Chemometrics, physicochemical and sensory characteristics of pearl millet beverage produced with bioburden lactic acid bacteria pure cultures

Abstract

The aim of this study was to evaluate the physical, chemical and sensory characteristics of non-alcoholic pearl millet beverage produced using isolated and purified cultures of bioburden lactic acid bacteria (LAB). Traditional non-alcoholic pearl millet beverage (TNAPMB) was produced through spontaneous fermentation. The slurry was fermented for 36 h at 37°C while monitoring the microbial growth at 3 h interval. LAB were grown on deMan, Rogosa and Sharpe agar and identified using Vitek 2 system. The initial numbers of LAB were 7.04 log cfu/ml and increased to 8.00 log cfu/ml after 21 h. The beverage was dominated by LAB and contaminants and their survival was in succession. LAB from the genera Leuconostoc, Pediococcus, Streptococcus and Enterococcus were the main fermenting species in TNAPMB. Pearl millet extract (PME) was produced by hydrating pearl millet flour (PMF) with water (1:10, PMF:Water). To the mixture sprouted rice flour (10%), ground ginger (10%) and pectin (0.6%) were added. Stable PME was used in the production of plain non-alcoholic pearl millet beverage (PNAPMB). PME was pasteurized at 98°C for 30 min, hot filled and cooled to 25°C. The fluid was inoculated with Leuconostoc mesenteroides, Pediococcus pentosaceus and Enterococcus gallinarum each at 0.05, 0.075 and 0.1%, respectively, using factorial design and fermented for 18 h at 37°C. The pH of the beverage ranged between pH 3.32 and pH 3.90. L. mesenteroides, P. pentosaceus, E. gallinarum, the interaction between L. mesenteroides and P. pentosaceus and the interaction between L. mesentoroides and E. gallinarum had a significant effect (p ˂ 0.05) on the pH of PNAPMB except the interaction between P. pentosaceus and E. gallinarum (p = 0.631). The total titratable acidity (TTA) of the beverage ranged from 0.50 to 0.72%. All cultures had a significant influence (p ˂ 0.05) on the TTA of the beverage with the exception of the interaction between L. mesenteroides and E. gallinarum (p = 0.102). However, Monte Carlo simulation showed that E. gallinarum caused an increase in the pH and a decrease in the TTA of the beverage. During fermentation, the pH of the beverage is desired to decrease while the TTA increases, hence E. gallinarum was removed. The interaction between L. mesenteroides and P. pentosaceus at 0.05% and 0.025%, respectively produced an acceptable PNAPMB with potential for commercialization. Furthermore, moringa supplemented non-alcoholic pearl millet beverage (MSNAPMB) was produced by adding 4% of moringa (Moringa oleifera) leaf powder extract during the production of PNAPMB. The physicochemical, nutritional, microbial (LAB) and sensory characteristics of the PNAPMB, MSNAPMB and TNAPMB were determined. LAB were significantly (p < 0.05) affected by the fermentation period and increased from 3.32 to 7.97 log cfu/ml and 3.58 to 8.38 log cfu/ml in PNAPMB and SNAPMB, respectively. The pH of PNAPMB decreased from pH 5.05 to pH 4.14 while the pH of MSNAPMB decreased from pH 5.05 to pH 3.65 during the 18 h fermentation. The growth of LAB during fermentation had a significant effect (p < 0.05) on the pH of the beverages. The TTA increased from 0.14 to 0.22% and increased from 0.17 to 0.38%, in PNAPMB and MSNAPMB, respectively. The TTA of the beverage was affected significantly (p < 0.05) by the 18 h of fermentation. The protein content was 1.62, 2.17 and 1.50% in PNAPMB, MSNAPMB and TNAPMB, respectively. PNAPMB sample was deemed acceptable in comparison to the MSNAPMB. The total colour difference (ΔE) was 5.91 and 10.60 in PNAPMB and MSNAPMB, respectively in comparison to the TNAPMB. Volatile compounds with beneficial effect such as anti-inflammatory and anti-pathogenic properties were identified in the beverages. Principal component analysis indicated that the variations in characteristics of PNAPMB and MSNAPMB could be explained using total fat, saturated fat, total sugar, ash, moisture, proteins, chroma (C), hue and b*. The results showed that isolated pure cultures could be used as starter cultures in the production of non-alcoholic cereal beverages at a commercial level with predictable quality and safety properties.