Evaluation of physicochemical, textural and sensory properties of fortified yogurt with zinc and iron micronutrients

Yogurt is the most frequently consumed fermented milk product with a positive impact on human health due to its high nutritional value. Characteristics such as acidity, amount of free fatty acids with sensory characteristics and nutritional properties of yogurt are very important. Food fortification is defined as addition of one or more essential nutrients at levels higher than naturally presence in food products. The World Health Organization estimates that at least 1 of every 5 people in the world suffer from lack of iodine, zinc, iron, folic acid, calcium, vitamin A and B. The aim of this study was to evaluate the changes in physicochemical, textural and sensory properties of iron and zinc enriched yogurt during the shelf-life period and to assess the extent and the effect of these supplements on these properties.
To perform enrichment, milk was divided into seven parts: 3 containers for fortification with iron, 3 containers for fortification with zinc and 1 container as the control sample. Fortification with iron and zinc was performed by concentrations of 20, 40 and 60 mg per kg of milk (according to the daily requirement of iron and zinc). The samples were then transferred into the container of 100 grams which were incubated at 45 °C, and after reaching the pH of 4.4 to 4.5, they were transferred to cold storage at 4 °C. To investigate the effect of enriching on the produced yogurt properties, the samples were evaluated after a period of one, seven and fourteen days. The syneresis of yogurt samples was determined by Salvador and Fiszman (2004) method, with slight modification. Water holding capacity was determined by centrifuge (Hettich Universal 320R - Germany) as reported by Sahin et al (2008) with slight modification. The acidity of the yogurt samples was measured by Kim and Lee (2002) method based on the percentage of lactic acid. Sensory characteristics of our samples in terms of taste, odor, color and texture and overall acceptability were evaluated by 12 Panelists (6 males, 6 females, 45- 21 years) by using a five-point hedonic test. Texture analysis was performed using Texture Analyzer (Brookfield Model CT3 Texture Analyzer – USA) based on Supavititpatana et al (2008) method.
Syneresis of product decreased from 0.24 to zero in yogurt samples containing iron and from 1.20 to 0.81% in zinc treatments. Increasing the storage time in the presence of iron causes a sharp decrease in the amount of the syneresis up to zero. The main reasons for syneresis in fermented products include high incineration temperature, low total solids and inadequate storage temperature. The water holding capacity of the product was shown a relatively increase over time and the highest amount was observed in the treatments containing high concentrations of iron during the early days of storage, while at the same high concentrations in the final days of storage, the lowest water holding capacity was observed, which could be due to the adverse effect of the storage time on this factor. The viscosity increased over time, and the rate of increase in the early days was far higher than the final days (in both iron and zinc treatments). In iron and zinc treatments respectively, water holding capacity increased from 50.08 to 55.50, and 43.57 to 55.47 % and viscosity from 855.55 to 961.11, and 677.78 to 833.30 mPa s. By analyzing data obtained from texture analyzer, important properties such as firmness and springiness increased and the lowest cohesiveness and the highest adhesiveness were observed in the middle of storage time. Both iron and zinc treatments increased the hardness of the product compared to the control samples and the amount of this increase in iron containing yogurt samples was slightly higher than that of zinc. In both treatments, there was a significant decrease in the product adhesion force compared to the control sample. In contrast, the highest continuity of product was observed in the middle days of the storage period.The highest levels of flavor were detected on the seventh day of storage, and changes in the zinc and iron content of the products were almost ineffective in the desirability or taste loss, and only control samples were evaluated with a little higher flavor than other treatments. Sensory properties were not significantly different (P