Microorganisms have traditionally been selected for fermentation of milk on their ability to grow in milk. However nowadays the trend is towards including probiotic bacteria, i.e. those bacteria that have been suggested to provide additional health benefits to the consumer. Probiotic microorganisms have beneficial effects when ingested such as lowered incidence of colon cancer, the suppression of putrefactive and pathogenic bacteria by competitive exclusion and the production of organic acids, diacetyl and bacteriocins and a hypocholesterolemic effect, to mention but a few. To be more effective, the proposed microorganisms must be of human origin. Lactobacillus acidophilus and Bifidobacterium bifidum, which are two of the most successful probiotic bacteria used commercially, are normal inhabitants of the intestine of many animals including humans. They must however maintain their viability and activity in the bio-product in which they are formulated to be available at the right level at the time of consumption. A number of factors affect their viability, including low pH, the type of culture used and availability of essential amino acids. The choice of ingredients in the mix is crucial since it affects a number of mix properties such as pH and availability of nutrients. The demand for low fat products is a very strong driving force on the market place. However, reducing the fat content of such complex products as frozen dairy desserts is very difficult, since fat forms an integral part of the product. Removing fat poses a number of challenges as to which other ingredients to add in its place. Microparticulated whey protein concentrates have been formulated by food technologists to mimic the functionalities of fat especially as far as creaminess is concerned. Little information is available on fat replacement in frozen yoghurt, in terms of both its effects on the survival of probiotic bacteria and on some important quality parameters. Likewise, little research, if any, has been done on the perception of strawberry flavour in fat-free ice-creams and similar products. The purpose of the present study was thus to provide information on fat replacement by a microparticulated whey protein concentrate (Simplesse® 500) in frozen yoghurts and its influence on the survival of 1. acidophilus and B. bifidum. Several quality parameters of the frozen yoghurts were also evaluated. Frozen yoghurts were prepared by inoculating the mix with an ABT (acidophilus, bifidum and thermophilus) culture. Four mixes were formulated to contain 10% (m/m) milk fat, 5% (m/m) milk fat, 5% (m/m) of the microparticulated whey protein concentrate (WPC) and 3.4% (m/m) of the microparticulated WPC respectively. Decreasing milk fat from 10% to 5%, together with the addition of more milk solids-not-fat, did not lead to a significant increase (p > 0.05) in the acidity of the mix. Likewise the buffering capacity in both alkaline and acidic conditions were not significantly different (p > 0.05). Substituting fat with Simplesse® 500 at 3.4% levelled to a significant decrease in the pH of the mix (p < 0.05), which was due to the presence of more weak acids. The addition of the microparticulated WPC also led to an increase in the buffering capacity of the mix, which was due to an increase in weak acids and other buffer systems present in milk such as the citrate and phosphate systems. The addition of more Simplesse® 500 did not result in a significantly higher acidity (p > 0.05) since whey proteins are only weak acids. The viability of all three the bacteria types present in the ABT culture did not seem to be related to the presence or absence of the microparticulated WPC. The numbers of Streptococcus salivarius subsp. thermophilus, 1. acidophilus and of B. bifidum did not differ significantly (p > 0.05) between the four yoghurt mixes after incubation, ageing, whipping and freezing and three weeks of storage. However, only S. salivarius subsp. thermophilus and 1. acidophilus increased in numbers during fermentation in all the yoghurt mixes and only S. salivarius subsp. thermophilus increased significantly (p < 0.05) as a result of ageing in all the yoghurt mixes. Although the addition of the microparticulated WPC led to an increase in the acidity of the yoghurt mixes it also led to an increase in the buffer capacity, which thus helped to maintain the numbers of S. salivarius subsp. thermophilus and 1. acidophilus at the same level as in the mixes containing milk fat. While S. salivarius subsp. thermophilus and 1. acidophilus grew to numbers greater than 107 cfu/g, B. bifidum did not grow at all in any of the yoghurt mixes and the results suggest that they might not be available at the right level to have any therapeutic benefits to the consumers. Decreasing fat content led to an obvious increase in coarseness of frozen yoghurts (p < 0.05). The increasing perceived coarseness could be related to the amount of ice nuclei formed during whipping and freezing. The addition of the microparticulated whey protein concentrate could have resulted in a decrease in the freezing point, low enough to lead to a decrease in the amount of ice nuclei formed as a result of whipping and freezing. Therefore, the amount of unfrozen water available to freeze during hardening increased, thereby leading to an increase in the size of ice crystals. Fat is also known to decrease the size of ice crystals and the presence of more fat could also have led to an increased perception of smoothness. Increasing fat content led to a decrease in the perception of strawberry flavour and an increased perception of an aftertaste (p < 0.05). Fat is known to bind lipophilic compounds as well as decrease the melting rate and mass transfer, both of which will decrease the release of flavour compounds.
Dissertation (MSc (Food Science))--University of Pretoria, 2006.