RESUMEN DE LAS ACTIVIDADES DE SUPERVISIÓN REALIZADAS EN EL PERÍODO

Fresh milk is the major ingredient in yogurt manufac- ture. Its chemical composition fluctuates depending on various factors as species, breed, and season of the year. This can affect the fermentation, as well as the properties of the yogurts.

Effect of the Species of Mammals on Yogurt Properties

The gross composition of the milk of different species of mammals used for manufacture of fermented milks is given in Table 10. 2. The dry matter is very differ- ent according to the species, from 18.8% for sheep milk to 10.8% for mare’s milk. Carbohydrate con- tent ranges from 4.6% to 6%. They are in excess for fermentation, so their variation does not affect the acidification of the milk. However, naturally present inhibitory substances in milk can affect the rate of acidification. It has been reported that camel milk

exhibits a slower acidification rate than in cow, sheep, or goat milk (Fig.10.1). This could be due to a higher concentration of lysozyme in camel milk as com- pared to the other milks (El-Agamy, 2000).

The texture and the flavor of the fermented milks are dependent on the protein and fat content, which shows strong differences according to the species. Sheep and buffalo milk exhibit very high fat con- tent, more than 7%; whereas horse milk contains less than 2% fat. Sheep milk has the highest protein con- tent (4.6%), and mare’s milk has the lowest (1.3%). This leads to differences in the quality of the yo- gurts. For example, a yogurt from sheep or buffalo milk will present a creamy texture and a buttery fla- vor associated with the high fat content (Aneja, 1991; Anifantakis, 1991). If the milk is not homogenized, a layer of cream will occur in the manufacture of set- type yogurt (Anifantakis, 1991). Yogurt from sheep milk, because of the high protein content, does not require milk fortification (Muir and Tamime, 1993). On the other hand, a yogurt from mare’s milk will have a very thin texture, and the blending with cow or sheep milk, or the addition of caseinates or thick- eners, is recommended to afford a convenient texture (Di Cagno et al., 2004). Figure 10.2 illustrates the ef- fect of milk source (cow, sheep, and goat) on yogurt viscosity and syneresis.

Finally, the content of minor components also has an impact on the yogurt flavor. For example, a “goaty”

Table 10.1b. Heat Classification of Nonfat Dry Milk

Classification Whey Protein Nitrogen Index (mg/g) Solubility Application

High heat <1.5 Least Baked goods, meats confections

Medium heat 1.51–5.99 Average Ice cream

Low heat >6.0 Most Recombined milk dairy products,

Used in Yogurt and Fermented Milk Manufacture

Specie Dry Matter Fat Casein Whey Protein Lactose Ash

Sheep 18.8 7.5 4.6 1 4.6 1 Buffalo 17.5 7.5 3.6 0.7 4.8 0.8 Camel 13.4 4.5 2.7 0.9 4.5 0.8 Goat 13.3 4.5 3 0.6 4.3 0.8 Cow 12.7 3.9 2.6 0.6 4.6 0.7 Horse 10.8 1.7 1.3 1.2 6.0 0.5

Source: After Walstra et al., 1999.

Figure 10.1. Effect of milk source on yogurt acidification. ——, cow milk; – – – –, sheep milk; ---, goat milk;

——, camel milk. After Jumah et al., 2001.

Figure 10.2. Effect of milk source on yogurt apparent viscosity (A) and syneresis (B). Yogurt viscosity was

measured by Brookfield LV viscometer using spindle n◦3 at 0.6 rpm. Yogurt syneresis was determined by draining 180 mL of yogurt on stretched cheese cloth. , sheep milk; , cow milk;, goat milk. After Kehagias et al., 1986.

flavor noticeable in the yogurt from goat milk, has been associated with a high amount of free fatty acids in goat milk (Abrahamsen and Rysstad, 1991). Ac- etaldehyde is one of the major aromatic components characteristic of yogurt flavor. It is produced mainly from the conversion of threonine into acetaldehyde and glycine during fermentation. Threonine aldolase is the key enzyme in this conversion. Low amount of acetaldehyde observed in goat milk yogurt has been attributed to high amount of free glycine in goat milk, which causes a feedback inhibition of the threonine aldolase (Rysstad et al., 1990). The addition of threo- nine to milk has been recommended in the manufac- ture of yogurt from goat milk (Rysstad et al., 1990), as well as mare’s milk (Di Cagno et al., 2004,) in order to improve yogurt flavor.

Effect of the Breed and Genetic Variant on Yogurt Properties

Table 10.3 shows the gross composition of milk pro- duced by four breeds of cows, Friesian, Holstein, Brown, and Jersey. Fat and protein content varies de- pending on the breed. This affects the textural prop- erties of the yogurt resulting due to the relationship between protein content in milk and yogurt viscos- ity (Schkoda et al., 2001b). For example, Allmere et al. (1999) observed a strong difference in elastic modulus for yogurts made with milk from individual cows from two breeding selection lines. An increase of 40% was observed with yogurt made with milk from one selection line as compared to the other. This was correlated to a difference in protein con- tent (3.71% versus 3.37%). Furthermore, an effect of the genetic variants on the physical properties of yo- gurt has been demonstrated for␤-lactoglobulin and ␬-casein variants. Allmere et al. (1998) and Bikker et al. (2000) reported higher elastic modulus with milk gels containing␤-lactoglobulin B and C than the ones containing␤-lactoglobulin A. For instance, Allmere et al. (1998) observed a 30% higher storage modulus in acidified milk gels containing only the B

Table 10.3. Approximate Average Composition (%, w/w) of Milk of Different Breeds of Cow

Breed Dry Matter Fat Crude Protein Lactose Ash

Friesian (in the Netherlands) 13.3 4.4 3.4 4.6 0.75

Holstein (in the US) 12.1 3.4 3.3 4.5 0.75

Brown Swiss 12.9 4.0 3.3 4.7 0.72

Jersey 15.1 5.3 4.0 4.9 0.72

Source: After Walstra et al., 1999.

variant of␤-lactoglobulin compared with those con- taining only the A variant. The use of milk containing ␬–casein variant AA or BB does not affect the vis- cosity or the texture of yogurts (Allmere et al., 1998; Muir et al., 1997). However, Muir et al. (1997) ob- served that the serum leakage was lower for yogurts made from milk with the␬-casein variant AA than yogurts containing the␬-casein variant BB. Effect of the Seasonal Variation in Milk Composition on Yogurt Properties

The composition of milk can vary across the seasons. For instance, approximately a 10% variation in fat and protein is observed in milk received in July and August (lowest level) compared to that received in October and November (highest level) in the United States (Chandan, 1997). These variations of compo- sition are known to affect the consistency and the quality of the manufactured dairy products. Seasonal variation of sheep milk in Scotland has been shown to change viscosity, serum separation, and acidity in yogurts (Muir and Tamime, 1993). Seasonal variation of cow milk in Australia has been reported to affect the viscosity and serum separation in both set and stirred yogurts (Cheng et al., 2002). Standardization of the protein content by addition of milk protein in various forms (powders or concentrates, fractionated or whole milk protein) reduces the effects of milk seasonality in yogurt manufacture.

Cream

Yogurt can have a fat content ranging from 0% to 10%, with most common values comprised between 0.5% and 3.5% (Tamime and Robinson, 1999). In the United States, regulations distinguish three types of yogurts: regular yogurts (more than 3.25% milkfat), low-fat yogurts (between 0.5% and 2% milkfat), and non-fat yogurts (less than 0.5% milkfat).

The effect of cream addition on yogurt texture is linked to the integration of the fat globules into the

Figure 10.3. Simplified structure of full fat yogurt issued from homogenized milk. Note that fat globules are

integrated into the gel structure. After Schkoda et al., 2001a.

gel structure. This integration does not occur if the cream is added after the fermentation (Schkoda et al., 2001a), or if the milk is not homogenized (van Vliet and Dentener-Kikkert, 1982). In this case, addition of cream decreases the viscosity of the yogurt, because milk fat globules act as “structure breakers” (Schkoda et al., 2001a; van Vliet and Dentener-Kikkert, 1982). On the other hand, when the cream is added in milk before fermentation, and when milk is then submit- ted to homogenization before inoculation with starter culture and acidified, which is the usual practice in yogurt manufacture, the addition of milk fat increases the yogurt viscosity and firmness, and decreases the serum separation. For instance, Martens (1972) re- ported an increase of 44% in the consistency score of stirred yogurt when the fat content varied from 0% to 3.9%. Becker and Puhan (1989) found that gel firmness was increased by 23% in whole milk yogurt (3.5% fats) compared to nonfat yogurt. De Lorenzi et al. (1995) observed a higher (23%) apparent vis- cosity at 100 s−1in full-fat yogurts (4% fat content) as compared to a nonfat yogurt. Finally, Becker and Puhan (1989) observed that yogurts made from whole milk did not show any whey separation, while in 63 nonfat yogurt samples, 15 showed a whey layer on the surface after 14 days of storage.

The effect of cream addition on yogurt physical properties can be explained by the integration of the milk fat globules in the gel network. During homog- enization, the native milk fat globule membrane is

removed and a new membrane is formed, which sta- bilizes the homogenized fat globules. The new layer covering the fat globules is predominantly composed of micellar casein. Cano Ruiz and Richter (1997) de- termined the percentage distribution of proteins in the milk fat globule membrane of homogenized milk and found a repartition between caseins, whey protein, and proteins from native membrane equal to 67%, 10%, and 13%, respectively. The new layer of the milk fat globules interacts with the casein micelles during acidification (Barrantes et al., 1996; Lucey et al., 1998) and acts as a “structure promoters” in this case (van Vliet and Dentener-Kikkert, 1982), as reported in Figure 10. 3.