LABAN RAYEB AND YOGHURT: SENSORY, RHEOLOGICAL, CHEMICAL AND MICROBIOLOGICAL PROPERTIES

Discussion

All the different parameters contributing to the determination of sensory properties (color, appearance, texture, aroma…etc) are equally important for the product acceptability (Pagliarini et al., 1991). As the overall acceptability (OAA) is the resultant of the examined 3 main sensory attributes, the average OAA scores of Laban Rayeb and yoghurt samples were 4.96 and 5.99, respectively (Table 4). These obtained data referred to the sensory quality of the examined products was slightly good.

Although 66% of Laban Rayeb samples had a relatively high score ranged from 5 to 9 (Table 2), the average OAA (4.96) referred to neither like dislike according the 9 points hedonic scale. Moreover, 78% of yoghurt samples had a relatively high score ranged from 5 to 9 (Table 3), but the average OAA (5.99) referred to like slightly according the 9 points hedonic scale.

With regards to the 1^st examined sensory attribute i.e. visual, Tables 2 and 3 showed that most of the examined Laban Rayeb and yoghurt samples were of good visual that appeared normal for the consumers. With pointing to the 2^nd examined sensory attribute i.e. texture, most of the highest percentages lied within a high score range; for Laban Rayeb samples, 30% with creaminess score 5 - <6; 26% with viscosity score 7 - <8; 52% with mouth feel score 6 - <8; 32% with consistency score 7 - <8; 28% with smoothness score 7 - <8 (Table 2); and for yoghurt samples, 26% with highest firmness score 8 - 9; 34% with creaminess score 6 - <7; 24% with mouth feel score 4 - <5; 28% with consistency score 6 - <7; 42% with highest smoothness score 8 - 9 (Table 3). Viscous properties are of primary importance with respect to the quality of the products (Magenis et al., 2006). As the flavor represented the 3^rd examined sensory attribute, 20 and 26% of Laban Rayeb samples with taste and aroma score 5 - <6, respectively (Table 2); 30 and 26% of yoghurt samples with a very high taste and aroma score 7 - <8, respectively (Table 3).

Why rheological properties were studied in the present investigation? Because of they are important for foods (such as fermented dairy products) in the design of flow processes, quality control, storage and processing and in predicting the texture of foods (Benezech and Maingonnat, 1994; Aichinger et al., 2003).

Figure 1 presented the syneresis values of Laban Rayeb samples along the total examined 50 samples with a minimum value of 9 ml in 3 samples and a maximum value of 15.5 ml in 1 sample and the average value was 10.4 ml. For yoghurt samples, Figure 2 illustrated the syneresis values along the total examined 50 samples with a minimum value of 3.25 ml in 1 sample and a maximum value of 16 ml in 1 sample and the average value was 10.429 ml. Mahdian and Tehrani (2007) concluded that degree of syneresis decreased with increasing TS significantly. Samples with higher TS had better textural properties than those with lower TS. Haj et al. (2007) concluded that the overall picture of stirred yoghurt quality evaluation needs emphasis on quality control during processing and storage.

In the present investigation, the chemical composition was of interest to be analyzed, not only for the nutritional value significance but also for the effect on the sensory and rheological properties. Robinson (1983) reported that the aim of TS content is consistency improvement imparted to the yoghurt coagulum. Mahdian and Tehrani (2007) found that texture acceptability increased with increasing TS significantly. It could be due to the effect of high TS content on increasing firmness of the gel and decreasing degree of syneresis (Mohammeed et al., 2004). More focusing, the firmness of yoghurt is dependent on TS content (Tamime and Deeth, 1980; Gastaldi et al., 1997; Penna et al., 1997; Kristo et al., 2003); and that was clear in the obtained result, as the average value of the firmness of the examined yoghurt samples as 6.03 (Table 4) was in compatible with the average value of TS content as 14.02% (Table 5). The obtained TS% was in agreement with Musaiger et al. (1998); El-Bakri and El-Zubeir (2009) as presented in Table 6, as yoghurt usually contains 12-14% total milk solids and has soft friable custard like consistency and a clean distinct flavor.

Mahdian and Tehrani (2007) found TS content of the yoghurt samples had significant effect on degree of syneresis. Reduction of free water and increasing the proportion of TS content, which occur during concentration, are 2 main factors which decreased rates of wheying off in the samples with high TS. Similarly, Shaker et al. (2000) indicated that the increase in viscosity of yoghurt with highest fat content may be due to increase of TS of the milk which has a significant effect on the firmness of yoghurt gel and decreasing degree of syneresis.

According to the some recorded studies shown in Table 6, the average fat% (3.34%) of the examined yoghurt samples was in accordance to most of the listed references. While, Musaiger et al. (1998) mentioned the yoghurt available in Bahrain had higher fat content than the yoghurt available in Egypt (Dagher, 1991), but lower level of moisture. These data are consistent with the obtained result of the present study (Table 6); but these chemical differences are mainly due to the method of preparation of yoghurt in both countries.

Also, Musaiger et al. (1998) analyzed 4 types of fermented dairy products commonly consumed in Bahrain for physical, proximate and mineral composition. The findings revealed that acidity, TS and SNF were found to be higher in yoghurt and Labenah (thick yoghurt) compared to low and full fat Laban (diluted yoghurt). These findings were in somewhat agreement with the obtained result in the present work (Table 5), if we put in consideration the comparison between yoghurt (as thick) and Laban Rayeb (as drinkable).

Regarding acidity%, it has a direct effect on the flavor of the examined products. Through our community contact, many people refuse Laban Rayeb and yoghurt consumption because of their sourness. Therefore, El-Bakri and El-Zubeir (2009) mentioned the kind of uses of yoghurt in Sudan (particularly in Khartoum) requires yoghurt with such mild acidity as the consumer desires.

As presented in Table 5, the average value of titratable acidity (TA) for both the examined products was about 1.2%. These obtained findings were higher than all TA% of the recorded studies listed in Table 6; and also higher than those of Madeha (1991) who found the average TA% of Laban Rayeb and plain yoghurt were 0.956 and 1.049 %, respectively. Also, lower results by Kim et al. (1998) who found TA% was 0.95 - 0.99% of Yam–yoghurt and Collado et al. (1994) who found the yoghurt drink and yoghurt like products had acidity% of 0.56 and 0.58%, respectively. The relatively high acidity in the examined Laban Rayeb and yoghurt samples may be due to increased starter culture and/or long incubation period.

When through the light towards the microbiological examination, Tables 7, 8, 9 and 10 gave a picture about the degree of microbiological contamination especially with Enterococci, coliforms, fecal coliforms, yeasts and molds. Most of the examined samples appear to be microbiologically not acceptable when compared to the Egyptian and International Standards. This may be attributed to the examined products were purchased from dairy shops whose using the traditional processing in manufacturing of these products. Dardashti et al. (2001) found that the rate of contamination in traditional processing with coliforms was higher than in industrial processing because of the differences in the practices between different manufacturers.

The examined Laban Rayeb and yoghurt samples (30 and 28%) were contaminated with Enterococci, with average counts of 1.19×10³ and 9.38×10⁴ cfu/g, respectively (Table 7). Higher incidence of Enterococci in yoghurt was detected by Al-Hawary (2000) but Abdel-Aal (2008) found lower incidence as stated in Table 11. The obtained results expressed that about third of the examined samples were contaminated with Enterococci. These bacteria are comparatively heat resistant, salt tolerant, grow at wide range of temperatures, low pH and more resistant to drying, detergents and disinfectants; and considered a definite index of fecal contamination (ICMSF, 1982), and useful indicators of the possible presence of enteric pathogens (Rao et al., 1986). Additionally, some of them may help in assessment of the hygienic standard in dairy farms and factories of fermented milks as they are sometimes causing food poisoning (Sinigaglia et al., 1997; Roushdy et al., 1998).

Table 8 showed high incidences of coliforms as 28% of Laban Rayeb samples and 22% of yoghurt samples were contaminated. Moreover, high frequencies distribution was prominent in Table 9 as 6 and 8% of the examined samples of Laban Rayeb and yoghurt, respectively, had coliforms count in the range 10³ - >10³. Madeha (1991) found higher incidences of coliforms and E. coli in plain yoghurt and Laban Rayeb than the obtained findings, as shown in Table 11. El-Bakri and El-Zubeir (2009) found 43.75% of samples had coliforms count lower than 10² which is the maximum determined in most of the International Standards (Kucukoner and Tarakci, 2003).

Egyptian Standards (1990; 1991; 2005) reported that yoghurt must be free from coliforms; therefore, 22% of examined samples were not statutory with this standard and of inferior quality. Presence of coliforms in yoghurt is considered as an index of unsatisfactory sanitation and possible presence of enteric pathogens (Frazier and Westhoff, 1983). The non-complying samples might indicate the low level of hygiene during processing of yoghurt (Birollo et al., 2001).

Our microbiological findings of coliforms and Enterococci in both of the examined products were in agreement with the results of Khalafalla et al. (1988), as they examined Laban Rayeb, Zabady, yoghurt, Laban Zeir and Kishk for chemical and microbiological analysis and found high incidence of coliforms, in addition to, Enterococci were identified as the most predominant species.

Coliforms and Enterococci are considered as indicators of insufficient sanitation and the carelessness for both principles of hygiene and good manufacturing practices (Batish et al., 1980; ICMSF, 1982). While, a greater resistance of Enterococci when compared with classical indicators of coliforms under unfavorable condition has led to an increasing tendency to include Enterococci in microbiological criteria as an indication of direct fecal contamination in various food products (Jay, 1992; Knudtson and Hartman, 1993; Audicana et al., 1995).

Madeha (1991) reported a presence of negative correlation between titratable acidity of fermented milk products and coliforms count. This correlation was obvious in the 4 Laban Rayeb samples had coliforms in the range 10² - >10³ (1 sample in the range 10² - <10³ + 3 samples in the range 10³ - >10³) (Table 9), in which all these 4 samples with titratable acidity <1%. But this correlation did not match with the yoghurt samples which had high coliforms range. Therefore, their presence indicates faulty hygiene in production, handling and plant sanitation. Furthermore, the use of milk with high coliforms count may become endogenous source of coliforms in dairy products in the absence of proper sanitary measures.

Laban Rayeb and yoghurt should be free from fecal coliforms, but the revealed results in Table 8 showed that 4 and 2% of the examined products, respectively, had fecal coliforms with a count lied in frequency distribution of the range 3 - <10 for both products (Table 10).

According to the revealed results in Table 7, Staph. aureus was detected in low incidence and count values in the examined products. These results may be attributed to that yoghurt culture reduces the concentration of Staph. aureus (Pazakova et al., 1997). Moreover, such organism was inhibited after few days during storage of fermented product (Estrada et al., 1999).

Yeasts and molds are the most predominant spoilage organisms that tolerate the low pH. Unfortunately, it was noticeable high average counts of yeasts and molds as listed in Table 7 in both the examined Laban Rayeb and yoghurt products. The obtained high counts may be attributed to the acidic conditions that favor the growth of spoilage yeasts and molds. Economically, presence of yeasts and molds in dairy products is undesirable even found in few numbers because they rapidly grow in a wide range of temperature, pH and humidity resulting in objectionable changes that render the product of inferior quality or even unmarketable (Mossel, 1982). Fungal growth predominate in dairy products with high water activity, acidity, processing or packing conditions enhance their growth over bacteria (Cousin et al., 1992).

Table 7 showed that 68 and 92% of Laban Rayeb samples were contaminated with yeasts and molds, respectively. These obtained incidences were higher than those obtained by Ahmed and Abdel-Sater (2003); Salama and Enan (2005) as mentioned in Table 11. While, 24 and 60% of yoghurt samples were contaminated with yeasts and molds, respectively. El-Bakri and El-Zubeir (2009) found 68.75% of samples had yeasts & molds count lower than 10³ determined in the International Standards. When the yoghurt are produced under good manufacturing conditions, it should contain <10 yeast/g (but preferably <1 yeast/g); and if refrigerated at 5ºC or less, it should not undergo spoilage by yeast (Fleet, 1990).

Consuming yoghurt containing yeasts and molds constitutes a public health hazard among consumers (Varnam and Sutherland, 1994). Fungal contamination indicated improper plant sanitation and/or neglected hygiene during production, packing or storage.

Concerning Cl. perfringens, Table 8 showed that none of the examined samples recovered it. Our obtained results were in harmony with Abdel-Aal (2008) who examined 50 yoghurt (25 plain+25 fruit) samples in Germany and failed to isolate Cl. perfringens.

The microbiological picture of the present examined Laban Rayeb samples may be attributed to the fact that no heat treatment is applied during its preparation. Also, poor hygiene during milking, storage and handling of Laban Rayeb are factors responsible for its contamination (Madeha, 1991).

The conclusion of the present investigation was in agreement with the conclusion of the study of El-Bakri and El-Zubeir (2009); who concluded that the commercially available yoghurt has a good chemical quality when compared to the International Standards; however, the microbiological quality was lower than that required by the International Standards in most of yoghurt samples.

More emphasizing to the aforementioned conclusion, a recent study in Sudan by Mohammad and El-Zubeir (2011) concluded that the plain set yoghurt agrees with the Sudanese Standards for yoghurt chemical composition.

Hence, it is recommended the implementation of HACCP (Hazard Analysis Critical Control Point) practices during processing and storage to obtain good quality products.

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