Physicochemical, microbiological and sensory characteristics of yoghurt produced from goat milk

E A Eissa, I A Mohamed Ahmed*, A E A Yagoub** and E E Babiker***

Ministry of Science and Technology, Food Research Centre, Khartoum North, Shambat, Sudan
* United Graduate School of Agricultural Sciences, Tottori University, Tottori, Japan
** Faculty of Agriculture, University of Zalingie, P.O. Box 6, Zalingie, Sudan
*** Faculty of Agriculture, Department of Food Science and Technology, University of Khartoum, Khartoum, Sudan
elfadilbabiker@hotmail.com

Abstract

Physicochemical, microbiological and sensory attributes before and after storage for different periods of time (5, 10 and 15 days) of yogurt produced from goat milk with reference to cow milk were analyzed.

Compared to cow fresh milk, goat milk was rich in fat, protein, ash and total solids but had low pH.  The gross nutrients of goat fresh milk were slightly changed after processing compared to that of cow milk. Yogurt processing significantly (P ≤ 0.05) decreased the initial pH of fresh milk in both yogurt types. Cow milk yogurt is more viscous than goat milk yogurt. Storage of both yogurt types resulted in significant changes in gross composition, especially after 10 days of storage. Yogurt processing of goat and cow milk based on Lactobacillus spp. and Streptococcus spp. as the main active organisms. Total bacterial and yeast increased significantly (P ≤ 0.05) on storage within 10 days. Thereafter, their numbers dropped off. Staphylococcus aureus and Salmonella spp. were absent. Coliforms and faecal coliforms were detected in both yogurt types but they disappeared after 5 days of storage. Goat milk yogurt had significantly (P ≤ 0.05) lower sensory acceptability than cow milk yogurt. Both yogurt types can withstand storage to a maximum of 10 days, thereafter sensory scores decreased.

Key words: Goat, nutrients, quality attributes, shelf life, yogurt

Introduction

The milk of small ruminants such as goats is of particular economic interest in the developing countries. The production of this type of milk has to be a useful strategy to tackle the problems of undernutrition (Haenlein 2004). Although the world production of goat milk has been relatively minor compared to that of bovine milk (2.1% versus 84.6% of the total milk production, respectively), the worldwide goat population has reached 758 million heads with 55% increase during the last 20 years, and goat milk production has reached 12.2 million tones with 58% increase during the same period (Haenlein and Abdellatif 2004). Goats’ milk has special nutritional properties that make it attractive to some consumers. It is easier to digest than cows’ milk and may have certain therapeutic value (Park 1994a; Haenlein 2004).

The use of goat becomes an opportunity to diversify the dairy market since it allows us to develop added value to the fermented products with particular characteristics compared to cow’s milk. The major differences between goat’s and cow’s milk are related to the different proportions of the different kinds of casein, and also to the different structure and size of fat globules and protein micelles (Tziboula-Clarke 2003). All these differences could lead to the milk behaving differently during processing that could affect the final quality of goat’s milk dairy products (Vargas et al 2008). The special characteristics concerning the composition of goat milk mean that its nutritional utilization is markedly higher than is the case with cow’s milk. Thus, the protein of goat milk is more digestible (Park 1994b; López-Aliaga et al 2003; Haenlein 2004), and less allergenic (Lara-Villoslada et al 2004). Similarly, the fat of goat milk is more digestible (Alférez et al 2001; Haenlein 1996), and it may be considered an excellent source of energy for use in various metabolic processes and even for combating metabolic diseases (Sanz -Sampelayo et al 2007). Goat milk and its products of yogurt, cheese and powder have three-fold significance in human nutrition: (1) feeding more starving and malnourished people in the developing world than from cow milk; (2) treating people afflicted with cow milk allergies and gastro-intestinal disorders; and (3) filling the gastronomic needs of connoisseur consumers (Haenlein 2004). 

Sudan is a country endowed with diversified animal wealth most of which is owned by nomadic sector scattering all over the country. Milk from these animals plays an important role in the nutrition of both rural and urban societies. Sudan is one of the leaders in goat’s milk production with 1.2 million tonnes per year. This amount represents 16% of the total milk production of the country and provides protein supply (milk + meat) of 22.0 g per person per day (FAO 2001). Possibly, more than half the quantity of milk produced in Sudan is processed into some fermented dairy products, such as Robe, Gariss, Mish, Jibna-Beida and Yogurt (Dirar 1993).  The source of the goat’s milk from different breeds is of greater importance considering the genetic protein polymorphysim on the manufacture and functionality on goat’s milk-based products (Vargas et al 2008). However, studies on yogurt from milk of the local breed goats were scanty. Therefore, in this study we would like to evaluate the nutritional, microbial and sensory quality before and after storage of goat milk yogurt with reference to cow milk yogurt.

Materials and methods  

Materials

Goat and cow milk were obtained from University of Khartoum farm, Sudan. Milk was collected from number of lactating animals, mixed well, and divided in to 3 lots each represent a sample. Then samples stored in a refrigerator for subsequent processing. All chemicals and media used in this study were of reagent grade.

Yogurt making

Yogurt was prepared as described by Dirar (1993). Raw milk of goat and cow (control) was heated in a water bath at 85°C for 30 min, cooled to approximately 42°C, inoculated with commercial yogurt culture (5g/Kg milk), transferred to plastic cups, incubated at 43°C for 4 h, and stored at 4°C overnight before testing at different periods of time (0, 5, 10, 15 days).

Gross composition

Total nitrogen was measured by micro-Kjeldahl method (AOAC 1990). Protein was calculated as N x 5.38. Fat was determined by the Gerber method (Bradly et al 1992). Ash content was determined by dry ashing of the samples for 24 h at 550°C. Moisture content was determined by drying samples overnight at 105°C (AOAC 1990). Total solids content was determined by gravimetric method by drying the samples in an oven at 105°C for 24 h (AOAC 1990). Crude fiber content was determined according to the acid/alkali digestion method of Southgate (1976). Analyses were performed in triplicate.

Total titratable acidity, pH and viscosity

Total titratable acidity was determined by AOAC (1990) method. The pH was measured using a pH-meter (HANNA-pH 210, Germany) and the viscosity was measured by a viscometer (Haake georz auee, Germany).

Sensory analysis

Sensory profiling of the yogurt samples was conducted, using conventional profiling, by a trained panel. Ten judges were selected among the faculty, staff, and students of the Department of Food Sciences, Khartoum University who had successfully passed standardized tests for olfactory and taste sensitivities as well as verbal abilities and creativity. The panellists were given a hedonic questionnaire to test taste, texture, color, flavor and overall acceptability of coded samples of cow milk yogurt as a control and goat milk yogurt. Both fresh samples and those stored for different period of time (5, 10 and 15 days) of both yogurt types were tested.  They were scored on a scale of 1–5 (1 = poor, 2 = fair, 3 = good, 4 = very good and 5 = excellent). Each attribute was evaluated in triplicate and the values were then averaged.

Microbiological tests

Ten grams of yogurt sample were placed in 90 ml sterile 0.1% peptone water and shaken to prepare 10^-1 dilution. Then a decimal dilution series was prepared in 0.9% NaCl saline solution. For Salmonella, 25g of yogurt sample were placed in 225ml of buffer peptone water and incubated for 24h at 36°C. One ml of this solution was transfered in to 10ml of selenite cystine broth and incubated as before. Aliquots (0.1 ml) were used to inoculate on to the surface of agar media a spread plate technique. Additionally, aliquots (1.0 ml) were used in an agar pour plate procedure for total viable count of bacteria, lactic acid bacteria, Streptococcus; Salmonella; Staphylococcus aureus, spores, and yeast and moulds. The agar media employed were: plate count agar incubated both aerobically and anaerobically; MRS agar containing 10mg ml^-1 nystatin selective for lactic acid bacteria; M-17 agar selective for Streptococcus, Brilliant green, xylose lysine decaeboxylate, and bismuth sulphite agar selective for Salmonella spp.; Mannitol salt agar- selective for staphylococci; Baird Parker agar selective for Staphylococcus aureus, nutrient milk agar for total spores; malt extract agar containing 100 μg ml^-1 chloramphenicol selective for yeasts and moulds. Media after inoculation were incubated at 37°C (malt extract agar was incubated at 28°C and Salmonella spp. agar media at 36°C) and examined after 24-48 hours. Colonies on the agar plate were counted (cfu g^-1) and a proportional subsampling procedure was used to select colonies of bacteria for identification. Gram staining, spore staining, presence of active enzymes, and growth in air tests were employed to identify the genera of bacteria. For identification of Salmonella the ATP system was used to identify the bacteria in the rapid ID 32 strip. Viable cells and spores were enumerated as cfu g^-1. Total coliforms count was done by inoculating 1.0 ml aliquot from a suitable dilution in MacConkey broth and incubated at 37°C for 48 hours. Tubes producing acid and gas were used for further tests. Aliquots from acid and gas positive test were inoculated into brilliant green bile lactose broth, one set of tubes was incubated at 37°C for 48 hours, and the other at 44.5°C for 24 hours. For further confirmation of faecal coliforms (Escherichia coli), tubes giving positive reaction at 44.5°C were streaked onto eosin methylene blue (EMB) agar. Positive test colonies were then counted as cfu g^-1 (Harrigan and McCane 1976).

Statistical analysis

Means of 3 determinations were analyzed using analysis of variance (ANOVA) of the SAS Institute–version 6.3 (1997). Significant differences between means were determined at P ≤ 0.05.

Results and discussion 

Gross composition of fresh milk and yogurt

Raw milk and yogurt of goat and cow (control) were analyzed for nutrients content and pH as shown in Table 1.

Compared to the composition of fresh cow milk, goat milk had higher fat, protein, and total solids. Moreover, the total protein content was found higher than values reported for goats’ milk of different worldwide breeds (Guo et al 2001; Hadjipanayiotou 2004; Stelios and Emmanuel 2004; Güler 2007; Pirisi et al 2007) while, fat was comparable to goat milk from Turkish breed (Güler 2007). In general, Goat’s milk reported to provide higher proportion of total solids, protein and fat than cow milk (Haenlein 1996). The nutrient compositions of goat milk can be greatly influenced by several factors such as season, stages of lactation, breed, diet, individual animal and environmental management conditions (Haenlein 2004). Preparation of yogurt slightly changed the level of protein, fat, ash, total solids and moisture for both goat and cow milk products, suggesting the effect of the indigenous microflora on such constituents. The pH of the fresh milk was remarkably decreased from 5.98 to 5.71 for goat yogurt and from 6.31 to 5.60 for cow yogurt. Constituents in yogurts were influenced by the fermentation process, draining of yogurt, cooking, and manufacturing utensils (Güler 2007).

Effect of storage on yogurt gross composition

Table 2 illustrates the behaviour of goat and cow milk yogurt in terms of nutrient composition under cold storage (4°C) for different periods of time (5, 10 and 15 days).

The results obtained revealed that storage of yogurt for 5 days significantly (P≤0.05) increased fat, protein, ash, and total solids for both goat and cow milk yogurt. Further increase in fat, ash and total solids of both yogurt types was observed after 15 days of storage. Proteins and moisture of both yogurt types significantly (P≤0.05) decreased after 15 days of storage. 

Figure 1 and 2 show changes in pH and acidity of yogurt samples during storage.

The pH (Figure 1) and total acidity (Figure 2) of both yogurt types were 5.71 and 5.60 and 0.74 and 0.68%, respectively. After 5 days of cold storage a significant (P≤0.05) increase in acidity and a decrease in pH for both yogurt types were observed. The magnitude of increase in acidity is high in goat yogurt. After 15 days of storage goat milk yogurt had a higher acidity (2.58%) and a lower pH (2.67) compared to cow milk yogurt. A faster acidification and lower pH values in goat milk yogurt was reported by Bozanic et al (1998). Different behaviour could be explained by the enhancement of the microbial growth, acidity progress and peptidase activity of Lactic acid bacteria in goats’ milk (Tamime and Robinson 1999). Moreover, the activity and growth rate of the starter cultures are strain dependent. Hence, the acidification rate of lactic acid bacteria varied with the type of milk (Vargas et al 2008).

Effect of storage on viscosity of yogurt

Figure 3 shows changes in viscosity of yogurt as affected by type of milk and storage period.

A higher significant value for viscosity was obtained for cow milk yogurt (950.12 cp) compared to that of goat yogurt (820.14 cp). Similar differences were found by Jumah et al (2001) who ascribed this difference to the variation in chemical composition of milk, especially total solids and protein content. Viscosity of both yogurt types significantly (P≤0.05) decreased with increasing storage time. After 15 days of storage the initial viscosity of goat and cow fresh yogurt decreased by about 85%. Continuous activity of microflora in yogurt suggests changes in the micro-structure of the media and hence affecting viscosity. The viscosity is affected by the state and concentration of fats, protein, temperature, pH, and milk age (Park 2007).

Microbial succession during storage of yogurt

Table 3 shows changes in total viable count of goat and cow yogurt during storage.

The results obtained revealed that goat and cow milk yogurt contained total number of 1.3 x 10⁵ and 2.6 x 10⁵ cfug^-1 viable cells of bacteria, respectively. Initial bacterial count increased significantly (P ≤ 0.05) during storage of both yogurt types. Bacterial count reached its maximum increment at the 10^th day, and thereafter significantly (P ≤ 0.05) declined to 1.25×10⁶ and 1.9×10⁶ cfug^-1 for goat and cow yogurt, respectively. The increment of the acidity of the growth media (Figure 2) with the storage time may retard the bacterial growth. These results are in agreement with the findings of Masud et al (1991). Lactobacillus count in goat yogurt (1.45×10⁶ cfug^-1) was observed to be higher than that of cow yogurt (1.7×10⁵ cfug^-1).

Throughout the storage period, the numbers of Lactobacillus spp. in cow yogurt surpassed that of goat yogurt. Fresh goat milk yogurt had lower number (1.4×10⁶ cfug^-1) of Streptococcus spp. than that of cow milk yogurt (1.6×10⁶ cfug^-1). Streptococcus spp. load in both types of yogurt significantly (P ≤ 0.05) increased as the storage time prolonged, reaching its maximum at the 5^th day, and then declined. Tamime and Robinson (1999) reported that yogurt should contain 10⁷ viable cells of lactic acid bacteria per millilitre. On the other hand, yeasts and moulds steadily increased with increase in storage time with maximum value of 8.0×10⁷ for cow yogurt and 6.6×10⁷ cfug^-1 for goat yogurt at the 5th day. An increase in acidity (Figure 2) and/or reduction in potential oxygen during fermentation process may provide suitable conditions for growth of yeasts and moulds. Yeasts are commonly associated with traditional fermented dairy products and have been reported earlier (Beukes et al 2001; Isono et al 2001). Contamination by yeasts and moulds in traditionally processed yogurt was reported by Dardashti et al (2001).

Staphylococcus aureus and Salmonella spp. were absent in fresh yogurt and also throughout the storage period. Coliforms and Escherichia coli viable cells were detected in both goat and cow yogurt. In cow yogurt they significantly (P ≤ 0.05) increased during the first 5 days of storage and thereafter completely disappeared. In goat yogurt coliforms increased but Escherichia coli decreased after 5 days of storage and thereafter both organisms disappeared. The presence of coliforms gives clues to unsanitary conditions of processing. However, it was reported that coliforms, if present, in yogurt could survive a maximum of 3 days (Dardashti et al 2001). Moreover, Escherichia coli was observed to survive the low pH of domestic yogurt developed during cold storage and could tolerate lower acidity up to 6 days (Morgan et al 1993; El-Kosi et al 2000; Quinto et al 2000).

Sensory evaluation

Colour, flavour, texture, taste and overall preference scores of yogurt samples during storage are shown in Table 4.

The scores of all sensory attributes of goat milk yogurt are significantly lower (P ≤0.05) than those of cow milk yogurt. Goat milk yogurt was evaluated as less consistent and more acid, with a non-typical yogurt taste and flavor. Similar sensory characteristics were reported by many researchers (Abrahamsen and Rysstad 1991; Alichanidis and Polychroniadou 1996; Duboc and Mollet 2001; Vargas et al 2008) of yoghurt manufactured from goat milk. Up to 10 days of storage both yogurt types retained their sensory characteristics but signs of deterioration appeared after day 15.  Microbial hydrolysis of yogurt component during storage was found to be the key deteriorating factor to taste, color, flavor, and texture (El-Gazzar and Hafez 1992) which hence affect the overall preference of the product.

Conclusions 

• Yogurt prepared from goat milk was rich in nutrients compared to that prepared from cow milk.

  
  

• Nutrients composition of both yogurt types changed during storage.

  
  

• Streptococcus spp and Lactobacillus spp. dominate  in both yogurt.

  
  

• Contamination by yeast and moulds characterized the processed yogurt.

  
  

• Goat’s milk yogurt was inferior with respect to overall preference compared to that of cow milk and the sensory attributes of both yogurt decreased after 15 days of storage.

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Received 9 March 2010; Accepted 26 March 2010; Published 1 August 2010

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