INTRODUCTION
Rhizopus oryzae (Rhizopus arrhizus) is one of the most economically important members of the zygomycete group of fungi. It has long been used for enzyme production (e.g., glucoamylase and lipase), organic acid synthesis (mainly lactic acid), and various fermented food applications (oriental foods and alcoholic beverages) [1].
Potato peel waste (PPW) is the main and zero-value waste from potato processing plants, generating about 8% of the waste by weight. The problem of the disposal of PPW causes considerable concern to the potato industries in Europe and is becoming a major issue in the USA, thus implying the need to identify an integrated, environmentally-friendly solution. One such alternative is to convert PPW into mainly lactic acid with microorganisms in fermenters [2,3].
PPW has a high moisture content (80-90%). The chemical composition of the dry matter reported in the literature differs greatly, however, it is certain that PPW has a high carbohydrate content (mainly starch followed by dietary fiber such as cellulose/hemicellulose/lignin), which makes it a good basis for fermentation. Overall protein and lipid contents are generally low [2–5].
R. oryzae was known to secrete a large number of carbohydrate-digesting enzymes including amylases, cellulases, and hemicellulases, which makes it a candidate microorganism for exploiting PPW fermentation. The cellulase system in R. oryzae comprises two hydrolytic enzymes: extracellular endoglucanase and exoglucanase, and thus cellulosic wastes could be easily and rapidly converted into glucose without the requirement of alkali or acid pretreatments. Xylanase is an enzyme that catalyzes the hydrolysis of 1,4-beta-D-xylosidic linkages in xylans that are constituents of hemicellulose and was shown to be readily produced by R. oryzaefrom different xylan-containing agricultural byproducts. Finally,R. oryzae also possesses a starch-breaking ability, it has been shown to produce extracellular isoamylase and glucoamylase to saccharify starch from potato, tamarind, tapioca, and oat [1].
PPW needs to be dried and grounded before being used as a substrate in a fermentation process. Here, one of the substrate-related process parameters is the PPW particle size. As the particle size decreases, the surface area available for the hydrolysis reaction increases, which can lead to a higher production rate and yield [6]. The solids loading rate is another important parameter associated with the effective fermentation of PPW, it has been previously shown to significantly affect lactic acid, acetic acid, and ethanol yields in an undefined mixed microbial culture [7].
Available studies in the literature either focus on the utilization of other agro-wastes by R. oryzae or on the utilization of PPW by other microorganisms. This study intends to fill this gap, by addressing the use of PPW, which is a no-value material, to obtain lactic acid and ethanol via fermentation by R. oryzae . In this context, the effects of loading rate and particle size of PPW on fermentation were investigated. The intermediary and final products obtained were defined and quantified. Rate and yield values were calculated to compare and assess the performance of the bioprocess.