Valorization of soy waste through SSF for the production of compost enriched with Bacillus thuringiensis with biopesticide properties

Highlights

• A soil amendment with biopesticide effect can be obtained from soy waste by SSF.

• Bacillus thuringiensis (Bt) is able to grow on solid soy residue.

• Sterile and non-sterile culture conditions have been investigated at lab scale.

• Bt is able to survive under non-sterile and thermophile conditions.

• This bench-scale research is the first for Bt culture under non-sterile conditions.

Abstract

There is a growing generation of biodegradable wastes from different human activities from industrial to agricultural including home and recreational activities. On the other hand, agricultural and horticultural activities require significant amounts of organic amendments and pesticides. In this framework, the present study evaluates the viability of soy fiber residue valorization as organic soil amendment with biopesticide properties through aerobic solid-state fermentation (SSF) in the presence of Bacillus thuringiensis (Bt). The experiments were performed first under sterile and non-sterile conditions at lab scale using 115 g of sample and controlled temperature (30 °C). Bt growth was successful in sterile conditions, obtaining 6.2 × 10¹¹ CFU g⁻¹ DM and 8.6 × 10¹⁰ spores g⁻¹ DM after 6 days. Bt survived on solid culture under non-sterile conditions (3.8 × 10⁹ CFU g⁻¹ DM and 1.3 × 10⁸ spores g⁻¹ DM). Further, the valorization process was scaled-up to 10 L reactors (2300 g) under non-sterile conditions obtaining a final stabilized material with viable Bt cells and spores (9.5 × 10⁷ CFU g⁻¹ DM and 1.1 × 10⁸ spores g⁻¹ DM in average) after 9 days of SSF. These results confirm the possibility of managing biodegradable wastes by their transformation to a waste derived soil amendment with enhanced biopesticide effect, in comparison to traditional compost using a valuable and low-cost technique (SSF).

Introduction

Organic wastes are worldwide produced in increasing amounts from different sources. Many industrial wastes contain an organic fraction or are mainly organic in nature. From those wastes, some are biodegradable. In that case, preferred management options include their valorization to obtain biogas, valuable products or a soil organic amendment among others (Murthy and Naidu, 2012). A good example are wastes from agro and food industries that are hardly contaminated by substances that can prevent them from being recycled into valuable products such as fertilizers, closing in this manner the cycle: raw materials for agro and food products come from agriculture and agricultural soils, thus valorization of organic wastes from these industries into soil will contribute to maintain soil fertility and compensate the loss of nutrients (Chiewa et al., 2015, Paradelo et al., 2013). Specifically, a local factory produces soy milk and tofu and generates 20 t per week of soy residues following grain processing. These soy residues are currently treated by composting and/or used for livestock feed. Furthermore, these soy residues are rich in water-insoluble ingredients including fiber in its majority, but also protein, fat, starch and sugar. Thus, they can be potentially used as high quality media for fermentation (Hiesh and Yang, 2004). About 1.1 kg of fresh residue has been reported to be produced from every kilogram of soybeans processed into soy milk or tofu (Khare et al., 1995). In fact, different researchers explored the possibilities of soy wastes as raw material in the production of organic acids, acetone, butanol, ethanol or enzymes from fermentable sugars (Karki et al., 2011).

There is also the novel possibility of obtaining a soil amendment with added biopesticide properties through solid-state fermentation processes in presence of Bacillus thuringiensis (Bt). Solid-state fermentation (SSF) has been defined as the fermentation process that involves a solid matrix and it is performed in the absence or near absence of free water on a substrate possessing enough moisture to ensure microorganisms’ growth and metabolism (Pandey, 2003). The SSF process has been used in different studies for the production of high value added products such as enzymes, biofuel, biosurfactants and biopesticides from many residues, mostly working at laboratory scale (Murthy and Naidu, 2012, Singhania et al., 2009). Thus, SSF is also presented as a viable technique for waste valorization as a source of valuable products to be considered in a biorefinery scheme (Forster-Carneiro et al., 2013).

On the other hand, B. thuringiensis (Bt) is a spore former, facultative anaerobic gram-positive bacterium present in soil, water and plant surfaces. It is a producer of a paraesporal crystal protein also called δ-endotoxin. The toxin has a great potential to cause mortality to insects belonging to different orders such as Diptera, Coleoptera and Lepidoptera, pests that destroy more than 40% of the world's food, forage, and fiber production. Conversely, these toxins are innocuous for plants, animals and human beings. The biopesticides used in biological control of plagues are an environmentally safe alternative to synthetic pesticides. They have been used worldwide for many years for food crops and forestry pests (Chandler et al., 2011).

The production of Bt based-biopesticides has been studied mainly by submerged fermentation and applied at industrial scale, with few studies in solid-state fermentation. In these cases, different wastes have been used as substrates, such as soy residues, wastewater treatment sludge, kitchen waste, wheat bran, among others (Devi et al., 2005, Zhang et al., 2013, Zhuang et al., 2011). So far, all the SSF studies have been performed under sterile conditions and mesophilic temperatures (Pham et al., 2010, Smitha et al., 2013).

Taking advantage of the ability of Bt to produce spores in adverse conditions, the aim of this study is to valorize soy fiber residue (from food industry) by SSF to obtain a soil amendment with the biopesticide effect of Bt. In this sense, the challenge is to make this specific bacterium grow in soy fiber residue under SSF process without sterilization and under self-heating conditions (which is the case of SSF at real scale with significant amounts of waste) including two scales (500 mL and 10 L reactors). This is, to our knowledge, the first study conducted under these conditions, as a viability waste valorization test for future application at industrial scale as a management option for different biodegradable organic residues including industrial, agricultural and municipal wastes.