Lactobacillus plantarum is the starter culture frequently used for the lactic acid fermentation of sweet potato as well as other plant materials (Ray and Panda, 2007). Mostly acids are produced during lactic acid fermentation and hence the sourness increases and the sweetness decreases, as the sugars are fermented to acids. The lacto-pickles have been prepared both from β-carotene, as well as anthocyanin rich sweet potato roots. Anthocyanin-rich sweet potato root cubes were pickled through lactic acid fermentation by brining the cut and blanched cubes in common salt (NaCl) and then the probiotic strain of L. plantarum MTCC 1407 was inoculated. The fermentation was allowed for 28 days. The lacto-pickle obtained after the fermentation had a pH (2.5–2.8), TA (1.5–1.7 g/kg), LA (1.0–1.3 g/kg), starch (56–58 g/kg) and anthocyanin content (390 mg/kg) on pickle fresh weight basis. Sensory evaluation rated the anthocyanin-rich pickle acceptable based on texture, flavour and after taste (Panda et al., 2009). Likewise, β-carotene-rich sweet potato pickle has been prepared and the sensory analysis showed the acceptability of the product (Panda et al., 2007). Pickled sweet potato petioles have been commercialized in Japan (Panda et al., 2009). The preservative and other additives used are soy sauce, sugar, sesame seeds and chillies respectively (Woolfe, 1992).
Lacto-juice Lacto-juices processed by lactic acid fermentation bring about a change in the beverage assortment for their high nutritive value, vitamins and minerals, which are beneficial to human health when consumed (Ray and Panda, 2007). Lacto-juice is prepared by fermentation of β-carotene and anthocyanin-rich sweet potato cultivars by inoculating LAB, Lb. plantarum MTCC 1407 (Panda and Ray, 2007; Panda et al., 2008). Sweet potato roots (non-boiled/fully-boiled), rich in β-carotene are fermented with Lb. plantarum at 28 ± 2 °C for 48 h to make lacto-juice. During fermentation, both analytical (pH, TA, LA, starch, total sugar, reducing sugar (g/kg roots), total phenol and β-Carotene (mg/kg roots)) and sensory (texture, flavour and after taste) analyses of sweet potato lacto-juice were evaluated. The fermented juice was subjected to panellists’ evaluation for acceptability. There were no significant variations in biochemical constituents (pH, 2.2–3.3; LA, 1.19-1.27 g/kg root; TA, 1.23-1.46 g/kg root, etc.) of lacto- juices prepared from non-boiled and fully-boiled sweet potato roots, except β-carotene concentration (130 ± 7.5 mg/kg (fully-boiled roots) and 165 ± 8.1 mg/kg (non-boiled roots) (Panda and Ray, 2007)).
Sweet Potato Curd and Yoghurt Generally curd and yoghurt are produced by lactic acid fermentation of milk and are reported to possess several nutritional and dietary advantages over milk (Berger et al., 1979; Younus et al., 2002). Curd with 12–16 % sweet potato pulp was most preferred by consumer panellists (Ray et al., 2005). There are also some instances where milk is fermented along with dietary fibres, starch, minerals, vitamins, vegetables like French bean, lemon, soybean and sweet potato for the production of better enriched curds and yoghurts. Curd is popular in Asian countries, while yoghurt is popular in American and the European countries (Younus et al., 2002). A yoghurt-like product has been prepared from sweet potato puree, milk, sucrose and freeze-dried yoghurt inoculums. The product had 0.85 % titratable acidity (TA). Rates of TA development decreased as levels of sweet potato and sugar were increased. Time of fermentation ranged from 6.25-7.25 h. The fermented mixture became slightly darker and more orange in colour as the level of sweet potato was raised. A trained panel gave a mean score of 7.7 (scale 1-10) for flavour, 3.9 (scale 1–5) for body/texture and 3.8 for appearance/colour (Collins et al., 1991). Similarly, sweet potato curd was prepared by using anthocyanin-rich sweet potato. The curd was prepared by fermenting boiled anthocyanin rich sweet potato puree (0-24 %) and cow’s milk with starter culture (Lactobacillus bulgaris, Streptococcus lactis and Diacetic lactis). Addition of anthocyanin-rich sweet potato puree improved the quality of the curd in various attributes such as flavour, texture, minerals, nutrients, anti-diabetic substances, anthocyanin pigments, dietary fibre and starch. The curd prepared by using 8-12 % of sweet potato puree was the one most preferred by the tasting panel (Panda et al., 2006). In a similar process, curd was prepared by using β-carotene-rich sweet potato puree, cow’s milk and curd starter. The curd prepared by the addition of 12–16 % of β-carotene-rich sweet potato puree was the most preferred among the other combinations. The addition of β-carotene-rich sweet potato puree (12–16 %) made the curd firm and imparted flavour, body/texture, minerals, nutrients, anti-diabetic substances, β-carotene pigments (antioxidant), dietary fibres and starch (carbohydrate) (Mohapatra et al., 2007).
Acidophilus Milk Acidophilus milk is a product prepared by the fermentation of the milk by the bacteria, Lactobacillus acidophilus. The acidophilus milk has certain therapeutic value, as it possesses the capacity of neutralizing the adverse effects of toxin-producing organisms in the intestine of human beings, especially in infants (Perez and Tan, 2006). It is known to have beneficial effects on the maintenance of normal intestinal microflora by producing inhibitors, stimulating the host immune system and reduction of serum cholesterol levels. It also helps in nutritional enhancement by reducing the levels of toxic substances (Ray and Panda, 2007). Acidophilus milk enriched with purees from anthocyanin-rich sweet potato varieties (kinampay and RC 2000) was developed. Addition of sweet potato puree to the acidophilus milk improved the sensory qualities and nutritional values (Perez and Tan, 2006). The finding suggests that the optimum conditions for the production of anthocyanin sweet potato-rich acidophilus milk are 1 % inoculum, temperature of 37 °C and incubation time of 14 h. The acidophilus milk enriched with anthocyanin-rich sweet potato was most accepted at the 6.25 % sugar level.
Table 5.2 Biochemical composition of wine and medicated wine prepared from sweet potato
Parameters | Wine | Medicated wine
TSS (Brix) | 2.25 | 4.0
Total sugar (g/100 ml) | 1.35 | 0.95
Titratable acidity (g tartaric acid/100 ml) | 1.34 | 1.25
pH | 3.61 | 3.34
Lactic acid (mg/100 ml) | 1.14 | 1.92
Phenol (caffeic acid equivalent)/100 ml | 0.3 | 0.19
Anthocyanin mg/100 ml | 55.09 | 59.90
DPPH scavenging activity (at a dose of 250 pg/ml) | 58.95 % | 51.35%
Ethanol(v/v) | 9.33 % | 8.61%
Source (Ray et al., 2012; Panda et al., 2013)
Wine Yellow, red and black coloured beverages like beer (sparkling liquor) and wine are being sold in the Kyushu Province in Japan, prepared from anthocyanin-rich sweet potato (Yamakawa, 2000). Ray et al. (2012) demonstrated the production of wine from sweet potato (a special type of sweet potato having high anthocyanin pigment in the root). The starch contents of purple sweet potato (root: water homogenized in 1: 1 ratio) were enzymatically saccharified (using commercial thermostable enzymes Termamyl® (0.2 %) and Dextrozyme®GA (1 %)) to fermentable sugars and the filtrate was ameliorated with cane sugar to achieve 20°Brix, for subsequent fermentation into a red wine using 2 % yeast (Saccharomyces cerevisiae) as the starter culture. The bio-chemical composition of the wine was comparable to that of traditional grape wine (Table 5.2). Sensory evaluation was carried out by 16 trained panellists on various attributes such as clarity, colour, viscosity, odour, flavour, spritz and finish. An independent t-test confirmed that all the sensory attributes of the PSP wine scored closely (within 10 % variation) to that of a commercial grape wine. The red wine produced contains essential antioxidants and acceptable sensory qualities.