Cassava lactic fermentation in Central Africa : microbiological and biochemical aspects | [Fermentacion lactica de la yuca en Africa central : aspectos microbiologicos y bioquimicos]

Retting is a lactic fermentation during which cassava roots are soaked for long periods in water. Despite the importance of this fermentation, no kinetic study of it has been undertaken. Our study therefore examined the biological and physical changes of cassava roots during retting to provide a basis for its possible mechanization. The study was carried out to (1) enumerate and characterize the main microorganisms of the process; (2) determine the evolution of physicochemical parameters during retting; and (3) measure the production of organic products and some principal enzyme activities. Retting can be characterized by three essential transformations of the roots: (1) a degradation of endogenous cyanogenic compounds (e.g., concentration decreased from 400 ppm in fresh cassava to 20 ppm in fermented mash); (2) a significant lysis of cassava cell walls due to the simultaneous action of endogenous pectin methylesterase and bacterial pectin lyase; and (3) the production of organic acids (C2 to C4), mainly lactate and butyrate, that contribute to the typical flavors of chikwangue and fufu. In the study, most microflora involved in retting were facultative, anaerobic, fermentative bacteria, among which lactic bacteria were predominant. From the second day of fermentation, endogenous Lactobacillus species were totally supplanted by Leuconostoc mesenteroides and Lactococcus lactis. Anaerobic bacteria such as Clostridium butyricum were also found and seemed responsible for initiating butyrate production. Yeasts played no significant role, but their increasing number at the end of the process (Candida species) probably influenced the conservation of end products. Despite the significant number of amylolytic bacteria (10(5)-10(6) b/ml), the amylase activity found in the retting juice came from the roots and disappeared after 48 h of fermentation. The main enzymes of this process were cassava pectin methylesterase, bacterial pectinase, and endogenous linamarase. The pH became stable at about 4.5 after 48 h and the partial oxygen pressure dropped to 0.2 mg/L after 10 h. These results suggested that retting is a typical heterolactic fermentation with a significant production of butyrate.