Fungal inhibition by lactic acid bacteria (LAB) is modulated in vitro by cocoa fermentation-related conditions: towards a biocontrol of fungi in processing cocoa

Abstract

Fungal growth impair strongly the quality of processed cocoa, leading to low crops value for farmers and manufacturers. The antifungal activity of lactic acid bacteria (LAB) from fermenting cocoa was analyzed and the conditions of fungal inhibition were evaluated. Fungi were isolated from stored fermented and dried cocoa beans on sabouraud plate medium. The results showed that, LAB strongly inhibit fungi isolated from fermented cocoa beans notably Mucors and Aspergilli, with Lactobacillus plantarum inducing the stronger inhibition whereas Leuconostoc mesenteroides produced a weaker inhibition. Acids production assayed by HPLC was found to be not related to antifungal activity, since LAB strains producing strong antifungal activity were not necessarly the best acid producers. Maximum fungal inhibition occured at 35 °C in Lactobacillus plantarum and Leuconostoc mesenteroides but also at 30 °C in  Lactobacllus casei. Likewise, a narrow acidic pH range (3.0-4.0) allowed full expression of fungal inhibtion in LAB analyzed that decreased progressively toward pH 7.0 and failed at alkalin pH. However, sucrose even at high content (8%) was found to have no effect on antifungal activity of LAB, but its bioconversion product glucose and fructose decreased this activity when concentrations were set at 6 et 2 %, respectively. All together, the results suggest that, LAB may effectively exert fungal inhibition in a narrow timing of cocoa fermentation, with pulp contained sugars as metabolic regulators of this inhibition. These results may contribute to a better management of LAB as starter culture for an efficient inhibition of fungal growth and prevention of cocoa contamination from mycotoxins.

Key Words : Antifungal activity ; lactic acid bacteria ; cocoa fermentation ; inhibition variation; pH and temperature ; sugars content.