Effect of using date molasses on the quality properties of set-type non-fat yoghurt

Introduction: Yoghurt is a well-known dairy product manufactured by fermentation of milk with lactic acid bacteria (LAB). Due to production of bioactive peptides by LAB during fermentation, yoghurt has higher biological activity than milk (Pessione and Cirrincione, 2016). Specifically, some peptides in yoghurt have antioxidant activities and are effective in inhibition of lipid peroxidation and removal of free radicals (Nielsen et al., 2017). Milk acidification induces the formation of continuous gel networks by incorporation of aggregated protein molecules and fat globules. The fat globules serve as structure modifiers and influence the rheological properties of the composite gels. Gel strength depends on fat globule size and the extent of their surface interactions with the gel network (Kirimlidou et al, 2017). The physical properties and microstructure profile of the gels are reflected onto texture and sensory properties of yoghurt (Öztürk et al. 2018). Development of more nutritious yoghurt is an ongoing topic in dairy science. Incorporation of technically-emulsified oil formulations rich in mono- and polyunsaturated fatty acids in yoghurt may help to increase its health benefits (Baba et al, 2018).
Reduction in consumption of dietary animal fat has been recommended by nutritionists due to the proven relationship between fat consumption and heart diseases. In this context, consumption of low or nonfat dairy products has increased because of recognition of their health benefits (Kucukoner and Haque 2003). Consumer trends have shown that consumption of healthful food supports healthy lifestyles and reduces the risk of disease (Asioli et al., 2017). These changes in consumer demand have driven the food industry to develop functional foods with health-beneficial effects (Vecchio et al., 2016). Low-calorie or low-fat dairy products have been available in EU or USA markets for a long time and their consumption have been increasing (Kucukoner and Haque 2003). Milk fat has an important role in the texture, flavour and colour development of dairy products. Fat reduction can cause some defects such as lack of flavour, weak body and poor texture (Haque and Ji 2003). Although the manufacture of low- or nonfat dairy products has been possible for many years, the use of fat replacers in the manufacture of dairy products is still novel. Fat replacers, which decrease the calorific value of food, can be used to solve some physical and organoleptic problems originating from low-fat levels in the final products. Fat replacers consist of mixtures of lipid originated fat substitutes, protein or carbohydrate originated fat mimetic or their combinations (Huyghebaert et al. 1996).
 Using fat replacers as improver of texture and flavor have developed in the formulation of dairy products. Date molasses is one of the most valuable secondary products of the date that rich in natural sugars such as fructose and glucose. This product contains significant amount of iron which can increase red blood cells and reduce the risk of anemia. In this context, the aim of this study was to investigate the effect of using different rates of date molasses as a fat replacer on the physicochemical, texture and sensory properties of yoghurt manufactured from non-fat milk.
Material and methods: In this research, two yoghurt samples were manufactured from whole milk (3%) and skim milk (<0.1%) as control samples and three yoghurt samples were manufactured with addition of date molasses in different concentrations of of 5, 10 and 15 % (w/w) to non-fat milk. All yoghurt samples were manufactured in three replicates. Skim milk powder (2% w/v) was added to all treatments in order to increasing the total solid of final product. The mixtures were then pasteurized at 90°C for 5 min and cooled to 43 ± 1°C, inoculated with 3% of starter culture (w/v), dispersed into plastic cups (200 ml), and incubated at 42± 1°C until pH 4.6. Following incubation, Yoghurt samples were stored in refrigerator (4±1°C) for 22 days and their physicochemical, texture and sensory properties were evaluated at 1^st, 8^th, 15^th and 22^th day of storage.
Results and discussion: Physicochemical analysis showed that increasing the rate of date molasses in yoghurt samples caused to decrease in acidity, viscosity and water holding capacity and increase in pH and syneresis (P<0.05). The yoghurt samples manufactured from different rates of date molasses had higher pH and syneresis and lower acidity, water holding capacity and viscosity compared with yoghurt samples manufacture from full-fat milk (P<0.05). On the other hand, with increasing the rate of date molasses in yoghurt samples, sensory properties including appearance and colour, texture and consistency and odour and falvour decreased (P<0.05). Increasing the amount of date molasses caused to decrease in hardness, cohesiveness and adhesiveness and increase in springiness of yoghurt samples (P<0.05). Results of fat, protein and dry matter analysis on the first day of storage in the yoghurt treatments indicated that, as expected, an increase in date molasses in the yogurt treatments resulted in increase in the dry matter. On the other hand, protein content in yoghurt treatments decreased with increasing the rate of date molasses. Also, according to the rate of milk fat used, control full-fat yoghurt had the highest fat content among the treatments.
 In terms of sensory properties, with increasing the date molasses, total sensory desirability of yoghurt samples decreased. In this context, the yoghurt containing 5% (w/w) of date molasses received the highest acceptability compared with other samples manufactured using date molasses. Moreover, increasing the rates of date molasses in the manufacture of non-fat yoghurt samples caused to decrease in hardness, cohesiveness and adhesiveness and increase in springiness values of them (P<0.05).
Conclusion: In this study, the effect of using different rates of date molasses on different quality characteristics of yoghurt manufactured from non-fat milk during 22 days of storage was evaluated. The results of different analysis showed that date molasses can be used in certain quantities in the manufacture of non-fat yoghurt. In conclusion, using 5% (w/w) of date molasses instead of milk fat led to the best result in manufacture of non-fat yoghurt in terms of physicochemical, texture and sensory properties. Analysis of texture characteristics is considered as a useful procedure for assessing the hardness, adhesiveness, cohesiveness and springiness in yoghurt. The results of texture analysis showed that increasing the date molasses rates in date molasses-enriched yoghurt samples resulted in increase in springiness and decrease in hardness, cohesiveness and adhesiveness.