Design of Dean flow Ultraviolet (UV) reactors and testing their efficacy for inactivation of Escherichia coli W 1485 and Bacillus cereus spores in milk

Author

Srinivasarao Bandla, Southern Illinois University CarbondaleFollow

Date of Award

12-1-2010

Degree Name

Master of Science

Department

Plant and Soil Science

First Advisor

Choudhary, Ruplal

Abstract

Consumer demand for fresher foods has necessitated the use of non-thermal technologies in processing milk. Two Dean Flow UV reactors (1/16" ID × 1/32" Thick & 1/8" ID × 1/32" Thick) were designed in the laboratory. The objective of this study was to examine the efficacy of designed UV reactors at four levels of Reynolds numbers (Re) on inactivation of Escherichia coli W 1485 cells and Bacillus cereus spores in raw cow milk (RCM), commercially processed skimmed cow milk (SCM) and raw soymilk (RSM). RCM, SCM and RSM were inoculated separately with E. coli W 1485 and B. cereus spores and were treated through the designed reactors for a residence time of 11.3 ± 0.1 s, equivalent to an UV dose of 0.05 J/ml in 1/16" ID reactor and 0.02 J/ml in 1/8" ID reactor. Four levels of Re were tested in the range of 181-1372. The influence of tube diameter (thickness of milk exposed to UV) and Re (indicator of turbulence) at constant residence time (11.3 ± 0.1 s) on inactivation of both the bacteria in both the UV reactors was analyzed using two-way ANOVA with proc GLM in SAS software. E. coli was inactivated to non-detectable levels (≥7.8 log10 CFU/ml) in SCM from the second level of Re (532.2) in 1/16" ID reactor. E. coli was also inactivated significantly (> 5logs) in RSM at the highest Re (1372) but this was not achieved in the case of RCM (712.7). Increasing the residence time to 14.2 s or greater (17 s) (equal to UV dose of 0.06 and 0.08 J/ml) inactivated E. coli cells to non-detectable levels in RCM using 1/16" ID reactor at the highest level of Re (712.7). Reduction of E. coli cells were in the range of 0.45-7.78 logs whereas B. cereus spores were in range of 1.06- 3.29 logs in all types of milk used in this study. The interaction effect of tube diameter and Re was statistically significant for E. coli cells in RCM, and SCM; B. cereus spores in SCM, and RSM (p < 0.05) whereas this was statistically non significant for E. coli cells in RSM and B. cereus spores in RCM (p > 0.05). Main effects of Reynolds number, and tube diameter were statistically significant (p < 0.05) on inactivation of B. cereus spores in RCM and E. coli cells in RSM. Inactivation efficiencies for both bacteria were higher in 1/16" ID reactor than 1/8" ID reactor. Using the 1/16" ID reactor at highest level of Re (RCM Re = 712.7, RSM Re = 1372), inactivation of standard plate count (SPC) present in RSM and RCM, and lipid oxidation during storage period (0, 1, 3 and 7 days) were measured. Inactivation of SPC in UV-treated RSM (3 logs) was lower than thermal pasteurization at 72°C for 20 s (7 logs). In case of RCM, the SPC was inactivated to 1.9 logs from 4.2 logs. Sensory evaluation (olfactory) of UV treated, untreated (milk passed through the 1/16" ID reactor while the UV lights turned off), and fresh RCM (control) suggested no change in flavor after treatment and upto 1 day after storage in refrigerated condition, but a perceivable change in the quality of UV treated and untreated cow milk were observed during the 3rd and 7th days when compared with fresh RCM (milked same day). RCM was treated with different UV dose levels (0.04, 0.05, 0.08, 0.12 and 0.16 J/ml) to examine the effect of UV light on malondialdehyde and other reactive substances using TBARS test kit. Reactive substances such as malondialdehyde content increased as the UV dose increased. The presence of malondialdehyde and other reactive substances were not significantly different (p < 0.05) in both thermal and UV-pasteurized soymilk; whereas these substances were found to be higher in UV-treated RCM after 7 days of storage than the untreated milk stored for 7 days at 4 °C and the fresh RCM. The designed reactors 1/16" ID and 1/8" ID reactors were useful to inactivate bacteria present in milk. But, the inactivation efficiency was more in 1/16" ID reactor than 1/8" ID reactor.