Courtesy of Wynboer

Ilse Fredericks, Maret du Toit & Maricel Krügel

Investigating the efficacy of UV-C radiation as an alternative technology to inactivate microorganisms in red and white grapes juices and wines.

INTRODUCTIONMicroorganisms certainly play a crucial role in wine production (Mauriello et al., 2009; Ruiz et al., 2010), however some yeasts and bacteria can have a negative impact on the quality of wine due to spoilage (du Toit and Pretorius, 2000). Good hygiene practices together with excellent winemaking procedures are recommended in the cellar to continuously control the growth of microorganisms throughout wine production. Traditionally, microbial growth is effectively controlled in grape juice and wine by the addition of sulphur dioxide (SO2). Since more consumers are allergic to SO2, its use is currently under review (Jackson, 1994). Besides SO2 usage, alternative additives such as potassium sorbate, Velcorin® (dimethyl dicarbonate), natamycin and lysozyme are also known to have inhibitory effects on microorganisms. Filtration is also very efficient in controlling microbial growth but, can however affect the colour, flavour and palate of wine negatively (Suárez et al., 2007). Ultraviolet (UV-C) radiation along with flash pasteurisation, pulsed electric fields (PEF) and high hydrostatic pressure systems are classified as innovative technologies with the potential of inactivating microorganisms in liquid food products without affecting the sensorial properties of the product (Sizer and Balasubramaniam, 1999; Puértolas et al., 2009). The effectiveness of UV-C radiation to inactivate microorganisms has already been investigated in a range of fruit juices, beer and milk (Koutchma et al., 2009; Lu et al., 2010). However, the feasibility of this technology to control wine-related microorganisms in grape juice and wine is still unclear. The aim of the study was therefore to investigate the efficacy of UV-C radiation as an alternative technology to inactivate microorganisms in red and white grapes juices and wines (Fredericks et al., 2011).METHODSPure cultures were provided by the Institute for Wine Biotechnology, Stellenbosch University, South Africa. Lactic acid bacteria and yeasts were cultivated in MRS and YPD broth at 30°C to start with a final concentration of ˜ 1 x 106 colony-forming units per millilitre (cfu/ml) (Fredericks et al., 2011). Brettanomyces bruxellensis ISA 1649 and Saccharomyces cerevisiae VIN13 were inoculated into 20 litre batches of grape juices and wines, while Pediococcus acidilactici were only inoculated in wines. A novel pilot-scale UV-C reactor system was designed by SurePure (Milnerton, South Africa) which consisted of one UV-C germicidal lamp (100W output; 30W UV-C output). The product was circulated at a constant flow rate of 4 000 (L/hr) in a closed system with applied UV-C dosages that ranged from 0 to 3 672 J/L. The control had represented wines containing SO2 that was taken after inoculation and not exposed to UV-C radiation treatment. Each experiment was performed in three replicates; followed by microbiological analysis.RESULTS AND DISCUSSIONThe effectiveness of UV-C radiation had been evaluated against a number of microorganisms in grape juices and wines. Only the results obtained for S. cerevisiae VIN13, P. acidilactici and B. bruxellensis ISA 1649 will be discussed further, yet the complete results are available in Fredericks et al. (2011). A significant difference (p<0.05) in microbial reduction were obtained for B. bruxellensis ISA 1649 (Figure 1) and S. cerevisiae VIN13 (Figure 2) in Chenin blanc juice and Shiraz juice. Even though both juices were unclarified, the Shiraz juice was more turbid compared to the Chenin blanc juice. According to previous reports, a turbid matrix causes shadowing and scattering of the UV-C rays and thus reduces the efficacy of the technology (Koutchma et al., 2004; López-Malo and Palou, 2005).

Based on the Chardonnay and Pinotage wine controls (data not shown), it is clear that UV-C radiation was the cause of microbial reductions obtained and not the SO2 present in wine. The reduction of B. bruxellensis ISA 1649 were also significantly different (p<0.05) in Chardonnay compared to Pinotage wine (Figure 1). Similarly, better reduction of P. acidilactici and S. cerevisiae VIN13 was found in Chardonnay wine compared to Pinotage wine (Figure 2). However, significant microbial reduction was obtained in Pinotage wine indicating that the penetration ability of UV-C light in red wine is not entirely limited.The UV-C resistance pattern for B. bruxellensis ISA 1649 was significantly different from P. acidilactici in Chardonnay and Pinotage wine. Yeasts contains less thymine or cytosine pyrimidines on their DNA strands which is essential for UV-C radiation to form dimmers; thus preventing microorganisms from reproducing (Bintsis et al., 2000; Thompson, 2003; Tran and Farid, 2004). This study showed different microbial reductions in different liquids such as red and white wine; indicating that UV-C efficacy is dependant on the physical appearance of a liquid such as colour, absorbance and turbidity (Keyser et al., 2008; Koutchma et al., 2009). A turbulent flow profile optimises the efficacy of UV-C radiation by facilitating better mixing and ensuring that each part of the liquid is equally exposed to the UV-C light. Therefore, the novel turbulent flow reactor of SurePure may significantly attribute to microbial reduction obtained in red and white grape juices as well as wines. CONCLUSIONUV-C radiation has effectively reduced wine-related microorganisms in grape juice and wine; and can thus represent an alternative technology to control microbial growth. However, the affect of the technology on the chemical and sensorial properties of wine needs to be investigated further.AUTHORSIlse N. Fredericks: Department of Food Technology, Cape Peninsula University of Technology, BellvilleMaret du Toit: Institute for Wine Biotechnology, Department of Viticulture and Oenology, Stellenbosch UniversityMaricel Krügel: Department of Food Technology, Cape Peninsula University of Technology, BellvilleACKNOWLEDGEMENTSWe would like to express our sincere gratitude to Mr Guy Kebble (SurePure, Milnerton, South Africa) for partial financial assistance and use the novel pilot-scale UV-C unit. Special thanks to, Ms Zaan Fourie (SurePure) as well as Mr Loftie Ellis (Wine Quality Consultants CC), for their technical input and assistance with obtaining the relevant grape juices and wines from the cellars. The authors further extend thanks to the National Department of Agriculture, Forestry and Fisheries for their continued financial support throughout the duration of this work. 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