UV-C light sterilization test: Yeast seemed unaffected?

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UVC light did not sterilize yeast in my test?

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Hi,

I have a 25w 254nm bulb ozone free UVC bulb (draws about 19watts from my kil-a-wat). This was purchased from amazon, produced in China.

This light definitely omits a funny smell, it pulls 19w, and the glow in the dark stickers are illuminated after my test (so I know it is omitting some sort of light). Nevertheless, I was curious if this light really ‘works’ for sterilizing so I tried to run a test:

I took two regular glasses. Filled both with about 200 ml of water, 1 tsb of regular yeast and 1 tsb of sugar. One glass was my control and one glass went into an aluminum lined tote with the UVC light for 20 minutes. After closing the light, I compared both glasses, both appeared identical. Few hours later they also appeared identical.

Why didn’t UVC light destroy, inactivate, or at least slow down the yeast? Is this just a case where the glass blocked the UVC and the light entering from the top was not enough for sterilization? Any recommendation for any other simple easy test I can run to 'verify' this bulb is actually working?
 

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  • #2
phyzguy
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When you say "Few hours later they also appeared identical.", did this mean there was obvious fermentation occurring in both cases? When I brew beer, it usually takes overnight before the fermentation get started.
 
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Yes, there was apparent fermentation in both. Both items had murky water, froth at the top and you could tell there was gas bubbling up (carbon dioxide)… So the ‘yeast’ become active under the UVC.

I think the reason beer (or in my case wine) takes hours to visually start is due to the ratio of liquid to yeast. This stuff would be the equivalent of adding about 10 cups of yeast in a 5 gallon carboy.

I’m starting to wonder if maybe the yeast built ‘foam’ at top of the glass as it disolved that protected itself from the UVC which allowed it to reproduce untouched in the liquid.
 
  • #4
BillTre
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If the UV had to go through the glass (as opposed to entering from the top), it will most likely be ineffective, since normal glass can filter out UV (quartz glass does not).
Similarly, for a UV bulb to be efficient for sterilization purposes, the bulb should be made of UV glass.
If the water was cloudy (as you indicated), the effectiveness of the UV will be reduced since the yeast in the water will be shaded from illumination. Stirring the cloudy water may overcome this limitation to some extent. If the cloudy water is not stirred, yeast at the bottom will not be very strongly illuminated.
UV will be more effective if the light's path through the water is shorter rather than longer.
Different organisms will be sensitive to UV at different doses (various yeasts at bottom of list).
The doses are measured in units involving the amount of UV, time of exposure and area illuminated and are expressed as µWs/cm2 to determine equipment sizing.

UV sterilization of water is fairly common in the aquaculture business.
It is not ever considered 100% effective.
 
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Although I was aware glass blocks UVC light before posting, I was not aware before my initial test. I have no quartz (only the bulb itself). The water at the start of the experiment was clear. Dry yeast tends to float, while some tend to sink to the bottom of the liquid. After a few minutes yeast at the bottom of the liquid almost seems to 'explode' or 'pop' into a cloud which creates foam. I suspect this process built up enough foam to act as a protective lid from the UV.

I wonder if using the bacteria in my mouth (saliva) would be a better test? Saliva mixed with water, 'sterilize it', seal it up, wait 24 hours and compare it to my control by smell.
 
  • #6
TeethWhitener
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Yeast isn't required for fermentation. Only the enzymes in yeast are required. (This discovery won Buechner a Nobel Prize.) UV light mostly damages DNA, not proteins. You may be killing the yeast but not denaturing the fermentation enzymes. The real test would be to see if the yeast could multiply.
 
  • #7
Vanadium 50
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The very best quartz windows are good to about 160 nm. 200 nm or even 250 nm is more common. Glass starts to shut down around 320 nm. zthe shorter the wavelength, the larger the expected events.
 
  • #8
BillTre
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Before you get too into this process you should probably want to figure out the kind of test you want to use to decide if your experiment has been successful or not.
I think it is a good idea to start with figuring out the test method you will use before anything else in an experiment.

In testing the effectiveness of UV sterilizers in water systems, we would take water samples, make a series of dilutions of the water and plate the water out on bacteriological plates. Comparing the number of colonies we could count after a period of time (taking into account the dilutions we made) would indicate how effective the process was.
You might be able to buy plates from some educational science website, I don't know. Or you could go through the process of making them. You could sterilize them in a pressure cooker.
An very expensive alternative would be a machine that determines the number of cells in a solution by optical properties. However, this could well get confused by the presence of dead cells.
Of all the different species of bacteria or other bugs you might find in the environment, only a small percentage of them can be easily cultured in a lab.
This are not the easiest things to do if you do not already have experience with them. Your idea of fermentation, if done in a way than allows some pre-planned level of quantification, might be fine. Perhaps expose defined volume of filtered solution to UV in a defined way, add a defined amount of sugar to solution and figure out a way to measure the volume of CO2 generated, at a defined temperature (water bath or incubator would be useful).

Initially, the more dilute you yeast (or whatever) and the more time you give it to grow, the more clear the difference will be between the control and experimental situation (if it exists).
That is until the population of the growing yeast hits the limits of the culturing situation and stops growing. A few survivors in the treated group could than grow in number and reach similar levels to the control group.

Before using the UV on your solution, you should probably filter the bacteria solution to remove undissolved particles and generally clarify it. Any kind of filtration would be better than none.
Mix up your yeast or whatever you are going to use first as a single solution, filter it and then split it up into your treatment and control groups. This should give you more consistent results. Filtration is the step immediately before a UV sterilizer in a well designed water system.

If you switch to another organism for you test, you should be aware that it may have a different sensitivity to UV (see chart linked to in previous post).

Although glass blocks UV, some plastics do not (but others will).
Google searches could provide more details.
 

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