Aroma characteristics in Ghanaian maize dough fermentations

Abstract

The dominant microbiota of Ghanaian fermented maize dough comprising lactobacilli and yeast have been identified and characterized for use as starter cultures in previous studies. The development of the starter cultures is aimed at enhancing the quality and safety of the dough as well as reducing fermentation time. Studies with use of legume combinations with cereals prior to fermentation have shown an acceleration of acid production. The aroma of Ghanaian fermented maize dough and fermentations of the dough in the presence of starter cultures or legumes has not been fully investigated. Volatile compounds associated with Ghanaian maize dough prepared by spontaneous fermentation and by the use of added starter cultures were identified and compared. The, starter cultures were added singly and in combinations in separate fermentation trials carried out in Ghana. The starter cultures examined were Lactobacillus fermentum, Saccharomyces cerevisiae and Candida krusei. Four combinations of starter culture were used as follows: S. cerevisiae + C. krusei; L. fermentum + C. krusei; L. fermentum + S. cerevisiae and L. fermentum + S. cerevisiae + C. krusei. Volatile compounds produced in fermented maize dough fortified with 20% blanched soybeans were also compared with the unfortified spontaneously fermented dough. Extraction of aroma volatiles-by Dynamic headspace and the Likens-Nickerson simultaneous distillation and extraction technique were compared in a preliminary evaluation and the method of Likens-Nickerson selected for use throughout all fermentation trials based on the ability to trap more volatile compounds which included volatiles identified by the Dynamic headspace method. Separation and identification of volatiles in fermented maize dough extracts were carried by GC-MS installed with a library of mass spectra for probability based matching. Further identification was conducted by matching spectra of compounds with mass spectra and retention indices. of pure standard reference compounds. Using a panel of four trained judges the method of Gas Chromatography Olfactometry (GCO) also referred to as 'GC-sniffing' was used to identify compounds with an impact on the aroma. They were identified as contributing to the characteristic aroma of maize dough fermented spontaneously or with the added use of starter cultures or soybeans. The organoleptic acceptability of a popular Ghanaian sour porridge, 'Koko' prepared from maize dough with added combinations of the starter cultures was evaluated using the Preference test based on a 9-point hedonic scale. The development of acidity and microbial population of yeasts and lactic acid bacteria were monitored' throughout fermentation trials. Data on the relative concentrations of aroma volatiles were analysed by Principal Component Analysis (PCA) and Partial Least Squares Regression (PLS-R) A total of 76 compounds were identified in spontaneously fermented maize dough by Likens-Nickerson technique compared to 30 by Dynamic headspace method. The compounds identified included 21 carbonyls, 19 alcohols, 17 esters, 12 organic acids, a furan, 2 phenolic compounds, an alkene and four unidentified compounds. The 'GC-sniffing technique showed 46 compounds as contributing to the aroma of spontaneously fermented maize dough. The aroma of spontaneously fermented Ghanaian maize dough could be described as a complex mixture of odours that showed a trend of development as green and musty in freshly prepared dough to alcoholic and fruity by 48 - 72 h of fermentation. The use of added single starter cultures showed 5 more compounds that could be identified by GC-sniffing. These compounds were mainly esters. A comparison of maize dough samples showed that spontaneously fermented maize dough was characterized by higher levels of carbonyl compounds while fermentations with added L. fermentum recorded the highest level of acetic acid. Fermentations with added S. cerevisiae produced higher amounts of fusel alcohols and increasing level of esters with fermentation time and C. krusei showed similarity to L. fermentum with lower levels of most volatiles identified. Using Principal Component Analysis (PCA) the aroma profiles of maize dough samples fermented with the various combinations of starter cultures could be separated into five distinct groups based on the type of fermentation. Fermentations where S. cerevisiae and C. krusei were combined produced the highest concentration of aroma compounds, particularly esters. Combinations of L. fermentum and S. cerevisiae produced high concentrations of ethanol and fusel alcohols while a more balanced profile that included alcohols, carbonyls, esters and acids was obtained with the combination of L. fermentum and C. krusei. Lactic acid and acetic acid concentrations were highest with the combined use of all three starter cultures. Sensory studies indicated scores of 'like very much' for 'koko' from spontaneously fermented maize dough and from combinations of L. fermentum and C. krusei as well from the triple combination of starter cultures. 'Koko' from L. fermentum and S. cerevisiae and from the combination of the two yeasts were liked 'moderately'. The aroma profile of soy-fortified fermented maize dough was not significantly different from the unfortified dough. A slight decrease in total concentration alcohols was observed in soy-fortified fermented maize dough. An acceleration in the production of lactic and acetic' acids occurred in soyfortified samples, however, the maximum concentration of acetic acid achieved did not significantly exceed the maximum concentrations obtained in the unfortified dough. The present study has given a detailed picture of the aroma compounds associated with Ghanaian maize dough fermentation and demonstrated that the predominant: microorganisms in the dough can be used as starter cultures to modify the aroma. The study has documented the advantage of using starter cultures in African traditional food processing and provided a scientific background for introducing better controlled fermentations