Optimisation of the growth of ‘A. oryzae’ in a koji model system

Abstract

Aspergillus oryzae is a filamentous fungus that can be used for rice fermentation to produce Asiatic food productslike koji. Fermentation is a metabolic process performed by organisms such as yeasts or fungi by breaking down a substrate into different metabolites. The breakdown of macromolecules such as carbohydrates, fats, sugars and proteins into smaller metabolites is performed by enzymes, the activity of which can be controlled by applied environmental conditions; such as temperature and water activity. The environmental conditions may be controlled to produce specific flavour imparting metabolites, which can be translated into distinct commodities. Different expression levels of key enzymatic genes will result in different metabolites and their respective quantities, which can be translated into a variety of flavours and aromas. By fermenting rice in different conditions, we expect to form an array of koji with a variety of flavours and aromas. This study aims at identifying the optimal environmental conditions for the growth of Aspergillus oryzae and the expression of key enzymatic genes by performing the fermentation process in different temperatures and water activity. This study confirmed that the highest growth rates for A. oryzae are achieved at temperatures varying from 32°C to 37°C at a aw of 0.970 in a rice agar model. The highest germination rates are achieved by the same range of temperatures at a aw of 0.990. Protocols for making model koji, RNA extraction and gene expression assessment were also optimised in this study, providing a tool that can be used by manufacturers to produce a bespoke product efficiently.