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The exceptional nutritional values of milk have long been recognized. However, important drawbacks of milk and dairy products are their relatively high cholesterol contents and their low proportion of polyunsaturated fatty acids. Hypersensitivity to milk proteins can also draw the public away from fluid milk consumption. The nonpathogenic ciliate Tetrahymena thermophilahas a unique set of enzymes that converts cholesterol into provitamin D3and related compounds. It also contains Δ12 fatty acid desaturases and releases various hydrolases, including proteases, lipases, and phospholipases. We investigated the potential use of these peculiar traits to improve milk by short-term incubation with live ciliate suspensions. We found that cholesterol can be reduced to 50% of its initial content in whole milk by a 12-h period of incubation with T. thermophilacells, mostly by conversion to Δ7-desaturated cholesterol derivatives, including provitamin D3. In addition, a considerable increase in γ-linolenate (18:3, n-6) takes place. The protein profile is markedly changed by proteolysis, with a striking reduction of casein and enhancement in total amino acid content. Despite these changes, no marked alterations in flavor or odor were observed at the end of 12h of incubation. Thus, Tetrahymenaappeared ideally suited to correct for milk drawbacks, making this important source of nutrients more acceptable to the health-conscious public.

A method for the determination of the settling rate distribution of yeast aggregates using a spectrophotometer is proposed. The reproducibility of the assay is 5 % in the following conditions: quadruplicate measurements, use of fresh culture medium (pH = 4.7) as diluent, fixed biomass concentration (0.Ds40nm= 1.000), fixed agitation speed (1 cycle/second during 30 seconds). The storage of samples led to a significant loss of the aggregative state. A simulated model was formulated predicting that the scattering range of the distribution would not be affected by the light beam width used or the continuous variation in the absorption coefficient, but a 10 % inaccuracy should be expected for the average distribution estimated.