Production of Vinegar from Omani Dates and Characterization of its Aroma Compounds

Abstract

Dates (Phoenix dactylifera) are carbohydrate rich fruits that occur in numerous cultivars. Date palms are grown in great numbers in Oman and are the country’s most important crop. The dominant carbohydrates in Omani dates are the simple sugars glucose and fructose which constitute about 56 -62% of the fruit’s weight. Their high concentration of sugars makes dates a potential raw material for many industries including vinegar production. The current method used to produce vinegar in rural areas in Oman results in low quality vinegar, in terms of sensory quality and hygiene. High quality vinegar can be produced by implementing well established techniques that yield products with high value, a delicate flavour and desirable functional properties. The overall aim of the current work is to optimize a process to produce high quality vinegar from Omani dates that will be well accepted in the marketplace.

In order to optimize the method of production, an experimental factorial design was applied to determine the factors affecting either alcoholic or acetous fermentation. These experiments showed that many factors affected the alcoholic fermentation including the temperature and initial total solids measured in brix. Temperature was found to be the sole factor that affected alcoholic fermentation of date extracts with initial total solids of 10°brix, while with higher initial total soluble solids of 20°brix, temperature, addition of NH4, initial pH, the combined effect of temperature + initial pH, the combined effect of NH4 addition + initial pH, and the combined effect of temperature + NH4 addition + initial pH were found to have significant effects on alcoholic fermentation. The highest ethanol production of 38.72 g/l was achieved at an incubation temperature of 35°C with initial total solubles of extract 10°brix. However with a more concentrated extract of 20°brix, a higher ethanol production of 77.60 g/l was achieved at an incubation temperature of 30°C. Initial ethanol and inoculum size of bacteria was found to significantly (p < 0.01 and p < 0.05, respectively) affect the acetous fermentation; the increase of initial ethanol from 64g/l to 77g/l increased the growth rate of the bacteria as indicated by an increase in absorbance at A600. The initial inoculum size also was also found to have a significant positive effect on bacterial numbers with absorbance values increasing from the initial value (A600) of 0.3 to 0.43, and from 0.3 to 0.63 for date mash with initial ethanol concentrations of 64 g/l and inoculum sizes of either 10 or 20 μl (cell count 7.25 ×105 cfu/ml) respectively during the fermentation. A similar trend was found for the date mash with an initial ethanol concentration of 77g/l where the absorbance was increased from 0.4 to 0.6, or from 0.4 to 0.8 with inoculum sizes of either 10 or 20 μl (cell count 7.25 × 105 cfu/ml) respectively. Vinegar was produced by two methods of acetification; with oak wood chips immersed in the fermenting medium (DVW), or without wood chips (DVP). By the end of two months aging the total acidity reached 5.52, and 5.58 % acetic acid, respectively. The acetification in oak wood chips resulted in the extraction of phenolic compounds such as furfural and benzaldehyde as shown by GC analysis. In order to assess the influence of the acetification method on the sensorial properties of date vinegar descriptive sensory analysis was carried out using a panel of seven assessors assigned from the Department of Food Science at the University of Otago. One way analysis of variance ANOVA and Tukey posthoc revealed that attributes associated with aroma, taste, after taste and the mouth feel of date vinegar significantly (p < 0.05) differentiated DVW from DVP in aroma attributes of pungency, woody, complexity, aroma intensity, wine character, in malty taste, in woody after taste, and in mouth feel. A novel extraction technique for sensory analysis was used in this study. Solid phase micro extraction was used to introduce samples to a GC-O system that was not reported within literature cited about sensory evaluation of vinegar. The technique allowed the sensory evaluation panel to describe the sensory quality of the eluting compounds with high frequency of up to 87%. The sensory panel provided a description of a total of thirteen odour active compounds that distinguished DVW from DVP; among them some compounds were associated with aging in wood casks such as whiskey lactone with a note of forest or coconut, eugenol with lavender or candy floss notes, and syringol that had a medicinal odour. Eight odour active compounds found exclusively in DVP were associated with a variety of odour qualities. These included isobutyric acid, cinnamyl alcohol, phenylethyl benzoate, nonanoic acid, 2ethyl-phenol, methyl vanillate, with odour qualities of cheesy, citrus/herbaceous, ripe fruit/strawberry, pastrami/brown, chemical, floral, caramel/coffee, and flowery/herbaceous, respectively. These findings established that dates are a valid raw material to produce vinegar with high quality sensorial and compositional properties. It was also shown that production steps such as thermal treatment, alcoholic and acetous fermentations affected the vinegar’s above mentioned properties. In addition, the acetification with oak wood chips affected the compositional structure of date vinegar and consequently the sensorial and antioxidative properties. Comparison with published data revealed that date vinegar has a comparable antioxidant activity to vinegars such as cider vinegar, however the production process imposes changes in antioxidant activity of the fermenting medium from the extraction through alcoholic and acetous fermentations. Nevertheless it was found that date vinegar had antioxidant activity that is comparable to wine vinegar and higher than cider and white wine vinegars.