Harnessing Waste Bread: From Potential Use in Microbial Growth and Enzyme Production to Techno-Economic Assessment

Abstract

This study highlights waste bread (WB) as a novel, cost-effective, and nutrient-rich substrate for microbial growth, offering a sustainable alternative to conventional media. As a renewable resource, WB promotes the circular economy by reducing food waste and encouraging biotechnological innovation. The incorporation of WB into microbial culture media enhanced the growth of various reference strains (E. coli, E. faecalis, P. aeruginosa, and S. aureus), with at least a two-fold increase compared to conventional Luria-Bertani (LB) medium. Moreover, combining 2% WB with diluted LB (1/10) reduced medium costs by up to 90%. Furthermore, it was confirmed that 1% WB can effectively replace starch during the screening of amylolytic strains. Applying a fractional factorial design, the production of amylase by Bacillus sp. BSS (Amy-BSS) was enhanced 15-fold. An analysis of the Pareto diagram revealed that WB was the most significant factor. Additionally, Amy-BSS was applied to hydrolyze polysaccharides in WB, enabling the generation of high-value-added products in food processing. This hydrolysis process yielded 4.6 g/L of fermentable sugars from 1% WB. Evaluating the economic feasibility of WB valorization into value-added products elucidates potential pathways for cost reduction and enhanced environmental sustainability, thereby positioning WB as a viable tool for sustainable development.

Keywords: Bacillus; amylase; microorganisms; techno-economic assessment; waste bread.

Figure 1

Bacterial growth of reference microorganisms in different culture media (C.LB, NC1, and NC2).

Figure 2

Screening assay of amylolytic strains after incubation at 37 °C for overnight. (a) Bacillus strains incubated on SWB plate. (b) Bacterial colonies on LBS plate.

Figure 3

Standard Pareto chart of the normalized effect for the Amy-BSS production.

Figure 4

Optimal factors setting for Amy-BSS production. Blue line represents the predicted maximum response. Red vertical lines indicate the optimal levels of each factor used in the solution; Black squares correspond to experimental design points.

Figure 5

Optimized response design and data responses resulting for the $2_{\mathrm{V}}^{4-1}$ plan. AA, amylase activity; TI, temperature of incubation; WB, waste bread; HT, hydrolysis time. Blue line represents the predicted maximum response; Red vertical lines indicate the optimal levels of each factor used in the solution.

Figure 6

The surface plot of the synergistic interaction (amylolytic activity × hydrolysis time). y is the hydrolysis yield.

Figure 7

Hydrolysis products of 3% WB using 10U of Amy-BSS: (A) glucose solution of 5 mg/mL, (B) maltose solution of 5 mg/mL, (C) 3% unhydrolyzed WB, and (D) hydrolyzed WB by Amy-BSS.