Valorization of exo-microbial fermented coconut endosperm waste by black soldier fly larvae for simultaneous biodiesel and protein productions

Highlights

• Exo-microbial fermentation to spur palatability of black soldier fly larvae (BSFL).

• Impacts of exo-microbes’ concentration and fermentation period on BSFL growth.

• Simultaneous production of biodiesel and protein from harvested BSFL.

Abstract

The conventional practice in enhancing the larvae growths is by co-digesting the low-cost organic wastes with palatable feeds for black soldier fly larvae (BSFL). In circumventing the co-digestion practice, this study focused the employment of exo-microbes in a form of bacterial consortium powder to modify coconut endosperm waste (CEW) via fermentation process in enhancing the palatability of BSFL to accumulate more larval lipid and protein. Accordingly, the optimum fermentation condition was attained by inoculating 0.5 wt% of bacterial consortium powder into CEW for 14–21 days. The peaks of BSFL biomass gained and growth rate were initially attained whilst feeding the BSFL with optimum fermented CEW. These were primarily attributed by the lowest energy loss via metabolic cost, i.e., as high as 22% of ingested optimum fermented CEW was effectively bioconverted into BSFL biomass. The harvested BSFL biomass was then found containing about 40 wt% of lipid, yielding 98% of fatty acid methyl esters of biodiesel upon transesterification. Subsequently, the protein content was also analyzed to be 0.32 mg, measured from 20 harvested BSFL with a corrected-chitin of approximately 8%. Moreover, the waste reduction index which represents the BSFL valorization potentiality was recorded at 0.31 g/day 20 BSFL. The benefit of fermenting CEW was lastly unveiled, accentuating the presence of surplus acid-producing bacteria. Thus, it was propounded the carbohydrates in CEW were rapidly hydrolysed during fermentation, releasing substantial organic acids and other nutrients to incite the BSFL assimilation into lipid for biodiesel and protein productions simultaneously.

Introduction

Latterly, the renewable-cum-sustainable energy has gained a great momentum to counteract the global warning threat by substituting the fossil fuels, catering for the increasing population. This green energy source can be reaped from hydropower, tidal power, wind power, geothermal power and lastly biomass especially in Asia countries such as Malaysia. Of late, Malaysia has tapped into the power generation derived from five major agricultural residual biomasses, namely, rubber, oil palm, cocoa, paddy and coconut (Chuah et al., 2006). However, only 27% of these residual biomasses is exploited as a fuel and the remnants are disposed mainly through burning (Zafar, 2018). Focussing on the residual coconut, annually, about 1.7 million boe (barrel of oil equivalent) of energy could be potential generated from coconut plantation. However, there are very limited studies reporting on the exploitation of coconut waste, possibly due to the poor infrastructure located in the rural plantation areas (Chuah et al., 2006; Zafar, 2018).

Today, the production of biodiesel in Malaysia is primarily derived from palm oil, contributing to the 75% of total cost being consumed by the feedstock production; besides, incurring food versus fuel trade-off, deforestation as well giving rise to the fluctuation of palm oil prices (AsiaBiomass, 2010). Hence, an alternative source for biodiesel production should be technically feasible, economically competitive, environmentally friendly and easily available. An extensive research in various insect oils were carried out recently and it had been confirmed that most of these feedstock could be plausibly used to produce biodiesel (Li et al., 2012; Yang and Liu, 2014). By comparing with feedstock as abovementioned early, the insect seems to be a better choice than others due to the reasons that they have a shorter life cycle, require a smaller land, are cost-saving and have comparable amount of fat content (Manzano-Agugliaroa et al., 2012). In addition, the protein yields from insect biomasses can accounted for about 42%–63% of total biomass with high achievable digestibility of 77%–98%, serving as a good protein source for farm animals (Makkar et al., 2014; Verkerk et al., 2007). Among the insects, the black soldier fly is deemed best since it is a non-pest to human population (Diclaro and Kaufman, 2009) and has a large size while reaching the prepupae stage with high content of stored fat. Besides, the black soldier fly larvae (BSFL) can naturally migrate itself away from the feeding medium for pupation, easing the larval harvesting process (Manzano-Agugliaroa et al., 2012).

The investigations on the potential of BSFL to be exploited for biodiesel production had ubiquitously targeted on valorizing of various low-cost media, including animal manure (Li et al., 2011a), food wastes (Surendra et al., 2016; Zheng et al., 2012), sewage sludge (Leong et al., 2016), etc. According to Wang et al. (2017), it had been vindicated that the physicochemical characteristics of BSFL lipid could be a potential source for biodiesel production. Likewise, the BSFL biodiesel had been compared with rapeseed oil-based biodiesel following the ASTM and EN 14214 standards. The results showed that the BSFL-derived biodiesel was determined to be similar to that of rapeseed oil-derived biodiesel; but possessed a slightly higher oxidative stability (Nguyen et al., 2018). Insect-based biomass can incontrovertibly serve as the sustainable source for energy in addition to the protein production. However, the fuel quality is largely depending on the taxonomic groups as different fats will yield different properties of biodiesel. Thus, oil mixtures from different insect species could allow the manipulation of fat qualities to the certain degree to suit the specific purpose (Manzano-Agugliaroa et al., 2012).

On another note, Yu et al. (2011) had discovered that with the supplementation of exo-microbes into BSFL feeding medium, a denser prepupal weight could be harvested; besides, promoting the survivorship from prepupae to pupae as opposed to the control medium administration. Nevertheless, the detailed impacts of exo-microbes on larval feeding medium that eventually influence the BSFL growth and valorization performances have never been specifically documented. Thus, the goodness of microbial supplementations in modifying the feeding medium for BSFL remains practically unwarranted. With that, in this study, the source of exo-microbes was initially inoculated into the larval feeding medium, i.e., coconut endosperm waste, to carry out the fermentation process. The impacts of various microbial inoculation concentrations and fermentation periods were then stepwise optimized in fortifying the physiological development of BSFL, inclusive of larval lipid (for biodiesel production) and protein contents, as well as inciting the waste valorization potentiality by BSFL. Moreover, the microbial profile demonstrating the thriving of various microorganisms along the fermentation period was finally unveiled to rationalize the performances of BSFL in bioconverting fermented coconut endosperm waste into larval lipid and protein sources. In fact, the bioconversion of fermented feeding medium into BSFL biomass served as a key player in determining the true benefit of inoculating exo-microbes to ferment the feeding medium prior to BSFL offering.