Changes in high and low molecular weight carbohydrates during Rhizopus nigricans cultivation on lemon peel

Abstract

Rhizopus nigricans was cultivated in a liquid medium using lemon peel as the carbon source. During cultivation, changes were measured in high and low molecular weight carbohydrates from the growth medium, as well as changes in the uronic acid amount of the alcohol insoluble solids from the lemon peel before and after fermentation. The initial amount of carbohydrates in the cultivation medium originated from the solubilisation of small sugars and partial hydrolysis of lemon cell wall polysaccharides in the acidic medium during the autoclaving step of the growth medium preparation. A complex mixture of monosaccharides (fructose, glucose, xylose, inositol), cellobiose, unknown oligosaccharides, galacturonic acid oligomers (penta-, tri-, and monogalacturonic acid), and polysaccharides was solubilised in the cultivation medium. During fermentation the fungus grew, produced the pectic enzyme endopolygalacturonase and consumed free sugars (fructose and glucose) and galacturonic acid. A carbohydrate polymer fraction remained resistant to fermentation, while a fraction of lower molecular weight was consumed.