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.