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The study was conducted to optimize the walnut addition level in order to obtain better quality yogurt as a functional food. In the results, it was determined that the addition of walnuts affects the gross chemical component of yoghurt samples (p < 0.05) It was found that pH and serum separation values decreased during storage. On the other hand, it was observed that titration acidity values increased.

Yogurt, which is considered the most consumed product of dairy products, has an important role in people’s nutrition due to its high nutritional value. Yogurt contains high levels of protein, lactose, water-soluble vitamins and calcium (Aryana & Olson, 2017). Food fortification, which means adding one or more ingredients to foods, is carried out regardless of whether the ingredient is present in the food. The purpose of food fortification is to add additional properties to newly designed food products (Świeca et al., 2014). Due to the bioactive components in it, functional foods are effective in physical and mental recovery of individuals and in reducing the risk of diseases. This is because bioactive compounds contain beneficial components (Biesalski et al., 2009). Since it is a frequently consumed dairy product with antioxidant activity, the researchers added some beneficial components to yogurt. These are useful ingredients such as apple polyphenol and paprike extract (Halah & Mehanna, 2011; Tamjidi et al., 2012). Yogurt can usually be consumed by fruits and vegetables (Kiros et al., 2016; Oliveira et al., 2015), or by adding herbal extracts, seeds or nuts (Boycheva et al., 2007; Boycheva et al., 2007a; Damyanova et al., 2009; Dimitrov et al., 2002; Todorova et al., 2009). Since fruits and vegetables contain rich phenolics, herbal extracts, seeds and nuts are also consumed with yoghurt due to their positive effects on human health. All these additions increase the quality of yogurt. Nuts contain high levels of vitamin C and vitamin E. Thanks to vitamin C, it has a powerful antioxidant feature. Thanks to vitamin E, it prevents tissue damage. It also takes part in the neutralization of free radicals. Hazelnut containing vitamin E is especially important in cancer and cardiovascular diseases (Giugliano, 2000). Apart from that, nuts also contain folic acid. Folic acid is the compound required for cell division and formation of red blood cells. Therefore, pregnants need high amounts of folic acid during pregnancy (Wardlaw, 1999). Walnut, on the other hand, has both polyunsaturated fatty acid and high nutritional value (Exler & Weihrauch, 1986; Savage, 2001). Due to its frequent consumption and high nutritional value, walnuts (Juglans regia L.) which are grown in many regions of the world, are a useful food source that people have been consuming since ancient times (Cheraghali et al., 2018). Walnuts contain high amounts of protein and fat. Due to this situation, it is of great value for human nutrition. Walnut, which is included in the priority plants list of FAO (Gandev, 2007), contains vitamins and minerals as well as high protein and fat content. Walnuts are also good sources of flavonoids, sterols, pectic substances, phenolic acids and related polyphenols (Çağlarmak, 2003; Crews et al., 2005; Martinez et al., 2010; Muradoğlu et al., 2010). Walnut, which contains high amounts of omega-6 and omega-3 PUFAs, has a critical role in the importance of coronary heart disease due to the omega-3 PUFA it contains (Davis et al., 2007). Walnut kernels have the highest antioxidant activity among 25 commonly consumed food types. This is because walnuts contain high phenolic levels (Gunduc and El, 2003). Walnut phenolics show high levels of antioxidant properties and scavenge free radicals (Colaric et al., 2005; Fukuda et al., 2003). The total phenolic in 50 grams of walnuts (about a handful) is higher than a glass of apple juice (240 ml), 43 grams of milk chocolate, or a glass of red wine (150 ml).

The aim of this study is to optimize the addition of walnuts to obtain a better quality yogurt as a functional food, and to predict the physicochemical, sensory, phenolic and antioxidant properties of yogurt enriched with walnut pieces.

Walnuts that were purchased from a local store in Zonguldak were cut into 3-5 mm. To obtain yoghurt, sterilized full-fat cow’s milk purchased from a local store in Zonguldak was used. Skimmed milk powder (NFMP) was obtained from Pınar Dairy Products in Izmir.

Strains grown prior to use were then reconstituted (11% w / w). After this period, it was pasteurized (85 ° C) for 20 minutes in skimmed milk. After pasteurization the strains were incubated at 42 ± 1 ° C and the incubated milk was immunized with this culture at a ratio of 2.0% (1: 1).

In the technique created by Tamime and Robinson (1999), cow milk with 3% fat, 3.6% protein, 12.1% solid content and 8 SH acid is used in yogurt production. Yogurt production in the study was also carried out according to this technique. By adding skimmed milk powder in yogurt, the solid content in yogurt was increased to 15% (15 grams per 100 grams). Then, at concentrations of 0%, 1%, 2.5%, 3.5% and 5% walnuts were added to the milk and the milk was was homogenized (Homogenizer T 65, Ika, Staufen, Germany). The milk was heated for half an hour and pasteurized (85 ° C). Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus was added to the mixture obtained as starter culture at a rate of 2% (w / v). After this period, the mixture was incubated at 42 ± 1 ° C and pH 4.6-4.7 for 3.5 hours. After the incubation period, the milk was rested at 4 ± 1 ° C for half a day (12 hours). Yoghurt to be used within the scope of the analysis was kept at 4 degrees for 28 days. As part of yoghurt production, the process was repeated twice and yoghurt was measured once a week (on the 1st, 7th, 14th and 28th days).

The whey syneresis, achieved through the mass and quantity of whey, was determined by weighing 25 grams of yogurt on filter paper placed in a funnel. After this process, whey was collected in a graduated cylinder. The drainage time is 2 hours and the drainage temperature

is +4 C (Atamer & Sezgin, 1986; Tamime et al., 1996).

Gallic acid was used as a standard during the determination of the phenolic content in yoghurt determined by the Folin-Ciocalteu method (Singleton and Rossi, 1965). The absorbance level was measured by spectrophotometer at 760 nm (Shimadzu Scientific Instruments, Inc., Tokyo, Japan). The gallic acid equivalent (GAE) determined with the help of the standard gallic acid curve gives the total phenolic content concentration in yogurt. The results obtained within the scope of the study are expressed as mg GAE / kg.

Sensory evaluation of yoghurt was performed by 10 trained panelists (4-5 males, 4-5 females) using sensory evaluation scorecards. These panelists consist of faculty members of the Food Engineering Department of Faculty of Engineering, Zonguldak Bulent Ecevit University. The panelists’ age range is 25-34. Yogurt has been evaluated in five different categories. After each weekly storage (1, 7, 14, 28 days) yogurts were evaluated in terms of appearance, consistency, taste-aroma, odor and general acceptability. The evaluation for four categories (appearance, taste-aroma, appearance and general quality) was evaluated on a 10-point scale (Lawless & Heymann, 1999; Altuğ-Onoğur & Elmacı, 2011; Özer, 2006).

The whole experimental process and subsequent analyzes were repeated twice. Data were analyzed with the help of SPSS for Windows 16.0. During the analysis process, repeated one-way analysis of variance (ANOVA) measurement was used. The differences between the repeated data were analyzed with Duncan analysis (SPSS, 2017).

Table 1 shows the average composition of yogurts fortified with walnut compared with control yogurt. It was observed that the gross chemical composition of the yogurt samples were affected significantly by the addition of walnuts (p <0.05) (Table 1). It was determined that the amount of solid content in the samples increased with the increase of the walnut level. pH and serum separation values ​​decreased during storage, but titration asthma values ​​increased. In addition, consistently low pH and increasing acidity were observed throughout the storage process. The aforementioned results are similar to the results of previous studies (Güler-Akın, 2005; Guven et al., 2005; Sahan et al., 2008; Ye et al., 2013; Dal Bello et al., 2015; Ozturkoglu-Budak et al., 2016; Su et al., 2017; Baba et al., 2018; Ertem and Çakmakçı, 2018; Ujiroghene et al., 2019).