Changes in volatile compounds during fermentation of nham (Thai fermented sausage)

Nham samples fermented by indigenous microorganisms and Lactobacillus curvatus starter culture were studied for their compositions of volatiles in comparison to a non-fermented sample which was prepared by adding antibiotics to the ingredients. The incubation for all samples was at 30°C for 3 days. Volatile compounds of nham were isolated and analysed using dynamic headspace sampling–gas chromatography-mass spectrometry (DHS-GC-MS). Fermentation of nham resulted in a decrease in aldehydes and sharp increases in esters and alcohols in the first day. Natural fermented nham and nham inoculated with 10^4 cfu/g L. curvatus starter culture (LC) were similar in their concentrations of total aldehydes, alcohols, esters and sulphur compounds. The most prominent aroma components identified by their odour active values (OAV) were ethyl butanoate (buttery, ripe fruit note) and sulphur-containing compounds from garlic. Other aroma impact compounds were ethyl 2-methylbutanoate (fruity), ethyl 2-methylpropanoate (fruity), 3-hydroxy-2-butanone (yoghurt-like), octanal (fatty-fruity) and hexanal (green). Ethyl butanoate, ethyl 2-methylbutanoate, ethyl 2-methylpropanoate and octanal were from microbial activity in the fermented samples whereas hexanal and 3-hydroxy- 2-butanone were also found in the non-fermented sample. The intensities of “fermented”, “acidic”, “garlic” and “overall flavour of nham” aroma evaluated by ten trained panelists, increased sharply on the first day. Acidic, garlic and overall flavour of nham aroma intensities then remained almost constant. The intensity of fermented aroma in LC nham, however, continued to increase on the second day. This was concomitant with the increase in ethyl butanoate (OAV = 9072). In comparison to the natural fermented sample, LC nham had a faster reduction in pH, higher fermented aroma on the second day, and higher concentrations of ethyl butanoate and 3-hydroxy-2-butanone.