Investigation on the physicochemical properties of fortified yogurt containing pumpkin powder

Today, consumers increasingly believe in the effects of food on their health, and food should provide the necessary nutrients to prevent nutrition-related diseases as well as improve their physical and mental condition. Therefore, functional foods have been considered with the aim of improving the nutritional status, most of which is related to low calorie foods. Like many popular dairy products, yogurt consumption shows significant growth; The popularity of this product is primarily due to its high content of calcium, vitamins, minerals and low levels of fat, and secondly due to its health-promoting effect and inhibition of harmful bacteria. In the fresh mass of the pumpkin (Cucurbita moschata), total carotenoid content, a major contributory factor in the high nutritional value of pumpkins, ranges from 2 to 10 mg/100 g, the content of vitamins C and E accounting for 9–10 mg/100 g and 1.03–1.06 mg/100 g, respectively (Hosseini Ghaboos et al., 2016; Nawirska et al., 2009). Pumpkin fruit is also a valuable source of other vitamins, e.g., vitamin A, B6, K, thiamine, and riboflavin, as well as minerals, e.g., potassium, phosphorus, magnesium, iron and selenium (Mirhosseini et al., 2015; Obradović et al., 2015). Since pumpkin is a valuable micronutrients source, dried pumpkin could be processed into powder for foods to increase fibers, vitamin A and mineral contents. In addition, pumpkin powder can be used in bakery products because of its highly-desirable flavor, sweetness and deep yellow-orange color (Salehi, 2020).

At first, the pumpkin and spinach were dried and powdered in controlled conditions. Fresh pumpkins (Cucurbita moschata) were purchased from local market. The pumpkin slices with 5 mm thickness were dried in an oven (45°C). The dried pumpkins were milled, powdered and passed through a 125 mesh screen (Hosseini Ghaboos et al., 2016). To prepare yogurts containing pumpkin, skimmed milk was used form yogurt production factory with 1.5% fat. In fact, milk that was homogenized with a homogenizer (at a temperature of 70-60 ° C and a pressure of 150-160 bar) and by a pasteurizer (temperature at 92 ° C for 5 minutes) was pasteurized in the required amount in 5 liter containers. The starter culture (2% at 42°C) and squash powders (0, 2.5, 5 and 7.5%) were added to the milk. Then they were filled and closed. Approximately 100 samples were prepared at each time. Then, incubation was performed until the sample acidity reached 65°Dornic. For this purpose, the acidity of the sample was measured every 5 minutes at the end of the predicted fermentation time (experimentally). The used cold-room was an industrial cell greenhouse. The samples were placed in such a way that all the samples were on the same level and position in the greenhouse, which is very important for the uniform heating of the samples. After incubation, the samples were refrigerated at 4°C. Viscosity was measured using a viscometer with an LV64 spindle at a constant shear rate of 40 rpm and a temperature of 8°C. For this purpose, 250 ml of the sample was poured into a 250 ml glass beaker and stirred with a plastic spoon for 20 seconds. After turning on the device, and 30 seconds, the viscosity in centipoises was reported. Acidity was measured every 30 minutes to measure the fermentation time since the samples were incubated. When the acidity reached 70 degrees Dornic, the acidity was measured once until the acidity reached 80 degrees of Dornic. For yogurt, 10 g of the sample was titrated in the presence of phenolphthalein using 0.1 normal NaOH and the result was expressed based on °Dornic degree.

In this study, pumpkin powder in the amount of 0, 2.5, 5 and 7.5% in the formulation of 1.5% fat yoghurt was used and its chemical and rheological properties were studied for 20 days of storage. The results showed that the highest rate of syneresis (41.68%) was related to the interactions of the control sample with a shelf life of 20 days. The highest viscosity (3194 cp) was related to 7.5% pumpkin powder sample. The highest pH (4.39) was related to pumpkin powder 5%. The results also showed that the highest amount of acidity (74 °Dornic) was related to the control sample. The highest amount of total phenol (10 mg/g) and antioxidant activity (75%) was related to pumpkin powder 7.5%. The results showed that by adding 2.5% pumpkin powder to the yoghurt, the gel strength of sample was decreased and with increasing concentration up to 7.5%, its gel strength was increased but it is still less than the control sample. The results of comparing the means, the interaction effects of shelf life and pumpkin powder show that the highest rate of syneresis (41.68%) was related to the interaction effects of the control sample with a shelf life of 20 days. The results also showed that the lowest amount (39.98%) was related to the interaction effects of 5% pumpkin powder at a shelf life of 1 day, which was statistically 7.5% with a pumpkin powder at a shelf life of 1 day and pumpkin powder 5 and 5. 7.7% had no significant difference in shelf life of 12 and 20 days. According to the results of comparing the means, the highest viscosity (3173 cp) was related to the shelf life of 20 days and the lowest (3170 cp) was related to the first day of shelf life, which is statistically different from days 7 and 12. Based on the results of analysis of variance, the data showed that the effect of different levels of pumpkin powder and shelf life in terms of total phenol in yogurt showed a significant difference (P<0.05). As the results of comparing the means shows the highest amount of total phenol (10 mg/g) was related to pumpkin powder 7.5% and the lowest amount (8 mg/g) was related to pumpkin powder was at 0%.

In general, adding pumpkin powder to yogurt formulation can reduce the syneresis of the product and also increase its nutritional value.