Five different batches of skim milk were prepared and fortified by the addition of skim milk powder (SMP) or sodium caseinate (Na-cn) or by concentration using a vacuum evaporator (EV), ultrafiltration (UF), or reverse osmosis (RO) to contain similar levels of protein (5.0-5.5%). Yoghurts were made by inoculating the milks with one of 3 commercial yoghurt starter cultures and by incubating the mixes at 42°C for 2.5 h. The following factors were found in this study to affect firmness of the yoghurts: (a) Lactic acid production (acidity) - Yoghurts containing 1.02% of lactic acid or more (pH 4.54 or less) were firmer than yoghurts having a lower lactic acid content and a higher pH value. (b) Casein to non-casein protein ratio - Firmer yoghurts were obtai ned at a ratio of 4.62 than at 3.20-3.4D.
Microstructure of the yoghurts as examined by electron microscopy was affected by the method of fortification of the milk. SMP-fortified yoghurt had the most dense matrix composed of short micellar chains and small micellar clusters. This was the softest yoghurt. Na-co-fortified yoghurt had the most open matrix consisting of robust casein particle chains and large clusters. This was the firmest yoghurt.
Appendages or spikes formed by heatdenatured B-lactoglobulin or by a complex consisting of B-lactoglobulin and K-casein were attached to casein micelles in all the yoghurts except the one fortified by the addition of Na-cn.
Void spaces (cavities) around lactic acid bacteria and filaments of mucous or slimy material produced by a ropy bacterial culture and attaching the bacterial cells to the protein matrix were additional microstructural features observed in the yoghurts under study
