yoghurt and probiotic bacteria in yoghurt made with commerical starter cultures. International Dairy Journal 7: 537–545. Dave RI and Shah NP (1998) Ingredient supplementation effects on viability of probiotic bacteria in yoghurt. Journal of Dairy Science 81: 2804–2816. Guzman-Gonzalez M, Morais F and Amigo L (2000) Influence of skimmed milk concentrate replacement by dry dairy products in a low-fat set yoghurt model system. 0049 0050 YOGURT/Yogurt-based Products Journal of the Science of Food and Agriculture 80: 433–438. Hekmat S and McMahon DJ (1997) Manufacture and quality of iron-fortified yogurt. Journal of Dairy Science 80: 3114–3122. IDF (1996) Oligosaccharides and Probiotic Bacteria. Bulletin 313. Brussels: International Dairy Federation. Kosikowski FV and Mistry VV (1997) Cheese and Fermented Milk Foods. Westport, CT: FV Kosikowski. Kurmann JA and Rasic JLJ (1991) Health potential of products containing bifidobacteria. In: Robinson RK (ed.) Health Potential of Products Containing Bifidobacteria: Therapeutic Properties of Fermented Milks. London: Elsevier Applied Science. Mazza G (ed.) (1998) Functional Foods: Biochemical and Processing Aspects. Lancaster, PA: Technomic Publication. Modler WH (1994) Bifidogenic factors – sources, metabolism and applications. International Dairy Journal 4: 383–407. Ordonez GA, Jeon IJ and Roberts HA (2000) Manufacture of frozen yogurt with ultrafiltered milk and probiotic lactic acid bacteria. Journal of Food Processing and Preservation 24: 163–176. Ravula RR and Shah NP (2000) Influence of water activity on fermentation, organic acid production and viability of yoghurt and probiotic bacteria. Australian Journal of Dairy Technology 55: 127–131. Robinson RK (ed.) (1986) Modern Dairy Technology, vol. II. Advances in Milk Products. London: Elsevier Applied Science. Shah NP (2000) Probiotic bacteria: selective enumeration and survival in dairy foods. Journal of Dairy Science 83: 894–907. Shah NP, Lankaputhra WEV, Britz M and Kyle WSA (1995) Survival of L. acidophilus and Bifidobacterium bifidum in commercial yoghurt during refrigerated storage. International Dairy Journal 5: 515–521. Tamime AY and Deeth HC (1980) Yoghurt: technology and biochemistry. Journal of Food Protection 43: 939–977. Tamine AY and Robinson RK (1999) Yoghurt: Science and Technology. Cambridge: Woodhead Publishing Ltd. Yogurt-based Products A Y Tamime, Ayr, UK Copyright 2003, Elsevier Science Ltd. All Rights Reserved. Introduction 0001 The transformation of milk into yogurt, with a starter culture composed of Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus, can easily extend the shelf-life of milk from a few days to about 3 weeks. However, the starter cultures employed in the manufacture of biofermented milks including 6259 yogurt-related products have been reviewed recently. (See Yogurt: The Product and its Manufacture.) The main organisms belong to the following genera: Lactobacillus, Bifidobacterium, Enterococcus, and Pediococcus. The application of the different preservation techniques known to humans for thousands of years could extend the shelf-life of yogurt to a few months, or even indefinitely. Examples of such postfermentation processes include heating, concentration, freezing, and/or drying, but it is evident that such treatments will alter the characteristics of the end product. Some of these processes are still carried out using traditional methods, but limited data are available in the literature; however, some of these processes have been mechanized and developed by industrial organizations, and as a consequence, sometechnical data are somewhat limited. (See Starter Cultures.) Methods of Manufacture Pasteurization Traditionally, natural yogurt in rural areas in the Middle East is heated gently for a few hours over low fires using a special wood, and the product is known as ‘laban mudakhan’ or ‘smoked yoghurt.’ The application of heat inactivates the starter culture organisms and their enzymes, as well as other contaminants, e.g., undesirable bacteria, yeasts, and molds. The use of such methods enables the nomads in that region to extend the shelf-life of yogurt for a few weeks, or until they reach a market to sell their product. Alternatively, the hot yogurt is placed in a clean jar and covered with a layer of olive oil or tallow, so that the ‘smoked yogurt’ is preserved over the winter months. In mechanized plants, natural/plain, fruit, flavored, or drinking yogurts are subjected to heat treatment after the fermentation stage to prolong the shelf-life of the product. The time–temperature relationships, which are used to achieve this objective, are dependent on: (1) the level of acidity, (2) the method of heating and packaging, and (3) the storage conditions. Set-type yogurt is heated between 60 and 85 C for up to 50 min, and in some instances, the heating is carried out under a pressure of about 0.2 MPa. The other types of yogurt are pasteurized/ultrahigh temperature (UHT), which is heated at 65 to > 100 C for up to 50 s. In general, the heating and packaging methods can be divided into the categories of pasteurized/UHT yogurts given in Table 1. (See Heat Treatment: Ultrahigh Temperature (UHT) Treatments.) Postfermentation heating of yogurt causes the separation of the aqueous phase from the suspended casein particles. This is mainly due to the 0002 0003 0004 本文献由“学霸图书馆-文献云下载”收集自网络,仅供学习交流使用。 学霸图书馆(www.xuebalib.com)是一个“整合众多图书馆数据库资源, 提供一站式文献检索和下载服务”的24 小时在线不限IP 图书馆。 图书馆致力于便利、促进学习与科研,提供最强文献下载服务。 图书馆导航: 图书馆首页 文献云下载 图书馆入口 外文数据库大全 疑难文献辅助工具