The effects of fermentation on phenolic content, in-vitro bioavailability and physical properties of black mulberry yoghurt

Mulberry (Morus atropurpurea Roxb.) is widely consumed both as fresh fruit and as an ingredient in food processed products, which is a rich source of bioactive compounds including vitamins, carotenes, flavonols, anthocyanins, phenolic and other phenolic compounds. Also milk and fermented dairy products are rich source of many nutrients like proteins, calcium, phosphorus, vitamin B2 and B12. Studies which shows health-promoting properties of yoghurt and milk has also increased the number of studies on fruit and fruit flavoured products that people consume on their diets. The objective of our study is to investigate the effects of fermentation on antioxidant capacity, total phenolics/flavovoids, in-vitro bioaccessibility and rheology of yoghurts which is prepared by the addition of black mulberry to milk. During production of yoghurt, fruits was added to milk in different proportions before and after fermentation, glukano-delta-lactone was used as an alternative to starter culture and also storage effects was investigated. Analysis were applied to milk, black mulberry, fruit milk and fruit yoghurt. It was aimed to contribute literature on protein-phenolic interactions, effects of fermentation, starter culture and storage for new resources and projects. Total phenolic compound, flavonoid and total antioxidant activity of milk, black mulberry, fruit milk and fruit yoghurt were analyzed spectrophotometrically. For determining antioxidant capacity DPPH (2,2-Diphenyl-1-Picrylhydrazyl), ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)), FRAP (Ferric Reducing Ability of Plasma) and CUPRAC (Cupric Reducing Antioxidant Capacity) methods were applied. Anthocyanin profile was investiagted with HPLC method and for determining bioavailability in-vitro digestion method was applied. In this method gastrointestinal system was simulated in a test tube in the laboratory environment by using stomach enzymes and bile salts and changes in the bioavailability of anthhocyanins was observed. Also for determining the effects of interactions on rheological properties of yoghurts, water holding capacity of gel, gel hardness and gel strength analysis were applied. Samples which were produced to investigate the effect of fermentation on the total phenolic content ranged from 62,8±9,7 to 89,5±5,5 mg GAE /100 g fresh wieght. Both yoghurts with starter cultures and glukano-delta-lacton which were produced with adding mullberries before fermentation has higher total phenolic contents than samples which were produced with adding mullberry after fermentation. Total phenolic content of yoghurt with GDL(glucano-delta-lacton) added 6% fruit before fermentation(H) was found to be 75,9±9,1 mg GAE /100 g fresh wieght which was found to be 21% higher than yoghurt with GDL(glucano-delta-lacton) added 6% fruit after fermentation(I) (62,8±9,7 mg GAE /100 g fresh wieght) and similarly total phenolic content of yoghurt with starter culture added 6% fruit before fermentation(J) was found to be 83,7±2,9 mg GAE /100 g fresh wieght which was 31% higher than yoghurt with starter culture added 6% fruit after fermentation(K) (63,8±10,1mg GAE /100 g fresh wieght) (p>0,05). Samples which were produced to investigate the effect of fermentation on the antioxidant capacity showed that samples which were produced by adding mulbery after fermentation had higher antioxidant capacity than before fermentation in CUPRAC and DPPH methods but had lower antioxidant capacity in ABTS method. The mean values of total antioxidant capacity of H analyzed with CUPRAC, DPPH and ABTS methods were 172,1±54,1, 73,3±7,7 ve 119,3±13,4 mg TE/100g fresh weight, respectively which were 14% and 26% lower according to the CUPRAC and DPPH methods and 36% higher than I according to the ABTS method (p>0,05). After 21 days of storage, decrease in total phenolic content were found to be 60% for yoghurt with starter culture (in the absence of added mulbery)(F), 153% for yoghurt with GDL (in the absence of added mulbery)(G), 34% for yoghurt with GDL(glucano-delta-lacton) added 6% fruit before fermentation(H), 53% for yoghurt with GDL(glucano-delta-lacton) added 6% fruit after fermentation(I), 33% for yoghurt with starter culture added 6% fruit before fermentation(J), 54% for yoghurt with starter culture added 6% fruit after fermentation(K), 6% for yoghurt with starter culture added 12% fruit before fermentation(L), 1% for yoghurt with starter culture added 12% fruit after fermentation(M), 1% for milk added 6% fruit(N) and 6% for milk added 12% fruit(O). On the other hand decrease in total flavonoid content were found as 31% for F, 34% for G, 47% for H, 45% for I, 22% for J, 36% for K, 34% for L, 27% for M, 83% for N and 24% for O. The mean values of total antioxidant capacity of samples analyzed after 21 days of storage showed significant difference (p<0.05) with CUPRAC method (except for J and Ksamples); with DPPH method (except for F, G and H samples); and with ABTS method (except for F, G, H and I samples). After the simulation of in vitro gastrointestinal digestion system, total phenolic content, total flavonoid content and total anthocyanin content as well as the total antioxidant capacity for each samples were evaluated. Total antioxidant capacity retrieval proportion (%) of J was found as 25% with CUPRAC method, 3,3% with DPPH method, 11% with ABTS method, total antioxidant capacity retrieval proportion (%) of K was was found as 14,2% with CUPRAC method, 3,7% with DPPH and 18,8% with ABTS method. pH values of fruit milks, fruit added yoghurts and plain yoghurts ranged between 4,2 and 5,9 in the first day; 4,0 and 6,1 after 21 days of storage. For samples all the samples-except N, pH values showed a decrase during storage. Samples which were produced by adding mulberry before fermentation showed higher L values and lower a and b values than the data obtained after fermentation. After 21 days of storage for all the samples a and b values increased and L values decreased. Sample which was produced by using starter culters had higher syneresis and viscosity values than the sample produced by using GDL and during storage increased more. Samples that were produced by adding mulberry before fermentation had higher synersis values and lower viscosity than the samples after fermentation. For all the samples syneresis increased, and the viscosity decreased during storage.