Reduction of Microorganisms in liquids by UV light, a prototype test

Abstract

Reducing microorganisms by UV light is has been commonly used for drinking water for many years. In other liquid products like apple juice it is a rather novel application. The advantages would be that quality parameters like aromas and vitamins are less affected by the UV treatment compared
to a heat treatment. Furthermore it would probably be cheaper, as the lamps need less electrical energy than the heating and cooling of the product.
In this work 3 strains of microorganisms were tested: Bakers yeast (Saccharomyces cereviciae) as a spoilage microorganism, a strain of
E. coli as a possibly pathogenous bacteria and a strain from the
group of lactic acid bacteria (Lactobacillus plantarum) as a spoilage bacteria. Additionally a sample of untreated juice was tested to see how the device performes with mixed wild type strains compared to single, pure lab strains. All three microorganisms were tested in 0.9% saline solution,
in clear and cloudy apple juice.
In saline solution, all three microorganisms were killed quickly after a few seconds of UV exposure.
In the clarified juice a treatment of around 33 seconds is necessary to reduce yeasts to an acceptable level and around 26 seconds for the lactic acid bacteria. In the cloudy apple juice around 130 seconds for yeast and 156 seconds for the lactic acid bacteria were needed. The E. coli strain could
not handle the low pH in the apple juice, so a more acid tolerant strain will have to be tested.
The effect of the UV light on vitamin C was evaluated in clear juice and as a control with vitamin C in water. In the clear juice around 81% of the natural vitamin C in the juice was left after a treatment that would reduce the microorganisms to a level safe level.
Further experiments will have to be performed with to round up this prototype test.

Popular Abstract

Many food products get heat treated to kill off microorganisms and thereby make them safe and prolong their shelf life. However, heat treatments like pasteurisation have their drawbacks, they can alter the taste and flavour of the products, reduce the amount of micronutrients like vitamin C and
the tend to be rather expensive. In recent years new techniques are emerging, like pulsed electric field treatment, “pasteurisation” by high pressure, Super critical gas treatment and treatment with ultra violet light. Such techniques have the advantage that they kill microorganisms without the
strong side effects on taste, flavour and micronutrients. However, one problem might be, that enzymes are usually not affected by such treatments, which could lead to problems with the shelf life of certain products, so it is especially fresh products that profit from these new techniques.
More and more applications of these novel techniques are accepted by law.
UV light has long been used in treating drinking water, as it does not leave any residues, like chlorine would. Its applications in food, like surface disinfection of meat or cut fruit are rather new.
Another application is the “pasteurisation” of liquid food products like fruit juices, beer, wine and eventually even milk, though for milk only heat pasteurisation is allowed for now. The advantages would be, that the taste and micronutrients are affected a lot less compared to heat treatment and
that the treatment is a lot cheaper, as the lamps need a lot less energy than heating and cooling large amounts of fluids.
In this master project a UV treatment prototype was tested on clear and cloudy apple juice. The two main aspects of the project were to determine how long the juice has to be treated to reduce the bacterial load of the juice to a safe level, and how the UV treatment affects the vitamin C content of
the juice. The efficiency against microorganisms was tested with bakers yeast as a spoilage organism, Lactobacillus plantarum as another organism that could have a negative effect on the shelf life and E. coli as a pathogenic bacterium of which certain types, like EHEC can cause serious diseases. There have been several outbreaks of EHEC connected to unpasteurised apple juices in the USA.
The UV device is made of two coated 25x25 cm quartz glass sheets with brass nozzles creating a matrix of 3 mm thickness and around 3.8 meters length. The UV light is created by six light tubes on each side. On the far sides the device is closed by dielectric mirrors, which reflect around 99% of
the UV light, increasing the effectiveness of the lamps. The optical wattage was calculated to be around 32 watt.
For the microbial experiments, commercial apple juices and 0.9% saline solution were inoculated with the before described microorganisms and then treated several times. Before the treatment and after different numbers of treatments samples were taken and plated to determine the amount of
living bacteria or yeast left in the tested product. The plating per sample was done in duplicate and over 3 dilution steps. The plates were incubated for 48 hours and counted both after 24 and 48 hours. For most experiment at least 2 replicates were performed. Additionally, most of the samples were stored in a refrigerator of one week to see if the product remains stable or if the microorganisms recover from the UV treatment.
In saline solution, the vast majority of microorganisms was killed after a single treatment, using a UV dosage of 2 to 4 J/ml.
In the clear apple juice, for yeast around 16 J/ml were necessary to reduce their amount to a level that would yield a stable product for longer storage.
L. plantarum only needed 4 to 8 J/ml to be killed off almost entirely. The strain of E. coli that was tested in this project could not handle the
low pH of the apple juice and the vast majority of them died already before the treatment.
In the cloudy apple juice, yeast needed 32 J/ml for a reduction by 5 log factors and L. plantarum needed 38 J/ml for a reduction by 5 log factors. Again E. coli could not handle the acidity of the
juice and most died before the treatment.
Additionally an unpasteurized, sedimentation filtered apple juice was tested. The initial bacterial load was very low (10 colony forming units per ml), so not much information could be won from this experiment. The idea was to see how wild type strains react to the UV treatment, as they can be more resistant than laboratory strains.
The vitamin C was measured using High performance liquid chromatography. In clear apple juice, after a treatment with 11.5 J/ml which would be enough to obtain a safe product, 81.1% of the vitamin C were still left, while in the commercial, heat treated apple juice, no vitamin C was found.
So the UV treatment has an effect on the vitamin C, but it is a lot smaller than the effect of the heat treatment. Unfortunately we did not have the time and material for testing cloudy apple juice.
Comparing the here tested UV equipment with those from similar projects published in scientific literature, it became clear, that the product layer thickness plays a vital role in the efficiency of the UV treatment. UV devices with a thinner product layer performed generally better than this device, while devices with a thicker product layer performed much worse. The UV output of the different UV devices was fairly similar.
There are a few aspects that should be investigated further. A more acid resistant strain of E. coli should be tested with this equipment, as of the tested microorganisms, E. coli is definitely the most dangerous, so killing them has the highest priority. Additionally a mixture of different
microorganisms could be tested. Another interesting trial would be long term storage tests, to see how long the shelf life could be for a UV treated juice.
More tests with vitamin C should be performed, especially with cloudy apple juice, as a stronger treatment is necessary here, but the turbidity of the cloudy juice probably also protects the vitamin C from the UV light. For orange juice this would be interesting, as it should have more vitamin C
than apple juice, and the consumers probably expect a high amount of vitamin C in orange juice. A quantification of the vitamin C would also be helpful.
Last but not least a sensory evaluation should be performed on UV treated juices, which was not possible in this project, as the experiments were performed in a pathogen lab, so consumption of the juice was not allowed. For the sensory a triangle test and a preference test would be ideal. A triangle
test against a heat treated juice from the exact same raw juice could determine if the difference in taste and flavour can be perceived. The preference test can then show which juice the consumers like more

• author: Leinweber, Yannick
• supervisor: Jenny Schelin
• organization: Applied Microbiology; Biotechnology
• course: KMBM01 20171
• year: 2017
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: UV, microbial reduction, food technology, applied microbiology, teknisk mikrobiologi
• language: English
• id: 8926759
• date added to LUP: 2017-10-16 16:27:27
• date last changed: 2017-10-16 16:27:27