Systematic investigation of the influence of suspended particles on UV‐C inactivation of Saccharomyces cerevisiae in liquid food systems

Abstract

AbstractUV‐C treatment of food products is a non‐thermal, energy‐ and cost‐efficient alternative to traditional thermal treatment. To ensure a shelf‐stable product, it is necessary to understand the UV‐C dose–response relationship of spoilage microorganisms in liquid food. Two product parameters in particular have a significant affect on the UV‐C propagation through liquid food: the absorption of dissolved substances and the absorption and scattering of suspended particles in the liquid. The aim of this study was to investigate the effect of suspended solids on the UV‐C inactivation of Saccharomyces cerevisiae in model suspensions with different opacifiers (silicon dioxide, titanium dioxide, microcrystalline cellulose, calcium carbonate, and polymethyl methacrylate [PMMA]). PMMA spheres with 11 different diameters ranging from 100 to 6 μm were used to determine the influence of particle size on the inactivation process. Particles with a high imaginary part with respect to the refractive index at 254 nm absorb the UV‐C light instead of scattering it. This energy loss results in less inactivation of S. cerevisiae. Non‐absorbing particles affect inactivation if their size results in Mie scattering. Suspensions of particles between 500 nm and 2 μm significantly reduce the efficiency of the process. The greatest influence is seen at a size of about 900 nm, where the Mie scattering efficiency of PMMA in water is at its maximum. Suspensions with turbidity of 1000 NTU, consisting of particles larger than 2 μm, had no effect on the inactivation of S. cerevisiae. Particle sizes that are in the Rayleigh scattering range (<500 nm) show little to no effect on inactivation. This correlation was confirmed by inactivation experiments of S. cerevisiae in cloudy and clarified apple and grape juice. The results show, that the determination of the particle size distribution and its ratio to the wavelength is crucial for predicting the efficiency of microorganism inactivation by UV‐C light.Practical ApplicationsUV‐C treatment is a cold pasteurization process. It is a cost‐effective alternative to the conventional heat treatment. The inactivation of organisms only occurs when they are directly exposed to the UV‐C light. Liquid foods rather are suspensions than solutions. The presence of suspended particles can significantly reduce the efficacy of the treatment. It is imperative to understand the effect of suspended particles on process efficiency.