Investigating the physicochemical and antioxidant properties of functional yogurt containing Malva Sylvestris extract

Abstract
Introduction:
In recent years, with the increase the of scientific research, human awareness of the importance of functional food and the relationship between the types of food consumed and human health has increased (Bulut et al. 2021). Malva sylvestris is a one-year, two-year, and sometimes multi-year plant. Aqueous extracts of leaves and flowers of Malva sylvestris contain various compounds such as phenols, flavonoids, terpenoids, catalase enzyme, sulfite oxidase, fatty acids, beta-carotene and vitamins C and E (Taha Nezhad et al. 2012). Yogurt is one of the most important fermented dairy products, which is attractive to consumers due to its long shelf life, high nutritional value, favorable sensory properties, and good digestibility. Yogurt is produceds from milk fermentation by Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus and produced (Soliman et al. 2022). Yogurt is a suitable carrier for functional food additives, probiotic bacteria, and bioactive compounds with healthiness properties (Barukcic et al. 2022). In many types of researches, plant extracts have been used as carriers of bioactive compounds to produce functional yogurt. However, so far, no studies have been conducted on the production of functional yogurt enriched with malva sylvestris extract. Therefore, the latest study aims to evaluate the effect of malva sylvestris extract on the physicochemical, rheological, and sensory properties of functional yogurt.
Material and Methods:
To produce malva sylvestris extract, 200 grams of dry leaves were crushed in a blender and mixed, and washed with 80% ethanol. After centrifugation at 1107 g for 10 minutes, extraction from 20 grams of dry leaves with hot water with a ratio of solvent to dry matter of 30% was carried out. Next, the precipitated material was dried in a vacuum at a temperature of 50 ºC (Jooyandeh and Samavati, 2017). To prepare yogurt, whole milk was standardized to about 14% by adding skim milk powder (3%). Then, it was heated at 90 ºC for 10 minutes and malva sylvestris (0, 0.5, 1, 1.5, and 2%) was added to the mixture. After cooling to 42 ºC, a 2% starter was added to the mixture. Finally, the samples were stored at 4 ºC for 21 days (Cho et al. 2020). The pH of the yogurt samples was measured using a digital pH meter and the acidity was assessed by titrating the samples with 0.1 sodium hydroxide solution (Bulut et al., 2021). To determine the percentage of yogurt syneresis, yogurt samples were centrifuged at 3075 g for 20 minutes (Rojas-Torres et al. 2021). Viscosity was evaluated using a viscometer, total phenol content was evaluated by the Folin-Ciocalto method, and antioxidant activity was evaluated based on free radical reduction of 2,2-diphenyl-1-picrylhydrazyl (Tizghadam et al. 2021; Atwaa et al. 2022). Sensory evaluation was performed by 12 trained panelists (including 6 women and 6 men, experts in food science, 25 to 35 years old) using the 5-point hedonic method in terms of taste, color, smell, texture, and overall acceptance (Rojas-Torres et al. 2021).
Results and discussion: The results showed that malva sylvestris extract had a significant effect on the pH and acidity of enriched yogurt samples (p<0.05). The presence of malva sylvestris extract caused a significant decrease in pH and a significant increase in acidity in treated samples. It seems that the addition of malva sylvestris extract stimulates the metabolic activity of yogurt starter, lactobacilli, and streptococci. The results show that in the presence of malva sylvestris extract the amount of total phenol (TPC) and antioxidant activity (AO) in yogurt samples increased significantly (p<0.05). It seems that the phenolic compounds in milk, which are influenced by animal nutrition, are the main factors of total phenolic content and antioxidant activity in the control sample of yogurt (Barukcic et al. 2022). Also, the presence of benzoic acid and its derivatives, cinnamic acid, ferulic acid, hydrocinnamic acid, scopoletin, tyramine, sesquiterpene lactone has been reported in the malva sylvestris extract are probably responsible for the antioxidant activity of malva sylvestris extract and treatment samples (DellaGreca et al. 2009). The results cleared that the presence of malva sylvestris extract in yogurt samples had a significant effect on increasing the stability and reducing syneresis in yogurt (p<0.05). It seems that the decrease in the syneresis in the treated samples is due to some interactions between yogurt proteins and malva sylvestris extract compounds, which strengthens the gel matrix of yogurt and the gel network can maintain more serum. Also, the interaction of polyphenols and yogurt proteins can change the structure of the protein in the gel network and improve the protein affinity (Kwon et al. 2019). The results revealed that with the increase in the concentration of malva sylvestris extract, the viscosity of the treatment samples increased significantly (p<0.05). It seems that the increase in the interaction between the phenolic compounds of the extract and casein in the gel network caused an increase in viscosity (Hassanpoor et al. 2016). According to fitting the rheological behavior of yogurt samples with Herschel Bulkley model and Power law, it was found that the behavior of yogurt samples has a high correlation coefficient with Power law model. In the sensory evaluation, it was found that the treatment with 1/5% malva sylvestris extract was the most acceptable among the panelists.
Conclusion: In this research, the formulation of a new functional yogurt was evaluated using cheese extract. The results of this study showed a significant increase in the amount of total phenol and antioxidant activity of yogurt in the presence of malva sylvestris extract. In general, our results recommend the consumption of this enriched yogurt as a functional product for improving consumer health. Also, the syneresis of enriched yogurt decreased and the stability of yogurt increased. The addition of malva sylvestris extract at the level of 1.5% had a good effect on the sensory characteristics of the yogurt samples, but the use of higher levels had a negative effect onhurt the sensory characteristics. Therefore, a study on the production of cheese extract encapsulated in functional yogurt is suggested.