Possibilities of using the continuous type of UV light on the surface of lor (whey) cheese: impacts on mould growth, oxidative stability, sensory and colour attributes during storage.

Fresh whey cheeses are susceptible to the growth of mainly moulds and yeasts, due to their high moisture, protein and lactose contents, low salt concentration, and pH around 6.0. Even with good hygienic practices and effective cooling, they have limited shelf life. Thus, the investigation of post-production decontamination methods, preferably alternatives to thermal and chemical treatments, is required to ensure the safety and quality of whey cheeses during delivery and storage. Non-thermal processing technologies have great potential, mainly due to their low temperature and short processing time, one being ultraviolet (UV) light which is comparatively new and not well-studied. Although promising, exposure to high light intensities may promote physico-chemical changes in cheese components, such as lipids and proteins, and also interactions of protein-lipid oxidation products. Thus, the purpose of the study cited was to address optimum doses of a continuous type of UV light application to the surface of a whey cheese, to maximise mould inactivation and shelf life while minimizing quality deterioration.   

Whey cheese samples were produced without adding any preservatives and the UV-C light treatments were performed at room temperature, using the continuous UV-C disinfection system. The treatment was carried out at three different band speeds of 4m per minute (29.74 seconds), 1.6m per minute (73.91 seconds), and 0.2m per minute (677.43 seconds), which corresponds to exposure times of 1.617, 4.018 and 36.832kJ per m², respectively. Mould counts were done from cheese samples diluted with peptone water, homogenised and plated on Potato Dextrose Agar. The plates were incubated for five days at 25^oC and the mould counts expressed as log cfu per g. The protein and lipid oxidation products, and the sensory and colour attributes of the samples were also investigated.

The results show that UV treated samples had lower mould counts than those of untreated ones. UV treatments at or more than 4.018kJ per m² had around 0.7 – 2.7 log reductions on mould growth during storage. The increase in UV light dose caused significant increases in primary and secondary lipid oxidation products. In particular, the highest doses applied to the surface of the cheese samples had the highest values of protein carbonyls, as well as lipid oxidation products. Strong positive correlations were recorded between lipid and protein oxidation markers, whereas the highest dose of UV light increased foreign flavour perception, probably due to the oxidative reactions.

In conclusion, the results show that UV light application to decontaminate the surface of highly perishable dairy products, such as whey cheese, can be a practical way to increase shelf life, provided the UV exposure in time and extend is moderate. High doses (in this case above 4.018kJ per m²) may lower quality, mainly associated with photo-induced modification of lipids and proteins.