Non-Thermal Preservation of Wine

Abstract

Brettanomyces bruxellensis is a fastidious slow growing yeast of concern for the wine industry worldwide, capable of spoiling wine and causing undesirable sensory properties. Sulphur dioxide (SO2) is an additive with antioxidant and selective antimicrobial properties that is widely used in the wine industry to inhibit the growth of moulds, bacteria and yeasts and prevent microbial spoilage. Nevertheless, this preservative can cause adverse effects in consumers, including allergic reactions, asthma and headaches. Consequently, the wine industry is interested in finding alternative methods to reduce SO2 levels, while maintaining wine quality. The objectives of this study were therefore to determine the viability of using non-thermal technologies including high pressure processing (HPP), pulsed electric fields (PEF) and ultra sound (US) for B. bruxellensis inactivation in wine and the effect on wine quality parameters directly after processing and during storage. The sensory difference between HPP and PEF wines and untreated wine, after one year's storagewas determined as well as the effect of SO2 (50 mg/L free SO2), HPP (400 MPa, 5 s) and PEF (46.8 kV/cm, 100 Hz) on B. bruxellensis inactivation and wine quality including colour density, total phenolic content, pH and volatile phenol concentration. It was also important to ensure that PEF treatment has no harmful effect on wine through the release of metal ions. HPP and PEF proved to be viable alternatives for the inactivation of B. bruxellensis in wine, with the potential to reduce the industry's reliance on sulphur dioxide. Important factors that influenced HPP B. bruxellensis inactivation were pressure, time, yeast strain and wine type/grape variety. HPP treatment at 200 MPa for 60 seconds applied to seven different wines led to log reductions ranging from 1.1 to 5.1 in the Dolcetto Syrah and Rosé wines, respectively. Alcohol concentration also had a significant effect on Brettanomyces inactivation, leading to increased inactivation from 3.1 in 12% to 4.2 in 14% v/v wine after 200 MPa for 180 seconds. The Weibull model successfully described the HPP 'Brett' inactivation. To achieve high throughput in the wine industry, HPP treatment at 400 MPa for 5 seconds, which achieved > 6.0 log reductions of B. bruxellensis, is recommended. With respect to PEF, electric field intensity was found to have a greater impact on inactivation than specific energy. Treatments of 31, 40 and 50 kV/cm resulted in D-values of 181.8, 36.1 and 13.0 μs, respectively. At 50 kV/cm, a temperature rise of almost 10 °C(overall temperature < 40 ºC), doubled inactivation to 3.0 log reductions. Yeast strain and alcohol concentration also influenced the degree of inactivation. PEF treatment produced wine safe for human consumption, with Fe, Cr and Ni ions contents well below dangerous levels. Ultrasound treatment of 16.2 W/mL for up to 40 min resulted in only 2.5 log reductions of B. bruxellensis in Cabernet Sauvignon red wine. Therefore, non-thermal (< 40 °C) ultrasound treatment proved to be ineffective for B. bruxellensis inactivation at the maximum energy delivered by the US unit used. Regarding the storage studies, no significant difference between the overall sensory quality of HPP vs untreated wines and PEF vs untreated wines were found after being stored for a year. Although both SO2 and HPP treatments led to > 5 log inactivation of B. bruxellensis in wine, the SO2 treated cells were able to recover after 10 months storage. SO2and PEF treatments had no significant effect on wine quality, while the colour and phenolic quality of HPP treated wine deteriorated after six months of storage. HPP was the only treatment which prevented Brett growth and the formation of off-flavours and -odours, with no 4-ethylguaiacol (4-EG) or 4-ethylphenol (4-EP) after one year storage. HPP was able to produce a microbiologically stable wine and had no effect on the sensory quality of wine after storage. Being a fastidious slow growing yeast, conventional enumeration of Brett using agar plates takes five days. Therefore, impedance methods were investigated. It was found that the 'indirect' impedance method was a good alternative for monitoring Brettanomyces bruxellensis contamination in wine, taking 0.9 and 57.7 hours for enumerations, depending on the yeast concentration.