Bioprotection of Nocellara del Belice table olives: effects of Candida boidinii LC1 and Candida norvegica OC10 under different thermal storage conditions

Abstract

Background. The Castelvetrano method for processing Nocellara del Belice table olives is a well-established Sicilian agrifood tradition, celebrated for producing olives with a distinctive sweet taste and bright green colour. This method has achieved notable commercial success at both national and international level. However, it faces challenges, particularly during warmer months, when the shelf life of the product is compromised due to the growth of spoilage and pathogenic microorganisms. To mitigate microbial spoilage and extend shelf life, producers often rely on refrigerated storage. Although partially effective, refrigeration significantly increases energy consumption and operational costs. To address these limitations and reduce the energy costs associated with cold storage, this study investigated the bioprotective potential of two yeast strains, Candida boidinii LC1 and Candida norvegica OC10, previously selected for their technological and antimicrobial properties (https://doi.org/10.1016/j.fm.2024.104477). Methods. Experimental batches of olives were processed using the traditional Castelvetrano method and inoculated with the selected yeast strains. These batches were then stored under three thermal regimes: constant refrigeration (8±1 °C); ambient temperature; and a combined regime: 90 days at ambient temperature followed by 90 days at 8±1 °C. Over a 180-day period, various aspects were monitored, including: dynamics of microbial groups such as lactic acid bacteria, yeasts, Enterobacteriaceae, Pseudomonadaceae, and Staphylococcaceae; kinetics of physicochemical parameters such as pH, salinity, temperature, pulp firmness, fruit colour; and finally sensory attributes evaluated by a trained panel using a structured profile method in accordance with ISO standards. Additionally, the presence of mycotoxigenic fungi was assessed to ensure food safety. Results. Yeast inoculation had a significant impact on controlling spoilage and pathogenic microorganisms, particularly under refrigerated and combined storage conditions. In these treatments, microbial populations such as Enterobacteriaceae and Staphylococcaceae were considerably reduced compared to uninoculated controls, and no Clostridia were detected. Conversely, olives stored solely at ambient temperature exhibited increased levels of pathogens, including Salmonella spp. and Escherichia coli, especially toward the end of the storage period. The inoculated batches also exhibited more stable pH values, better retention of green colour, and improved pulp firmness. Sensory evaluation revealed enhanced organoleptic qualities in inoculated batches, with C. norvegica OC10 outperforming in terms of colour intensity, texture, and overall appreciation. Importantly, no significant presence of mycotoxigenic fungi was detected, confirming the microbiological safety of the batches inoculated with the selected strains. Conclusions. This study confirms the effectiveness of bioprotective yeasts, particularly C. norvegica OC10, in enhancing both the microbiological safety and sensory quality of Castelvetrano-style olives. Notably, combining yeast inoculation with a reduced refrigeration period (from 180 to 90 days) resulted in a 50% energy cost reduction without compromising product quality. This approach offers a sustainable and cost-effective alternative for the olive processing industry, while also supporting the preservation and enhancement of traditional food products. Further research is recommended to explore the application of these yeasts in other processing styles and under extended ambient storage, with the aim of developing safe and functional table olives with a reduced environmental footprint.