9 The Microbiology Ethiopian Ayib
In Ethiopia, smallholder milk processing is based on sour milk resulting from high ambient temperatures, while meeting consumers' preferences and improving keeping quality (1). Ayib, a traditional Ethiopian cottage cheese, is a popular milk product consumed by the various ethnic groups of the country. It is made from sour milk after the butter is removed by churning. Traditional ayib making has been described by O'Mahony (1). Milk for churning is accumulated in a clay pot over several days. This is kept in a warm place (about 30°C) for 24 to 48 hours to sour spontaneously. Churning of the sour milk is carried out by slowly shaking the contents of the pot until the butter is separated. The butter is then removed from the churn and kneaded with water. The casein and some of the unrecovered fat in skim milk can be heat precipitated to a cottage cheese known as ayib. The defatted milk is heated to about 50°C until a distinct curd forms. It is then allowed to cool gradually, and the curd is ladled out or filtered through a muslin cloth. Temperature can be varied between 40° and 70°C without markedly affecting product composition and yield. Heat treatment does not appear to affect yield but gives the product a cooked flavor.
Ayib comprises about 79 percent water, 15 percent protein, 2 percent fat, 1 percent ash, and 3 percent soluble milk constituents. The yield should be about 1 kilograms of ayib from 8 liters of milk (1).
The safety of cheese with respect to food-borne diseases is of great concern around the world. This is especially true in developing countries, where production of milk and various dairy products often takes place under unsanitary conditions. Since there was no published information on the microbiology of milk and milk products in Ethiopia, a study was carried out in our laboratory to evaluate the microbiological quality of ayib as available to the consumer (2). One hundred samples of ayib were purchased at the Awassa market over 10 weeks. Since Awassa is an open-air market, ayib was generally handled at ambient temperatures (about 25° to 27°C during the study period). Samples were microbiologically analyzed within two hours of purchase.
Standard microbiological procedures were followed to determine the counts of aerobic mesophilic microorganisms, psychrotrophs, yeasts and molds, coliforms, bacterial spores, enterococci, Bacillus cereus, Listeria monocytogenes, staphylococci, and lactic acid bacteria. The pH of the samples was also measured.
Ayib samples showed high numbers of mesophilic bacteria, enterococci, and yeasts (Table 1). More than 90 percent of the samples had aerobic mesophilic counts of 10e8 cfu/g (colony forming units) or higher; more than 75 percent of the samples had yeast counts of 10e7 cfu/g or higher, and over 85 percent contained enterococci in numbers of 10e7 cfu/g or higher. The majority of the samples had mold and lactic acid bacteria counts of 10e5 cfu/g or higher, spore-formers of about 104, and psychrotrophs of about 10e6 cfu/g (Table 1).
Over 32 percent had coliform counts of more than 10e2/g, and about 27 percent contained fecal coliform loads of more than 10e2/g. Listeria spp. were not detected from the samples. B. cereus and S. aureus were isolated in 63 percent and 23 percent of the samples, respectively, but at very low numbers (10e2 to 10e3 cfu/g). About 40 percent of the ayib samples had pH values of less than 3.7, and 60 percent had values of 3.7 to 4.6.
Most of the production of milk and various milk products in Ethiopia is generally a household process that usually takes place under unsanitary conditions. However, despite its high moisture content, the low pH of ayib may prevent the further proliferation of various microorganisms. Yeasts, which can grow at lower pH values, may affect the flavor and keeping quality of ayib. In another study (3), proteolytic yeasts made up 47 percent of the total yeast isolates and all isolates showed lipolytic activities. Since traditional ayib making involves removal of fat from the sour milk, ayib contains only about 1 percent fat, and thus the lipolytic isolates may not play an important role in affecting the flavor or keeping quality of ayib.
TABLE 1 Frequency Distribution (Percent) of Aerobic Mesophilic Organisms, Yeasts, and Enterococci in Ayib Samples
(a)Colony forming units
Although proteolytic yeasts are important in cheese types that require ripening, their presence in a fresh product such as ayib is undesirable.
The findings in the previous studies indicated that ayib purchased from local markets was highly contaminated with various microorganisms. It was not known, however, whether these microorganisms were survivors of the heat treatment process or were postheating contaminants.
Another study was therefore conducted to determine the effect of cooking temperatures used in various parts of Ethiopia on the microbiological quality of the finished product and to recommend cooking temperatures that can decrease or destroy most microorganisms (4). Ayib was made in the laboratory using traditional methods. Pooled raw milk was allowed to sour naturally at room temperature. After removal of the fat by churning, the casein in the sour skimmed milk was heat precipitated at 40°, 50°, 60°, and 70°C in a water bath, and the curd was recovered by filtering through sterile cheese cloth.
Microbial analysis of raw milk, sour milk, and ayib indicated that heat treatment of the curd was effective at higher temperatures (Table 2). At these temperatures the time required for casein precipitation was also low. Heating the curd at 70°C for 55 minutes at pH 4 destroyed most of the microorganisms. The low pH also inhibited the proliferation of most surviving microorganisms. The high degree of contamination of market ayib could be due to either low curd cooking temperatures or addition of various plant materials to the finished product to give it desirable flavor, the packaging of ayib with Musa leaves, or other unhygienic handling practices. Thus, heat treatment of curd at 70°C and an appropriate handling of the product could result in a less contaminated and safer ayib.
TABLE 2 Frequency Distribution (Percent) of Lactic Acid Bacteria, Bacterial Spores, Molds, and Psychrotrophs in Ayib Samples
(a)Colony forming units
1. O'Mahony, F. 1988. Rural Dairy Technology Experiences in Ethiopia. ILCA Manual No. 4, International Livestock Center for Africa, Addis Ababa.
2. Ashenafi, M. 1990. Microbiological quality of ayib, a traditional Ethiopian cottage cheese. International Journal of Food Microbiology 10:263-268.
3. Ashenafi, M. 1989. Proteolytic, lipolytic and fermentative properties of yeasts isolate from ayib, a traditional Ethiopian cottage cheese. SINET: Ethiopian Journal of Science 12:131-139.
4. Ashenafi, M. 1990. Effect of curd-cooking temperatures on the microbiological quality of ayib, a traditional Ethiopian cottage cheese. World Journal of Microbiology and Biotechnology 6:159-162.
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