Plant Science Today

Sweet potato (Ipomoea batatas (L.) Lam.) is an important food crop with great source of starch. Sweet potato starch has inferior properties like high swelling power, soft gel texture and low paste clarity. Peracetic acid is an environmentally friendly oxidizing reagent without harmful effects to human health. This research evaluated the feasibility of peracetic acid concentration (2, 4, 6, 8, 10 ppm) and starch slurry ratio (1:8, 1:10, 1:12, 1:14, 1:16 w/w) to functional characteristics of the oxidized sweet potato starch. Results showed that the highest swelling power (57.34%), solubility (2.68%) and peak viscosity (6264 cP) were obtained by peracetic acid 6 ppm and starch slurry ratio 1:12 w/w. Peracetic acid could be successfully applied as a powerful oxidizing agent in starch modification.


Introduction
Hylocereus polyrhizus is extensively cultivated in Southeast Asia. H. polyrhizus fruit is rich in lycopene, anthocyanin, flavanoid, antioxidants and phytoalbumins (1)(2)(3)(4)(5). Its peel occupied 20-25% of the fruit with a great source of phenolics especially betalain but it is usually discarded as waste (6)(7)(8). Its peel can be utilized and converted into value-added product by pigment extraction. It can be considered as the natural food colorants not only healthy for human body, but also friendly for society and environment (9-11). Hibiscus sabdariffa is one of underutilized ornamental plants widely distributed in the subtropical and tropical regions of West, East Africa and South-East Asia. H. sabdariffa calyx has beautiful unique anthocyanin pigment with high antioxidant properties (12-14). Its flower contains valuable phytochemical bioactive constituents like phenolics, flavonoids, vitamins greatly contributing to different therapeutic advantages to cure hypertension, hyperlipidemia, cancer and inflammatory diseases; mild the blood viscosity; stimulate milk release and induce intestinal peristalsis (15)(16)(17)(18). It's commonly utilized as purple dye for colouring and flavouring agent to enhance the flavour, aroma and overall acceptance of wine, syrup, juice, jam, jelly, pudding, cake, ice cream, tea, confectionery, sauce, marmalade, chocolate (14, [19][20][21][22][23][24][25][26][27]. Peristrophe bivalvis is a herbaceous perennial plant widely distributed in cool and humid climate areas (28). It has numerous colors like purple, purple-magenta, red and yellow (29). It contains different photosynthetic pigments, anthocyanins, phenolics, flavonoids. It's useful in food, biological and pharmaceutical industries. It has various therapeutic functions such as antihypertension, anti-hyperlipidemia, fungistatic, antibacterial, antioxidant (30-33).
Lactobacillus acidophilus is a gram [+] bacteria with rod morphology, approximately 2-10 µm in dimension. L. acidophilus is homofermentative in glycolysis to ferment hexoses and produce D and Llactic acids (34). It grows well at 30-45 o C and pH 4-5 to produce a great amount of lactic acid, some acetic acid with no hydrogen and no catalase (35). L. acidophilus is proved to possess various beneficial effects against health disorders. It has ability to prevent blood cholesterol, mutagenicity, carcinogenicity, constipation, diarrhea, lactose intolerance (36,37). It's commonly used in the meat, milk, fruits, vegetables and cereal products (38). Yoghurt is a probiotic carrier originated from fermentation of milk. Yoghurt is nutritionally rich in available proteins, minerals, vitamins (39, 40). Yoghurt is extremely superior to milk, specially for people facing lactose intolerance because lactose has been converted to lactic acid by the bacterial starter culture (41,42). Yoghurt also has useful functional application as probiotic carrier against gastro intestinal disorders (39, 43). Yoghurt can improve gum health, facilitate calcium absorption, limit osteoporosis (44). There has a great demand of symbiotic yoghurt containing prebiotics and probiotics to boost human health and well-being. Different literatures mentioned to yoghurt production enriched by numerous phytochemical sources such as coconut (45), spirulina (46), dietary fiber (47), date (48), milk protein (49), strawberry pulp (50), apple pomace flour (51), artichoke flour (52), olive fruit polyphenol (53), soy bean flour (54), H. sabdariffa (55), H. polyrhizus (56). Yoghurts are prepared in various styles and varieties (57). However there was any report mentioned to the utilization of P. bivalvis to enrich yoghurt. Objective of our study was to examine the possibility of yoghurt incorporated with various pigment extracts from H. polyrhizus, H. sabdariffa and P. bivalvis.

Material
Pasteurized milk and skim milk powder were purchased from grocery store. H. sabdariffa (Roselle), H. polyrhizus (red pitaya), P. bivalvis (magenta plant) were collected from January to June of 2018 in local gardens of Ke Sach district, Soc Trang province, Vietnam. They were kept in fresh and quickly moved to laboratory for experiments. Lactobacillus acidophilus from Vinmec was kept at 20 o C during transportation and utilized as starter culture for Yoghurt fermentation. Chemical reagents were all analytical grade.

Researching method
H. sabdariffa calyx, H. polyrhizus fruit pulp and P. bivalvis leaf materials were preliminary dried by convective drying at 50 o C to 18 % moisture content and extracted by ethanol 70 % (at ratio 1:3, material: solvent) by hot extraction and then evaporated under a rotary concentrator to get pigment extracts. 1000 mL of pasteurized milk and 250 gm of skim milk powder were primarily mixed together. 5 gm of H. sabdariffa, H. polyrhizus and P. bivalvis pigment extracts were individually added into the prepared mixture. This mixture was heated at 85 o C for 20 minutes and cooled to 43.5 o C before adding 0.1% L. acidophilus (12 log colony forming unit/g or cfu/gm) as starter culture. The incubation was conducted at 43.5 o C for 8 hrs to produce yoghurt. The control group was similarly prepared as the above protocol without pigment extracts. All manufactured yoghurt samples were all examined total acidity, syneresis, viability of starter culture, total phenolic content, free radical-scavenging activity.

Physicochemical, microbial and antioxidant determination
Total acidity (%w/w lactic acid) was estimated by titration with 0.1 N NaOH using phenolphthalein as an indicator. Syneresis (%) was evaluated by draining 50 ml of unstirred yoghurt spread evenly on filter paper at 4 o C for 6 hrs. The obtained whey volume multiplied by 2 was considered as syneresis (58). Viability of L. acidophilus (log cfu/gm) was enumerated by 3M-Petrifilm protocol (59). Total phenolic content (mg gallic acid equivalent/ 100 gm or mg GAE/ 100 gm) was examined by the Folin-Ciocalteu protocol (60). Free radical-scavenging activity (%,DPPH) was determined by 2,2 diphenyl-1picrylhydrazyl supporting by UV/Vis spectrophotometer (60). Fig. 1 shows the effect of H. sabdariffa, H. polyrhizus, P. bivalvis extracts on the total acidity of the enriched yoghurt. Total acidity had an increasing trend during storage. During the fermentation of milk into yogurt, the high metabolic activity of yogurt bacteria decreased with cooling whereas enzymatic activity continued. Hence, an accumulation of total acidity was noticed during storage (61). Yogurt supplemented with H. sabdariffa had the highest total acidity (0.83÷1.13%). Meanwhile the yoghurt incorporated with P. bivalvis had the lowest total acidity (0.79÷0.99%). This may be attributed to the higher acidity of H. sabdariffa extract compared to other ones. According to one study, total acidity of H. sabdariffa extract was 4.20±0.01% (in fresh calyx), 12.73±0.09% (in concentrated fresh roselle extract) and 11.96±0.34% (in concentrated dried roselle extract) (62). Supplementation of H. sabdariffa calyx extract into the reconstituted low fat milk yoghurt resulted in significant reduction of pH 4.4÷4.16 vs. 4.54÷4.31 as control (63). H. sabdariffa L. flowers marmalade significantly increased titratable acidity of formulated yoghurt (64). Addition of H. sabdariffa calyx extract into the probiotic yoghurt resulted to total acidity 0.81÷1.14% (65). Yogurts enriched by H. polyrhizus resulted a higher lactic acid percentage (1.14÷1.23%) compared to plain yogurt (1.08%) (56).    (64). H. sabdariffa was reported as an excellent source of antioxidants and total phenolics (67)(68)(69). The total phenolic content in the yogurt enriched by H. sabdariffa was noticed at 15.21 mg GAE/100 gm (70). Yogurts enriched by H. polyrhizus resulted a higher total phenolic content (36.44-64.43mg/ml) compared to plain yogurt (20.25 mg/ml) (56). Our results were similar to the findings in another study (71). Yoghurt supplemented with aronia, blueberry and grape juice would receive great advantage of probiotic with excellent phenolic intake.

Conclusion
Yoghurt is one of the most common fermented dairy products with a great consumer acceptability due to its functional advantages. There is a trend of using yoghurt incorporated with plant pigments as bioactive constituents that are safe and potential health benefits.