**4.2 Microbial properties of milk and milk products**

306 Epidemiology Insights

awareness on hygienic handling of milk and milk products, which could mainly be attributed to inefficient extension services; shortage of capital to purchase recommended equipment (milk containers, and processing and packaging materials); lack of clean water for sanitation purpose and poor condition of barn or milking area that is directly related to

Milk, at its normal state, has unique physico-chemical properties, which are used as quality indicators. The density of milk, among others, is commonly used for quality test mainly to check for the addition of water to milk or removal of cream. Addition of water to milk reduces milk density, while removal of cream increases it (O'Connor, 1994). The solid constituents of milk make milk an important food item from nutritional as well as processing point of view. Milk fat and protein are most important components of different varieties of most shelf stable milk products. All the 22 milk cooperatives used specific gravity test as an indicator of milk quality. The average specific gravity of 72 milk samples tested was 1.028 and 1.029 for Holetta and Selale areas, respectively (Fig. 6). These values fall within the range between 1.028 and 1.032 given to unadulterated milk (O'Connor, 1994). About 21% of the same samples checked with alcohol test were positive, while only 14% of the samples were positive for clot-on-boiling test. The results indicate that certain farmers deliver milk to collectors that has undergone fermentation that occurred either through long time elapsed between milking and delivery; mixing evening and morning milk; or use of

shortage of capital and limited awareness of its implication on milk quality.

Fig. 6. Specific gravity/density of milk samples in Selale and Holetta areas

The mean pH values of whole milk ranged from 6.29 for Debre Birhan to 6.66 for Selale (data not presented). This variation and low pH may be attributed to the long time elapsed between milking and supply to collection centers. Three of the 24 samples of pasteurized milk checked with alcohol test were positive, while 5 of the samples were positive for cloton-boiling test. These results are consistent with the results of pH values observed for the

**4. Quality of milk and milk products 4.1 Density and freshness of products** 

insufficiently cleaned milk containers.

The microbial content of milk indicates the hygienic levels during milking that include cleanliness of the milking utensils, proper storage and transport as well as the wholesomeness of the udder of the individual cow (Spreer, 1998). The most commonly used microbial quality tests for milk and milk products include determination of total bacterial count (TBC) or standard plate count (SPC) and colifom count (CC). Estimation of yeast and mould counts is also useful for evaluating sanitary practices (O'Connor, 1994). Microorganisms can enter milk via the cow, air, feeds, milk handling equipment and the milker. Once they get into the milk their numbers increase rapidly. It is therefore more effective to exclude microorganisms than trying to control their growth once they get access into the milk.

There are varieties of traditionally fermented dairy products in Ethiopia, for which the exact type of desirable lactic acid bacteria responsible for fermentation is unknown due mainly to uncontrolled and spontaneous fermentation. Most of these products are produced by smallholder producers where access to the required dairy infrastructure is limited. Results of the present study and selected reports of relevant earlier research efforts on the microbial properties of locally produced milk and milk products that have been carried out in different parts of the country are briefly summarized below.


TBC: Total Bacterial Count; YMC: Yeast and Mould Count

Table 3. Overall bacterial and yeast and mould counts (log10) per ml/g of milk and milk product samples collected from different sources (sites and producer groups)

Although there are slight variations between sample sources (locations/producer groups) in microbial counts, the figures observed in the present study are generally much higher than acceptable limits (Table 3). TBC is generally high in samples of whole milk, *Ergo* and skim milk. Counts of *Enterobacteriaceae* and coliform counts are higher than acceptable limits: *Enterobacteriaceae* <1 and coliform <10 cfu/ml for pasteurized milk, and coliform <100 cfu/ml for raw milk intended for direct consumption (Council Directives 92/46/EEC, 1992). The higher count in milk indicates substandard hygienic conditions practiced during production and subsequent handling. The high count in fermented milk products, however, can be partly explained by lactic acid bacteria.

Microbial Properties of Ethiopian Marketed Milk and Milk Products and

*Total bacteria* 

*Enterobacteria* 

*Yeast and mould*

**4.2.2 Ergo – Naturally fermented milk** 

*Coliforms* 

cooperatives

source (producer type)

Associated Critical Points of Contamination: An Epidemiological Perspective 309

**Producer Whole milk Ergo\* Butter Arera Ayib Skim milk** 

Smallholder farmers 8.87 9.48 6.86 9.35 7.16 - Cooperatives 9.49 9.54 6.14 - 6.51 9.25 Overall mean 9.10 9.49 6.67 9.35 7.00 9.25

Smallholder farmers 5.51 4.94 4.97 4.94 4.85 - Cooperatives 5.45 4.98 4.90 - 4.82 5.30 Overall mean 5.48 4.95 4.95 4.94 4.84 5.30

Smallholder farmers 5.59 4.48 4.60 4.65 4.44 - Cooperatives 4.54 4.61 4.53 - 4.37 4.37 Overall mean 4.58 4.51 4.58 4.65 4.42 4.37

Smallholder farmers - 8.39 8.34 - 8.26 - Cooperatives - 8.34 8.27 - 8.27 - Overall mean - 8.38 8.32 - 8.26 -

\*Ergo refers to fermented whole milk for smallholder farmers, while it refers to fermented skim milk for

Table 4. Microbial count (log10) per ml/g of milk and milk products categorized by sample

Total aerobic plate counts were 1.1 x 105, 4 x 106 and 1.9 x 108 cfu/ml respectively for milk samples taken from milking bucket, storage container and processing plant on arrival (Godefaye and Molla, 2000). In the same study, mean coliform counts were reported to range from 1.3 x 104 cfu/ml (storage container) to 7.1 x 106 cfu/ml (on arrival at the processing plant). The hygienic quality of the milk from a collection center was poor with a mean total bacterial count of 1.3 x 107 cfu/ml. In another study, 4 x 107 and 1 x 109 cfu/ml total microbial counts were reported as lowest and highest values, respectively for raw milk samples at a processing plant in Addis Ababa. Of the total counts in raw milk, psychrophilic, thermoduric

*Ergo* is made by natural fermentation of milk under ambient temperature, without the addition of starter cultures (Assefa *et al.*, 2008). The use of a portion of ergo from a previous batch as a starter in highland areas where ambient temperature is relatively low is reported (Kassa, 2008). This practice is technically adapted to overcome the effect of low ambient temperature, which slows down the growth of lactic acid bacteria (LAB) in the absence of starter culture thus prolongs the fermentation time. The temperature and duration of incubation varies from place to place depending on the prevailing environmental conditions. According to a previous report, five LAB genera were identified from *Ergo* that include *Lactobacillus*, *Lactococcus*, *Leuconostoc*, *Entrococcus* and *Streptococcus* (Assefa *et al.*, 2008). They also indicated the LAB species identified in *Ergo* to be *Streptococcus thermophilus*, *Strep. acidominus*, *Enterococcus faecalis* var. *liquefaciens*, *Strep. bovis*, *Strep. mitis*, *Strep. agalactiae*, *Lactococcus cremoris*, *Leuconostoc dextranicum*, *Leuc. lactis*, *Lactobacillus xylosus* and

and thermophilic organisms made up 98.1, 1.4 and 0.5%, respectively.
