I sincerely thank all the teaching and technical staff of the Department of Dairy and Poultry Sciences for their help. Fermented milk products are part of the diet in many parts of the world and are consumed regularly. At the same time, some qualitative tests of dahi samples were performed according to the standard laboratory procedure.
Dahi, especially sweet dahi, is one of the most popular fermented milk products in Bangladesh. It is a food for which there seems to be no adequate substitute and is one of the most widely distributed products.
Difference between yoghurt and dahi
Dairy products such as yogurt, spicy yogurt milk, and buttermilk are also rich in protein, spices, and minerals, making them widely expected for their nutritional values. Milk and other fermented milk products are highly susceptible to contamination by microorganisms and it is also a suitable medium for the rapid growth and multiplication of bacteria at favorable temperatures (Megha S.V. and Annadurai B., 2014). Therefore, it is important to know which specific microorganisms can potentially contaminate dairy products, as improper processing conditions of dairy products are very common in South Asian countries, making the products unhygienic.
Food and nutritive value of dahi
Pazakova et al., (1999) investigated the sensory evaluation of yogurt produced from cow's and goat's milk. Karthikeyan et al., (1998) evaluated the performance of 2 types of yogurt culture in different combinations in buttermilk with sweet cream (SCBM) at different TS levels of the culture (9-24%) in terms of increasing acidity, syneroisy and curd and sensory quality of curd. Salem et al., (1995) reported that the fat content of frozen yogurt (7 and 10%) had good melting quality up to 60 days of storage.
Chemical and Microbiological quality of dahi or yoghurt
They found that yogurt made from the 1:1 mix of milk tended to have the highest viscosity throughout the 9-day storage period, while cow's milk yogurt had the lowest. Goat's milk yogurt also had the lowest lactic acid content, while cow's milk yogurt had the highest. However, the intron penetrance of the experimented milk in the yogurt gel form was lower compared to that of standard milk (measured just after fermentation).
Cardoso et al., (1991) reported that the firmness of yogurt made from whole buffalo milk (6.3% fat and 4.7% protein) was most acceptable when milk was pasteurized at 75°C for 5 minutes. Manjunath and Abraham., (1986) used goat's milk to make yoghurt using standard methods and compared to cow's milk yoghurt and goat's milk yoghurt, they showed higher lactic acid production and lower acetaldehyde production. similar for both yogurts. Goat's milk yogurt had a softer body, was whiter in color and less whey separation was observed. Members of a taste panel could not distinguish between the 2 yoghurts (taste and taste) and the characteristic taste of goat's milk was masked in goat's milk yoghurt with a typical yoghurt taste.
Yogurt made from (ii) was rated higher on the hedonic scale than the yogurt from (I). Duitschaever., (1978) reported that the microscopic appearance of goat's milk yogurt and the streptococci in it was different from that of cow's milk yogurt. The goat's milk yogurt was less viscous than that made from cow's milk and was less firm when stirred; during agitation it showed no whey during subsequent storage at 4°C.
In organoleptic tests, there was a 32% preference for goat's milk yogurt (72% when sugar and/or flavoring was added).
Standards and Standardization of yoghurt or dahi
14 Sreenivasan and Ranganathan., (1972) note that when market samples of dahi prepared by lactic acid fermentation are used from milk as a starter, the contaminating yeasts multiply during the daily transfer of dahi from previous days and impart an unpleasant taste to the product after storage for 72 hours. Indian Standard Institution (1973) prescribed the types, requirements, sampling method and test for fermented milk products (plain and flavored), i.e. i) sweet, dahi produced using a culture of streptococcus lactis, S. Eschmann., (1968 ) reported referring to Swiss standards that fermented milk products in Switzerland must be negative PR (Phosphate Reaction), lactic acid bacteria must be present at a dilution of at least 16c' and frozen bacteria not more than 50,000/ml in retail.
Diseases caused by the microbes
16 Escherichia coli is a gram-negative bacterium that can be found in the lower intestine of warm-blooded animals. Humans and animals can become infected by consuming food or drinking water already contaminated with E. Salmonella enterica can cause four distinct clinical signs: bacteremia, gastroenteritis, asymptomatic carrier state, and enteric fever.
Their main hosts are animals such as cattle, wild birds, flies as well as domestic animals. People become infected only when they consume contaminated water and food such as meat, milk, egg products, dairy products which are contaminated with infected feces as well as infected animals. Pseudomonas aeruginosa can cause cystic fibrosis, urinary tract infections and other external and internal infections of the human body.
Pathogenic strains can cause infection by producing a toxin protein on the cell surface that binds to antibodies and deactivates them. Staphylococcal scalded skin syndrome is a severe skin infection that can be seen in newborns (Akbar and Anal., 2013). Klebsiella is an opportunistic bacterium and infects mainly immunocompromised people suffering from various diseases such as diabetes.
Very limited work has been done in our country regarding the microbiological quality to isolate especially the foodborne pathogens such as Escherichia coli and Salmonella sp and evaluate the quality of commercialized dahi.
Materials
Acidity and protein percentage were determined by titration with 0.1 N sodium hydroxide solution or by the Kjeldahl method according to the procedure of Aggarwala & Sharma (1961).
Flowchart of the study design
Methods
- Collection of dahi
- Bacteriological analysis Serial dilution
- Acidity
- Protein Percentage
Nine grams of sample was taken into a conical flask and 9 ml of distilled water was added to the sample and mixed thoroughly using a glass stirrer. Nine grams of well-mixed dahi sample was weighed and introduced into a butyrometer using a 9 ml pipette. 9 ml of soft water was added and mixed thoroughly. After digestion, the digested solution was cooled with air and 100 ml of distilled water was added to the digested sample.
Several biochemical tests were performed to have a presumptive identification of the potential bacteria selected earlier. Most of the methods were performed according to the microbiology laboratory manual (Cappuccino and Sherman., 2005). Using sterile technique, a small amount of the experimental bacteria from 24 h old pure culture was inoculated into the vials using a streak inoculation method with an inoculation needle and the vials were incubated for 48 h at 37◦C (Cappuccino and Sherman. , 2005).
The indole production test is done to determine the ability of bacteria to degrade the amino acid tryptophan by the enzyme tryptophanase. Using sterile technique, a small amount of experimental bacteria from the 24 h pure culture was inoculated into tubes by the ring inoculation method with an inoculation ring and the tubes were incubated for 48 h at 37◦C. The urease test is done to simultaneously determine the ability of bacteria to hydrolyze urea by means of the urease enzyme.
Using a sterile technique, a small amount of the experimental bacteria from a 24-hour-old pure culture was inoculated into the tubes by a stab inoculation method with an inoculating needle and the tubes were then incubated at 37◦C for 24 hours (Cappuccino and Sherman ., 2005).
Microbial count
Total Viable Count (TVC)
The presence of E. coli and Salmonella sp in dahi samples indicates contamination of dahi during their production and handling, which may cause public health problems. In this study, no bacterial growth was observed in selective media such as Mac conkey, EMB for E. coli and XLD for Salmonella sp. Greenish metallic sheen and black red colonies in the center were absent in A) Macconkey, B) EMB and C) XLD agar.
Biochemical Characteristics of bacterial isolates of different samples
The effectiveness of hygienic practices and sanitary status in the production of dairy food products are reflected by its total bacterial count, the presence of E. The present study is agreed by Rashid and Miyamoto., 2005 that the fat percentage of dahi in Bangladesh ranged from 3.00 at 4.75%. Although, the fat percentage should be higher because some moisture is lost during heating, but the total percentage does not increase due to the use of skimmed milk powder. Variations in fat content between different dahi samples may be caused by lack of quality control or milk standardization. for the production of dahi, adulteration of milk, etc.
The result agreed with the results of Chakraborty., (1998) and Ali., (1998) who found that milk dahi contained 4.22% and 4.44% protein a respectively in low protein findings and the results of Chakraborty who found protein 5.1 % in buffalo milk dahi in case of high protein finding. Present study also showed presence of other pathogenic microorganisms such as Salmonella spp., Pseudomonas spp. Another study was conducted by Khan et al., (2008) on colostrum from balk tank milk before it is processed into other milk products such as buttermilk and curd.
The presence of these pathogenic isolates was also confirmed in the subsequent product where milk was used. Like the study by Khan et al., (2008), in this study, Escherichia coli and Pseudomonas spp. Another study was conducted by Beukes et al. (2000) where they investigated dairy products and fermented milk products.
During the study, all the samples were tested to confirm the presence of Staphylococcus aureus, Salmonella spp., Pseudomonas spp. All samples were negative for E. coli and Salmonella sp, indicating proper hygienic handling, preparation, marketing and storage of dahi. A study on production of Dahi from whole milk of cow, buffalo and their various proportional mixtures M.S.
Eosin Methyl Blue (EMB) Agar
Physiological Saline
Triple Sugar Iron Agar
The reagent bottle was then covered with aluminum foil to prevent exposure of the reagent to light and stored at 4°C.
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