Effect of heat treatment on the antimicrobial properties of tef dough, injera, kocho and aradisame and the fate of selected pathogens

Abstract

A known population from each of a 24 h culture of Bacillus cereus, Pseudomonas aeruginosa, Salmonella spp., Shigella spp., Klebsiella spp. and Staphylococcus aureus was inoculated into tef ¯our±water/kocho±water mixtures in screw-capped ¯asks and allowed to ferment for 30 h at room temperature (18±21 °C). The ¯asks were then heattreated. Cultures of the test bacteria were inoculated into tubes containing graded volumes of 30-h-fermented tef dough/kocho extracts which had been heat-treated at 45, 61 and 80 °C in assay broth containing aqueous extracts from injera and aradisame. They were incubated for 24 h at 32 °C and optical densities determined. Populations of the major indigenous bacteria, yeasts and moulds in fermented tef dough (30 h), kocho samples, injera and aradisame were determined from other control portions of the same samples. Higher temperature (80 °C) heattreatment promoted the inhibitory potential of extracts from doughs of both foods as compared with lower temperature heat-treatments (45 and 61 °C). Asporogenous test bacteria were affected more than the spore-formers. Better ef®cacy of extracts from injera and aradisame suggested improved antimicrobial properties of the baked products than in doughs. Heat of baking inactivated all vegetative cells although spores of B. cereus, the yeasts and moulds survived the heat (100 °C) applied for 5 min. The c.f.u./g of food for B. cereus was below the disease-causing level (0.5 ´ 101 and 1.5 ´ 103 , in injera and aradisame, respectively). Actual baking temperatures in homes are higher than the ones used here; if post-baking contamination is minimized or prevented, the products would be microbiologically safe with respect to the asporogenous pathogens when served fresh. Further studies on a¯atoxins and improved storage conditions for kocho are recommended.