Melese Temesgen, PhD fellow. Addis Ababa University Centre for Food science and nutrition,P.O.Box
Milk is a major component of the traditional diet in many regions of Africa. In these communities, most of the milk produced is consumed in the home and is rarely sold. However, high temperatures and lack of refrigeration facilities have led to the inability to process and store fresh milk (Ogwaro et al., 2002). Hence, conversion of any surplus liquid milk to relatively shelf stable products such as yoghurt, cheese, acidified milk, butter, and ghee has traditionally been done at the household level. Acidification of milk by fermentation is an old method of milk preservation. There are different methods of carrying out fermentation in various part of the world and these give rise to a range of fermented milk products including Kumiss, Kefir, acidophilus milk and yoghurt (Tamime and Deeth, 1980).
Product quality and consumer satisfaction are important for increasing the demand of various types of yoghurt. The increase in the per capita annual consumption of yoghurt in the majority of the countries has been attributed to increased knowledge regarding the health benefits of yoghurt, the ever-increasing availability of fruit or flavored yoghurt, and the diversity of presentations of the product (Kucukoner and Tarakci, 2003; Van de Water and Naiyanetr, 2008). The healthy food image of yoghurt is due to the probiotic effect of yoghurt bacteria.
According to Guarner et al. (2005) yoghurt bacteria are considered as “probiotics”, (i.e., live microorganisms which when administered in adequate amounts confer a health benefit to the host). The health promoting properties of live lactic acid bacteria in yoghurt include protection against gastrointestinal upsets, enhanced digestion of lactose by maldigesters, decreased risk of cancer, lower blood cholesterol, improved immune response and help the body assimilate protein, calcium and iron (Perdigon et al., 1998; Van de Water and Naiyanetr, 2008).
Background of yoghurt
Yogurt is derived from Turkish word “Jugurt “reserved for any fermented food with acidic test, Yogurt in different forms with appropriate local names is made throughout the world, in Botswana it is called Madila, Lesotho Amasi, Namibia Omashikwa, Omaze Uozongombe, Zambia MabisiSawa and in Ethiopia Ergo (Mburude Wagt,2011). Currently yogurt of many types including kefir, Greek style yogurt, Swiss and fruit yogurts can be found. Yogurt is one of the most popular fermented dairy product widely consumed all over the world. It’s obtained by lactic acid fermentation of milk by the action of a starter culture containing Streptococcus thermophilus and Lactobacillus delbrueckii ssp. Bulgaricus (Fadela et al., 2009) and Ergo a traditional femented milk in Ethiopia is semi-solid, smooth and uniform appearance and usually has a white milk color with pleasant odor and taste when prepared carefully. It constitutes primarily sour milk from which other products may be processed (Bereda et al., 2014). Yogurt has assumed different forms like stirred, set and frozen liquid yogurt (Anjum et al., 2007). The nutritive value of yogurt is attributed to the fat content, sugar and casein (El-Malt et al., 2013). Proteins in yogurt are of excellent biological quality, as are that in milk, because the nutritional value of milk proteins is well preserved during the fermentation process. Therefore, yogurt is recommended for sick and convalescent people (Ebringer et a l., 2008).
Proteins in yogurt are of excellent biological quality, as are that in milk, because the nutritional value of milk proteins is well preserved during the fermentation process (Gursoy et al., 2010). It has been argued that protein from yogurt is more easily digested than is protein from milk, as bacterial predigestion of milk proteins in yogurt may occur (FAO, 2013). Both the caseins and the whey proteins in yogurt are rich source of Amino acids (93%) and high in nitrogen availability. Amino acids like proline and glycine are present in free form and higher contents in yogurt than in milk (Damian, 2013). Although much of the yogurt sold in industrialized countries are produced from skimmed milk; traditional yogurt has always contained some 3-4 g per100 g milk fat; indeed, concentrated yogurt or Greek-style yogurts will contain 9-10 g per100 g fat (Tamine and Robinsion ,1999). Conjugated linoleic acid, a type of essential fatty acid found almost exclusively in the fat of dairy products, can be obtained only through the diet because it is not produced by the human body (Miller et al.,2006).
CLA has been shown to be a powerful natural anti-carcinogen that also can reduce the risk for cardiovascular disease, help fight inflammation, reduce body fat, especially abdominal fat, lower cholesterol and triglycerides, increase metabolism, lower insulin resistance and enhance the immune system (Lee and Lucey, 2010). In the case of natural yogurt, a number of mono and disaccharides are present in trace amounts, but lactose remains the dominant sugar in natural yogurt; even after fermentation, the product may contain some 4-5 g per 100 g lactose. The reason for this residue is that the processed milk is often fortified to 14-16 g per 100 g total solids (i.e. up to about 8 g per 100 g lactose), so that the lactose content of the end product is little different from normal milk (Tamine and Robinsion,2000). What is different, however, is the effect that these apparently identical levels of lactose can have on people who are so called lactose intolerant or lactose maldigestors and the nature of this reaction is of considerable medical interest (Damian, 2013).
Yogurt contains appreciable quantities of sodium and potassium which may not be suitable for feeding babies less than 6 months but, as shown in, the mineral salts in milk can be reduced prior to the production of yogurt (FAO, 2013). According to the report made by Mbaeyi (2010), a product called dahi which resembles plain yogurt in appearance and consistency and differ in having lesser acidity,100 g of dahi contains 0.8 g mineral matter, 149 mg Ca, 93 mg P, 102 IU Vitamin A, 49 μg Thiamine, 157 μg Riboflavin, 86 μg Nicotinic acid, 3.2 μg Biotin, 183 μg Pantothenic acid, 178 μg Folic acid and 1.3 mg Ascorbic acid. The cow milk dahi (1-day old) has 0.57 % total nitrogen, 0.497% protein nitrogen, 0.073 % non-protein nitrogen, 0.027% dialyzable nitrogen and 9.79% ammonia nitrogen (Vasiljevic and Shah, 2008)
Yoghurt probiotics, and prebiotics association
Probiotics are dietary supplements or food products that contain beneficial, friendly and good bacteria or yeasts normally found in human body (Roberfroid et al., 2010), and according to the definition of (Gursoy et al., 2010), probiotics are selected viable microorganisms used as dietary supplements having potential for improving health or nutrition of man or animal following ingestion (Mazahreh and Ershidat, 2009). Milk or milk products provide an excellent carrier for these probiotic organisms. Most of them can readily utilize lactose as an energy source for growth (Mckinley, 2005). Thus, an important requirement for the growth in the intestinal tract is provided by the milk. Milk proteins also provide important protection to the probiotic bacteria during passage through the stomach (Pereira et al., 2013).
The primary probiotic bacteria associated with dairy products have been Lactobacillus acidophilus, Lactobacillus casei and bifidobacteria. Species of Lactobacillus and Bifidobacterium are most commonly used as probiotics, but the yeast Saccharomyces cerevisiae and some E. coli and Bacillus species are also used as probiotics (Kun et al., 2008). Lactic acid bacteria, including Lactobacillus species, which have been used for preservation of food by fermentation for thousands of years, can serve a dual function by acting as agents for food fermentation and in addition, potentially imparting health benefits (Mazahreh and Ershidat, 2009).
The science of gelation and Syneresis in yoghurt
Leakage of whey (syneresis) from yoghurt is a common defect and must be controlled. Wheying-off may be indicative of faulty fermentation and off-flavors. Common reasons for the occurrence of whey on cultured products include the use of a high incubation temperature, excessive whey protein to casein ratio, low solids content (protein and fat if the milk is homogenized) and physical mishandling of the product during storage and retail distribution (Lucey, 2002). The type and concentration of added flavoring material and the storage time influence syneresis of yoghurt. Salvador and Fiszman (2004) concluded measured syneresis increased with storage time. Syneresis was observed to increase during storage of fruit flavored yoghurt (Kucukoner and Tarakci, 2003). Similarly, Salwa et al. (2004) showed that as the concentration of carrot juice added into the yoghurt increased the tendency of the resulting product to syneresis increased.
The use of natural stabilizers
In recent days, natural antioxidants, particularly in fruits and vegetables have gained increasing interest among consumers and the scientific community because epidemiological studies have indicated that frequent consumption of natural antioxidants is associated with a lower risk of cardiovascular disease and cancer (Thaipong et al., 2006; Temple, 2000). Thus, supplementation of yoghurt with products derived from fruit and vegetable is gaining the attention of both the processors to increase sales of yoghurt and the consumer for better health in addition to their role as flavoring agents. The defensive effects of natural antioxidants in fruits and vegetables are attributed to three major groups of compounds: vitamins, phenolics, and carotenoids. Ascorbic acid and phenolics are known as hydrophilic antioxidants, while carotenoids are known as lipophilic antioxidants (Halliwell, 1996).
Yogurt is also produced and consumed in flavored and supplemented forms. The flavoring pigment can be done by addition of natural ingredients or by addition of synthetic flavor compounds, adding fruit juices or fruit pulp (Vahedi et al.,2008). Many of these fruits are known as very good sources of anthocyanins, which display a wide range of biological activities including antioxidant, anti-inflammatory, antimicrobial and ant carcinogenic activities (Pereira et al., 2013). Although, in a wide assortment of flavors, typically fruit flavors such as strawberry or blue-berry and more recently cream pie and chocolate flavors are used (Mbaeyi and Anyanwu ,2010). Fruits and vegetables are not only good sources of carotenoids but also an important source provitamin A, i.e., β and α –carotenoids that are an efficient provitamin-As. Beta and α –carotenoids are the predominant carotenoids in fruits and vegetables, which account for 90% of the total carotenoids. Thus, incorporation of fruits and vegetables (juice stabilizers) into yoghurt could contribute to the prevention of diseases (e.g. sight problem) related to vitamin A malnourishment in addition to improving the antioxidant property of the yoghurt.
In summary, researches have indicated the health risk of consuming traditional fermented milk products (e.g., Ergo) which is made from raw milk without any heat treatment. Thus, there is a need to produce fermented dairy products such as yoghurt, which is safe as it is made from heat-treated milk. In addition, yoghurt production is a well-controlled process in which not only safe but also consistent product quality could be assured. However, in our country there is no published information on yoghurt production in such a way that it can be applied under small scale and household level production while safety and product quality are considered.
In addition, development of functional food from dairy products has proven to increase the demand of dairy products in many countries especially in the developed ones. However, there is no published research work in our country regarding development of functional dairy products, which are believed to stimulate the dairy market if in addition nutritional education is provided to the consumer.
Sweetening compounds are usually added into yoghurt either via the addition of fruit concentrate or in the initial milk base and after addition of fruits and flavoring materials, yoghurt may contain anywhere from 4 to 20% sweetener (Wilson et al., 1983). The main reason for addition of sweeteners in to yoghurt is to inhibit the level of acidity produced especially when high acid/low sugar content fruits are added (Staff, 1998). There are different types of sweetening compounds used for yoghurt preparation. They can be grouped into non-nutritional (such as;Saccharin, Cyclamate, Aspartame, Acesulfame-k) and nutritional (Sorbitol, Xylitol, High Fructose Corn Syrup,Glucose and Sucrose). Non-nutritive sweeteners (synthetic sweeteners) are mainly used to impart calorie reduction in fermented milks. Aspartame and Acesulfame-K individually and in combination have been used as sweeteners for the formulation of numerous low-calorie and sugar free yoghurts and cultured milk products (Lotz et al., 1992). Among all types of sweeteners, sucrose (sugar) is the most commonly used sweetener in yoghurt production especially in flavored yoghurt preparation.
While selecting sweeteners for yoghurt production, properties of the sweetener such as; stability to high temperature, the PH range at which the sweetener is stable, solubility in water, concentration at which it gives the desired sensory properties, inhibition of the starter culture, presence of lingering bitter after taste and stability towards microbial degradation by yoghurt starter during storage time should be considered (Pinheiro et al., 2005; King et al., 2000).
King et al. (2000) observed the stability of APM was influenced by pH, time and temperature. Fellows et al. (1991) reported sweetness perception (from Aspartame) decreases over time of storage and this was related to the growth and activity of the starter culture. Hyvoenen and Slotte (1983) investigated the effects of different sweeteners on yoghurt quality and they found that 15% Sorbitol added prior to incubation inhibited culture growth such that no acid, aroma, or coagulation is developed. The same results were obtained when yoghurt was sweetened with 7% Sorbitol. Saccharin-sweetened yoghurt was unacceptable due to an overpowering, bitter aftertaste. Similarly, Greig et al. (1985) demonstrated a slightly sweet, lingering aftertaste in yoghurt sample of 0.14% added aspartame which was not present in yoghurt samples of 2% added sucrose with equivalent degree of sweetness. McGregor and White (1986) concluded that High Fructose Corn Syrup (HFCS) did not adversely affect yoghurt quality and may have increased its acceptability. Nevertheless, due to its higher osmotic pressure, HFCS may inhibit the growth of the yoghurt starter culture (Keating and White, 1990). Therefore, amount and type of sweeteners used may be critical to culture growth and consumer
acceptance. According to McGregor and White (1986), regardless of the type of sweetener used, yoghurt made with 4 % sweetener was more liked by the consumer panelists. Even though, some workers showed the positive effect of sucrose levels on the growth of yoghurt starter and acetaldehyde production, Bills et al. (1972) reported that growth and acid production were inhibited in yoghurt containing 4% or more sucrose and lower acetaldehyde production was observed above 8% sucrose.
Rheological properties for foods, such as fermented dairy products, are important in the design of flow processes, quality control, storage and processing and in predicting the texture of foods (Shaker et al., 2000). The textural properties of acid milk gels can be assessed by a range of fundamental and empirical methods such as small amplitude oscillatory rheology (SAOR), large-amplitude oscillatory shear, penetration, texture profile analysis, rotational viscometry, flow through an orifice such as a Posthumous funnel, and various sensory methods (Lucey,2004).
Yoghurt exhibits a variety of non-Newtonian effects, such as shear-thinning, yield stress, viscoelasticity, and time-dependency (Benezech and Maingonnat, 1994). Almost every processing step in the manufacture of yoghurt can affect the viscosity of the finished product (Keating and White, 1990). O’Neil et al. (1979) concluded that the consistency of yoghurt was influenced by the composition, storage time and acidity.
According to Marshall (2006), it is of paramount importance to measure the firmness of the set-type yoghurt, which represents its resistance to rupture, i.e., the yield point of the structure as determined by a compression-type test. Measurement of gel firmness in yoghurt is achieved by means of constant speed penetration on universal testing machines or similar instruments, using cylindrical plungers and crosshead speed values ranging between 10 and 100 mm min−1 at <10°C. The force-response is monitored as a function of penetration depth (Jaros and Rohm, 2003). The plunger size and the penetration depth affect the force-response.
Gel firmness can be measured at a predefined time or at a predefined depth or until a point of gel breakage occurs i.e. yield point (Fiszman and Salvador, 1999; Harte et al. 2007). According to Ares et al. (2006), yield stress would be the most appropriate to characterize the texture of yoghurt in quality control or product development, since several methods that allow a quick and direct determination of yield stress are available. The main disadvantages of these empirical methods, when compared to fundamental measurement of rheological properties, are the use of relative scales and that results are for a given set of experimental conditions, making it hardly possible to compare results unless the same conditions are used (Benezech and Maingonnat 1994).
Sensory Properties of Yoghurt
Yoghurt can vary greatly in ingredient composition, so determining consumer acceptance for these products is necessary for commercial success. Therefore, high importance is placed on flavor and texture of yoghurt products. Consumer acceptance of yoghurt depends on acidity (sourness), sweetness, aroma perceptions, and textural properties of the product (Beal et al., 1999). Yoghurt aroma is generally ascribed to acetaldehyde produced by L. bulgaricus and S. thermophilus from threonine (Marschall and Cole, 1983). According to Vedamuthu (1991), the optimum range for acetaldehyde in yoghurt is 10-15 ppm. In addition, other volatile
organic aroma compounds have been identified for example, the diketones 2,3-butanedione (diacetyl) and 2,3- pentane Dione are also contributors (Ott et al., 1997). Nevertheless, the contribution of these compounds to yoghurt quality was not clearly demonstrated, and their role is minor when flavorings are added to the product.
According to Ott et al. (2000) the acidity of yoghurt plays a major role in yoghurt flavor and is perhaps more important than the concentration of acetaldehyde, diacetyl, or 2,3-pentanedione since the perception of acidity has influence on the perception of the other attributes. Barnes et al. (1991b) reported the overall liking of yoghurt by consumer was influenced by the degree of sweetness, sourness and above all, by the specific fruit flavor liking. Sensory attributes associated with plain yoghurt such as astringency, acetaldehyde and sourness were masked by the sweetener and fruit flavorings. Hence, to produce yoghurts having the highest overall liking rating, a balance between sweetness and sourness is necessary.
Texture is one of the main characteristics that define the quality of yoghurt and affect its appearance, mouthfeel and overall acceptability (Yoon and McCarthy, 2002). The consistency of yoghurt is probably as important as flavor. Adequate firmness without syneresis is essential for a top-quality product (Kroger, 1975).
The textural characteristics of yoghurt are generally studied in the cup, using a spoon, or in the mouth.
According to Cayot et al. (2008), in sensory evaluation of yoghurt, either consistency can be perceived visually or orally where oral consistency can be defined as Viscosity of the product on the tongue. The yoghurt sample is ‘fluid’ if it flows over the tongue immediately (non-viscous). The yoghurt sample is ‘‘thick’’ if the yoghurt sample stays on the tongue or flows slowly and is swallowed with difficulty (viscous) while visual consistency can be defined as Viscosity of the product when the spoon is slowly tilted up to 90°. The yoghurt sample is ‘fluid’ if the flow is easy, immediate and continuous from the spoon (non-viscous). The yoghurt sample is ‘thick ‘if the yoghurt sample flows slowly, with difficulty (viscous).
Mouth feel is another important sensory property of yoghurt products. According to Lawless and Heyman (1999), mouth feel is defined as ‘a category of sensations occurring in the oral cavity, related to the oral tissues and their perceived condition (e.g. drying, coating). Mouth feel, flavor, sweetness, sourness, and the balance between these factors have been shown to affect the overall preference for yoghurt (Barnes et al., 1991a). These characteristics, and many others, are important attributes for the acceptance of yoghurt products.
Shelf Life of Yoghurt
Product shelf life is the controlling factor in the distribution of dairy foods, which are highly perishable by nature.
Shelf life dictates the total elapsed time allowed from production to consumption. The shelf life of non-sterile dairy products such as yoghurt and fermented milk products, is generally limited to one to 3 weeks (Salvador and Fiszman, 2004), depending upon the quality of the raw ingredients, processing conditions, and post processing handling. The keeping quality of yoghurt can be improved by various methods such as gas flusing (carbon dioxide and nitrogen), use of preservatives and application of heat after incubation (Tamime and Deeth, 1980).
The drive to extend shelf life stems from increased distribution center demands due to consolidation of manufacturing facilities, product returns from code expirations, and interest in opening up new distribution channels and in expanding geographically.
Changes in the physical, chemical, and microbiological structure of yoghurt determine the storage and shelf life of the product. Alteration of these properties causes color, aroma, and texture deterioration of yoghurt, which are considered important quality criteria by consumers. Moreover, Salvador and Fiszman (2004) reported that studies of changes in these quality characteristics during storage would enable producers to predict the shelf life of the product more accurately.
Microbial activity is the first and most important limitation of a food’s shelf life in general and yoghurt in particular. The presence of live starter bacteria and yeast and mold contaminants coupled with packaging/storage conditions lead to the formation of off-flavors and other undesirable physicochemical changes that eventually lead to product failure (Muir and Banks, 2000; Salji et al., 1987). Yoghurt is notorious for its spoilage, especially in the development of harsh-acidic flavor and in thinning (low viscosity), when processing parameters, handling and cooling are not in control (Bille and Keya, 2002). According to Viljoen et al. (2003), mold and yeast growth and development of off-flavors can be a major determinant of shelf life of yoghurts. The off-flavor produced by those organisms can be described as yeasty, fruity, musty, cheesy or bitter and occasionally soapy-rancid which may be ascribed to their high proteolytic and lipolytic activities (Walstra et al.,1999).
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