Microbiological quality and safety issues with water reuse in the food processing industry: with focus on Reverse Osmosis membrane filtration and UV radiation

Research output: Book/ReportPh.D. thesis

  • Eirini Vitzilaiou
The food industry is adopting water reuse practices due to the rising water scarcity, sustainability issues, the increase in the price of potable water and the high cost of wastewater discharge.
Recovery of raw material is also a major driver. The water can be reclaimed from a food material or recovered from a processing step and reconditioned to reduce contaminants to acceptable levels. This PhD study investigates the microbiological risks associated with water reuse in the food processing industry, focusing on two common reconditioning technologies, Reverse Osmosis (RO) membrane filtration and Ultraviolet (UV) radiation. The microbial population levels and diversity of biofilms on RO membranes, RO permeate streams and UV-treated water in food processing water lines for reuse were investigated. The dominant microorganisms were characterized and their tolerance to relevant stresses such as heat, Cleaning-In-Place (CIP) and UV light was assessed.
Dense biofilms of the filamentous yeasts Saprochaete clavata and Magnusiomyces spicifer were detected on RO membranes for whey water reconditioning together with the budding yeast Sporopachydermia lactativora and the Gram-negative, slime-producing bacteria Pseudomonas sp., Raoultella sp., Escherichia sp., Enterobacter sp. and Acinetobacter sp. Biofilms were found on both the retentate and permeate side, despite regular CIP cleaning. The filamentous yeasts were dominating the biofilms with their long branching hyphae and large biomass, but they were only detected by microscopy and selective enumeration media since their presence would otherwise be overlooked due to the faster-growing bacteria. S clavata and M. spicifer have similar physiological and biochemical profiles. They metabolize urea and grow in the lownutrient RO permeate, they are heat tolerant (60 °C/20 min), UV tolerant (≤ 100 mJ/cm2), form biofilm readily, and survive routine CIP treatments as applied in a laboratory set-up. An additional heat sanitation step (78 °C/20 min) did decrease the filamentous yeast below the limit
of detection, but the large biomass will remain on the membrane surface and eventually affect the membrane efficiency.
UV disinfection is usually applied as the last step before water reuse. In an experimental meat processing water line for reuse robust slime-producing and UV tolerant bacteria such as Pseudomonas brenneri, Rothia mucilaginosa, Acinetobacter sp. and Deinococcus sp. were detected in the UV-treated water, surviving UV doses of 250 mJ/cm2. Heterotrophic Plate Count were less than 100 CFU/mL in the water and no E. coli or thermotolerant coliforms were detected, rendering the water safe for reuse according to international guidelines. The results were the same for the permeate streams after UV treatment in the whey water reconditioning line. However, low microbial numbers surviving UV disinfection due to high tolerance or shielding within aggregates could lead to proliferation during water storage, since UV has no residual action.
The UV inactivation kinetics of dominant microorganisms from both water recovery lines were therefore assessed and compared with their microbial aggregation potential. The aggregating bacteria had a higher UV tolerance than the non-aggregating. UV induced some aggregation in both Gram-positive and negative, aggregating, and non-aggregating strains, except for S. aureus. Aggregation was already induced at the lowest UV dose applied (25
mJ/cm2), at which a major part of microbial inactivation was observed. The aggregation rate was not, however, increasing further by increasing UV exposure. The aggregating and nonaggregating bacteria were still surviving the highest dose applied (240 mJ/cm2) with approx. 3.5-7-log10 reduction.
In the whey water reconditioning line, whey from cheese production is ultrafiltered to reclaim whey protein and the permeate stream is RO-treated to reclaim lactose. Bacteriophages of starter cultures, found in the whey solution, can be significantly up-concentrated in whey protein and lactose during UF and RO, leading to fermentation failure if the compounds are reintroduced in processing lines. Phages can also occasionally pass through to the permeate side and contaminate the water destined for reuse. The UV tolerance of two heat-tolerant phages of the 936-group of Lactococcus lactis (P680 and P008) was assessed. Among 265, 285 and 365 nm, the most efficient wavelength was 265 nm. P680 and P008 were highly UV tolerant, needing UV doses of 327 and 164 mJ/cm2, respectively, for a 4-log10 reduction. Their DNA was less damaged when their average particle size increased and P680, having a larger particle size, had less DNA damage. Some phage survivors from the first UV exposures were more UV tolerant when reexposed.
To ensure the microbiological quality and safety of food processing water reuse, regular sampling should be conducted on equipment surfaces and permeate streams, including selective methods for fungi and phages when relevant, since they can negatively impact process efficiency with high yield losses. Worst-case scenarios such as high microbial population levels, low solution %UVT and tailing effect at longer exposure times should be taken into consideration when designing UV disinfection models. Shorter UV exposure at higher intensity, followed by short water storage before reuse in dark and cool conditions may
efficiently avoid microbial regrowth or reactivation. Overall, a better understanding of the microbiological risks of food processing water reuse will enable the improvement of the reconditioning technologies and widen the reuse applications using a fit-for-purpose approach, thereby contributing to water sustainability practices worldwide.
Original languageEnglish
PublisherDepartment of Food Science, Faculty of Science, University of Copenhagen
Number of pages180
Publication statusPublished - 2022

ID: 310495316