SENSORY, RHEOLOGICAL, PHYSICAL, CHEMICAL AND MICROBIOLOGICAL PROPERTIES OF DIFFERENT TYPES OF YOGHURT

This study was designed to evaluate the quality of yoghurt sold in Assiut city, Egypt, in which a total of 60 samples of Baladi, pasteurized plain and pasteurized flavored (20 each) were collected randomly. The sensory evaluation was based on visual, texture and flavor. Syneresis was applied for rheological properties and pH for physical properties. Chemical analysis was applied through moisture, TS, fat and SNF%. Also, titratable acidity% and starch were detected. The microbiological examination for coliforms, fecal coliforms, E. coli , anaerobes and yeasts & molds were counted. The achieved results showed that the sensory evaluation of the pasteurized plain and pasteurized flavored samples was of higher scores than the Baladi samples, however on contrast, syneresis was higher in the Baladi samples. The average values of pH were 4.9, 4.89 and 4.83 for the Baladi, pasteurized plain and pasteurized flavored samples, respectively. For the fat content, the Baladi samples had higher fat%, followed by the pasteurized plain then the pasteurized flavored types. All the examined Baladi samples were starch free, while 70 & 65% of the pasteurized plain and pasteurized flavored types were positive, respectively. The average values of titratable acidity% were 0.91, 0.82 and 0.75, respectively. The microbiological examination cleared that the Baladi samples were more contaminated for coliforms, fecal coliforms and E. coli , while, the pasteurized plain and pasteurized flavored samples were more contaminated for anaerobes. It was found that 85% of the total examined samples were unacceptable according to the Egyptian Standards for their content of yeasts and molds.


INTRODUCTION
Yoghurts can be high in protein, calcium, vitamins, and live culture, or probiotics, which can enhance the gut microbiota. These can offer protection for bones and teeth and help prevent digestive problems. Low-fat yoghurt can be a useful source of protein on a weight-loss diet.
Probiotics may boost the immune system.
In order to manufacture a good quality product, the used raw milk must be of low bacterial count, antibiotics free, sanitizing chemicals free, not mastitic milk and not colostrum and the milk also should be bacteriophages free (Thapa, 2000).
Industrial yoghurt quality varied greatly with the chemical composition of the original milk, production method, type of added flavor and post-incubation processing. Yoghurt technology, starter culture microbiology and quality appraisal is the prime importance of any yoghurt type (Hui, 1993;Yadav et al., 1993).
Egyptian Standards (2005) stated that fungi must not exceed 10 cfu/g; in addition, the maximum count of coliforms is 10 cfu/g but should be E. coli free. Also, free from pathogenic bacteria & their toxins. In yoghurt, coliforms are contaminants and are used as indicators of hygienic conditions. According to the aforementioned, the current investigation was aimed to determine the keeping quality of different types of yoghurt sold in Assiut city; therefore, sensory, rheological, chemical and microbiological properties were evaluated.

Sampling:
A total of 60 Baladi, pasteurized plain and pasteurized flavored yoghurt samples (20 each) were obtained from different dairy shops and supermarkets in Assiut city, Egypt. The samples were transferred directly to the laboratory to be examined. All the available data were written in designed sheets including batch no., shelf-life etc.

Sensory examination:
The sensory evaluation was applied after direct transportation to the laboratory. All samples were scored by a regular score panel. The score was depending on a 9points hedonic scale (from 1 as dislike extremely to 9 as like extremely). Some texture attributes were described according to Gonçalvez et al. (2005) as: Creaminess means the time necessary to dissolve or mix the sample with saliva;

The sensory parameters
Consistency means a homogeneous structure, not watery and not fragile; Smoothness means the absence of gritty texture.
Rheological examination: 1) Syneresis (Dannenberg and Kessler, 1988)was done by placing 25 g of the yoghurt sample over What man filter paper on a funnel top in a graduated cylinder to collect whey (in ml) after 2 h at 5±2° C (refrigeration temperature). The estimated degree of syneresis is expressed as the drained whey amount.
Chemical examination: 1) Titratable acidity was done according to AOAC (2005), by titration of the sample against sodium hydroxide using phenolphthalein (indicator).
The total acidity was expressed as lactic acid% and was calculated according to the following equation: Acidity% = 0.009 × ml of N/10 NaOH × 100 Sample weight (g) 0.009 is equivalent to lactic acid normality 2) Moisture% was done according to AOAC (2000), by drying in an oven at 100° C until constant weight was recorded. Moisture%= (W2 -W3) / (W2 -W1) X 100 W1 = weight of the empty dish W2 = weight of dish with the sample before drying W3 = weight of dish with the sample after drying 3) TS% = 100 -moisture% 4) Fat% was estimated by Gerber method according to AOAC (2003). 5) SNF% = TS% -fat% 6) Starch test was done according to Kumar et al. (1998) by using 1% iodine solution.
Microbiological examination: Preparation of serial dilution : After thoroughly mixing of a sample, 10 g was transferred into a sterile wide-mouth container with 90 ml of 0.85% sterile saline solution to provide a dilution of 1:10 then ten-fold serial dilutions were prepared.

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No.                  (Pagliarini et al., 1991). The 3 main parameters of sensory evaluation (visual, texture and flavor) were studied in the current investigation as shown in Tables 1, 2 & 3 for the Baladi, pasteurized plain and pasteurized flavored yoghurt samples, respectively, showing high percentages of the examined samples with good scores for the sensory attributes especially in the pasteurized plain and pasteurized flavored yoghurt samples. Table 4 showed the syneresis values, in which the average syneresis value of the Baladi yoghurt samples was higher than those of the pasteurized plain and pasteurized flavored yoghurt samples. For the Baladi yoghurt samples, the syneresis values along the total examined 20 samples were of a minimum value of 6 ml and a maximum value of 9.7 ml with an average value of 7.77 ml. Higher syneresis average value was demonstrated by Sayed (2012)as 10.429 ml.
It was remarkable that some of the pasteurized flavored yoghurt samples were without whey as the syneresis value was zero ml (Table 4), and that may be attributed to the added flavoring agent which increased the total solids revealing no whey drainage.
The pH values of the three groups of the yoghurt samples were similar as cleared in Table 4, with averages of 4.9, 4.89 and 4.83 for the Baladi, pasteurized plain and pasteurized flavored yoghurt samples, respectively; which were low than 5.36 that obtained by Fahmid et al. (2016).
Regarding the acidity, which affects the flavor of the examined products, Table 5 showed the average values of titratable acidity for the Baladi, pasteurized plain and pasteurized flavored yoghurt samples as 0.91, 0.82 and 0.75%, respectively. Higher results were detected by Sayed (2012) as 1.2% and by Fahmid et al. (2016) at 1.21%. Dalles and Kechagias (1989) reported the acidity of commercial yoghurt ranged from 1.02 to 2.15%.
When through the light towards the chemical analysis of the examined yoghurt samples, it was found that the Baladi yoghurt was higher in fat content than the pasteurized plain and pasteurized flavored yoghurt (Table 5). It was noticed that the fat% was low or even undetected in the examined pasteurized flavored yoghurt samples with an average value of 0.65% (Table 5).
Unfortunately, starch was detected in 70% of the examined pasteurized plain yoghurt samples and 65% of the examined pasteurized flavored yoghurt samples; while all the examined Baladi yoghurt samples were starch free (Table 6). The Egyptian Standards documented that additives shall be in accordance with the legislation issued in this regard, and in the absence of decisions for any of the added materials, they shall be in accordance with what is issued by the International Alimentarius Committee.
Regarding the microbiological examination, Tables 7, 8, 9, 10, 11 and 12 gave a picture of the degree of microbial contamination with coliforms, fecal coliforms and E. coli.
Overall the examined yoghurt samples in the current study, the Baladi yoghurt was more contaminated than the pasteurized plain and the pasteurized flavored yoghurt. This result was similar to Dardashti et al. (2001) who found the contamination rate with coliforms in traditional processing was higher than in industrial processing because of practice differences among different manufacturers.
The Egyptian Standards (2005) stated that the coliforms count in yoghurt must not more than 10 CFU/g. Therefore, 15% of the total examined yoghurt samples were unacceptable (Table 8). Furthermore, the contaminated milk with high coliform count may become an endogenous source of coliforms in dairy products in the lack of proper sanitary measures. The presence of coliforms in yoghurt is considered an index of unsatisfactory sanitation and the possible presence of enteric pathogens (Frazier and Westhoff, 1983). Thenon-complying samples might indicate a low level of hygiene during the processing of yoghurt (Birollo et al., 2001). Table 13 reflected the presence of anaerobes in the examined yoghurt samples; where 35% of the pasteurized plain yoghurt samples and 30% of the pasteurized flavored yoghurt samples were contaminated with anaerobes; but found in 5% of the Baladi yoghurt samples. Moreover, overall the examined yoghurt samples, anaerobes were detected in 23.3% of the total samples; and according to the legal requirements of the Egyptian Standards (2005), all the examined samples should be free from anaerobes. Sadek et al. (2014) could not detect anaerobes in raw and pasteurized milk yoghurt.
For yeasts and molds (Tables 14 & 15), the obtained high counts may be due to the acidic conditions that favor yeasts & molds growth. Fungal growth predominates in dairy products with high water activity, acidity, processing or packing conditions encourage their growth over bacteria (Cousin et al., 1992). In a similar manner, yeasts and molds are the most predominant spoilage organisms that tolerate the low pH. The Egyptian Standards (2005) stated that yeasts & molds count in yoghurt must not more than 10 CFU/g. unfortunately, 85% of the total examined yoghurt samples were unacceptable (Table 15).