Production of fermented milk using a malty compound-producing strain of Lactococcus lactis subsp. lactis biovar. diacetylactis, isolated from Zimbabwean naturally fermented milk.

Malty compound-producing Lactococcus lactis subsp. lactis biovar. diacetylactis strain INF-DM1, originally isolated from naturally fermented milk in Zimbabwe was used to prepare fermented milk from ordinary milk, milk enriched with 2.5% (w/v) skimmed milk powder and by 2.5% (w/v) increase in dry matter by ultrafiltration. Inoculated milk was incubated at 22, 30 and 37 degrees C. Analyses were made after 0, 9, 18 and 24 h incubation and also after 24 h incubation followed by storage for one week at 4 degrees C. Samples were analysed for volatile compounds including malty compounds and for organic acids, pH and log cfu/g. All samples were also judged for sensory attributes. Products made from enriched milks showed increased viscosity which was most marked in ultrafiltrated milk incubated at 30 and 37 degrees C. The levels of certain compounds (lactic acid, citrate and diacetyl) were significantly affected by milk type. Incubation temperature had a significant effect on starter growth rate and the rate of production and amount of the malty compounds, lactate, diacetyl, ethanol, acetoin and acetaldehyde. 3-Methyl butanal concentrations were above the taste threshold level of 0.06 ppm in almost all products, including stored products. Although initial growth rate was fastest at 37 degrees C, an uncoupling of acid production and growth was observed after 9 h incubation, suggesting that this is above the optimum temperature for this strain. In addition, products incubated at 37 degrees C showed a tendency to whey separation, indicating that this temperature is also too high to give optimum product quality. All products attained good scores in sensory analysis provided that fermentation was complete. Variation in the levels of malty compounds during the fermentation had no significant effect on the sensory score for total flavour.