**4.1 Density and freshness of products**

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 insufficiently cleaned milk containers.

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 same samples that ranged from the lowest 5.46 to the highest 6.14 with an overall mean value of 5.87. The relatively low pH of *Ergo*, ranging from 4.3 to 4.5 retards the growth of undesirable microorganisms, such as pathogens and spoilage bacteria, and enables its further storage (Gonfa *et al.*, 2001).
