Wednesday, May 21, 2014

Sous vide and home pasteurization of shell eggs スービイと自家製滅菌卵

Salmonella contamination of hens eggs and chicken meat doesn’t appear to be a big problem in Japan. Raw and undercooked eggs and chicken meat are frequently consumed on such items as raw egg on rice (卵かけ御飯), moon-gazing soba noodle (月見そば) and "tataki" or " sashimi" of chicken tenderloins (とりわさ). In the US, salmonella food poisoning is a serous problem. Eating any product with raw egg in it is risky. Common sources of salmonella food poisoning are home-made mayonnaise, Caesar salad dressing, and raw cookie dough as well as sunny-side-up and soft boiled eggs with runny yolks, Some states (New Jersey being one) even outlawed restaurants from serving sunny-side-up eggs for a while. Certainly nobody in their right mind would consume raw or under cooked chicken meat.

If we want a dish with raw eggs or runny yolks, we use pasteurized shell eggs which we posted before. Low temperature pasteurization for shell eggs is a patented process by Dr. and Mr. Cox. Only one producer (Davidson's safe eggs) is using this patented method.

Recently I acquired a very reasonably priced home sous vide machine. There are at least three products in this category and after reading the review I settled for "Anova" sous vide machine (see below).

The submersible part contains a heating coil and agitating propeller under a metal cover with vertical slits and the  top has a good sized touch sensitive color LED screen for setting the temperature and time. It does quite well heating up the water, maintaining the temperature and providing good circulation. I double checked the temperature using a digital thermometer and found the temperatures registered in the water with the sous vide was within 0.7 F of the temperature registered on the thermometer so it was quite accurate. I am not sure, however, which one is more accurate. In addition the temperature was even thorough out the water bath.

The first thing I wanted to try was to pasteurize shell eggs at home. This has to be done at a  temperature low enough that the egg white and yolk don’t coagulate but high enough and long enough to reduce the salmonella to safe levels. I read about low-temperature pasteurization of shell eggs including the patent filing of Dr. and Mr. Cox and Food Lab's guide to slow-cooked. sous-vide-style eggs. Some suggestions regarding "home pasteurization of shell eggs" I found on the web appear to have no scientific basis and did not sound like they would accomplish adequate pasteurization at all.

I determined that  at 130F for more than 2 hours (2 hours 20minutes to be exact in the eggs I “pasteurized” but 3 hours will be on the extremely safe side) would be the most reasonable to accomplish the goal of pasteurization. But I do not have any microbiological proof to confirm this. So I have to add a word of caution that the results of this method are not guaranteed--use at your own risk.  If you are interested how I came up with this conclusion, please read below*.

I sous vided shell eggs directly out of the refrigerator for 2 hours and 20minutes at 130F. I thought that the long time at low-temperature would eliminate possible temperature gradient issues. I immediately soaked the sous vide eggs in ice cold water for 10-15 minutes, dried the surface and placed them in the refrigerator. My wife suggested that I mark the eggs before putting them with the others in the fridge. She helped me to write "P" for "pasteurized" on the eggs using a pencil (below). (She told me that when she was young this was a method her mother used to distinguish hardboiled eggs from raw ones—its no fun cracking the last egg in the house to make a cake only to discover it is hardboiled!)

When I cracked open the home pasteurized eggs after a few hours of refrigeration, the yolk and white were liquid and looked about the same as an unpasteurized egg.

The second egg (front) had a very slight cloudiness (Which I see occasionally even in the commercial pasteurized shell eggs) but was still liquid.

I scrambled these two eggs and made a stuffed omelet. It cooked exactly like any un-pasteurized egg but I did not have to worry about having undercooked egg in the omelet. Its texture and taste were fine as well.

* Sous vide low-temperature pasteurization of shell eggs.

Dr. and Mr. Cox patent application is a good example of lawyers writing a scientific/technical article. It is next to impossible to figure it out. This patent  contained not just pasteurization of shell eggs but other egg products, scrambling eggs in their shells (why do you need to do this is beyond me),  and a method of replacing naturally occurring gasses in shell eggs with mixture of  sterile air and gasses to prolonged the shelf life. This is a chart they included in the application.

shell egg pasteurization

I interpreted this chart this way; at 129.5F (54.4C), the egg white did not coagulate ("irreversibly damaged") at least up to 120 minutes but the yolk temperature reached 54.4C after 105 minutes and maintaining this temperature for 5 minutes would be "lethal" (for salmonella, I assume). At 140F (60C), eggs can be sterilized a bit shorter than 30 minutes  but before this happens "irreversible damage" of egg white will happen. Any other temperatures in between, pasteurization can occur before the egg white starts to "thicken before irreversible damage" but you need to stop heating before 120 minutes to accomplish pasteurization without "irreversibly damaging" the white. To my dismay, the document did not specify the initial temperature of the eggs in this experiment.The initial temperature could make a difference in the amount of time needed to eliminate a temperature gradient and have the yolk achieve a temperature high enough to be lethal to salmonella.

In any case, from this chart and Food Lab's guide to slow-cooked. sous-vide-style eggs, I concluded at 130F, you could keep the shell eggs for a long time (at least 2 hours to be safe) without coagulating the egg white or egg yolk and obtain the adequate pasteurization at the center of the egg yolk. I think that low temperature and long heating time should eliminate any variables stemming from the initial temperature of the eggs and temperature gradient.

Now that I think I can safely produce pasteurized eggs, I will still buy Davidson's Pasteurized eggs but, just in case, we now have a back up plan. More sous vide cooking is coming.

P.S. After receiving a comment from an anonymous reader, I am adding the new references below. Dr. Schuman's article has soild microbiological proof, albeit only 57C and 58C were tested in details. As per this paper, to be absolutely safe (non-detectable salmonella or total bacteriocidal level), 57C (134F) for 75 minutes is recommended. I am not sure if egg white will become cloudy at this temperature,  butI will try and let you know. The below is from the reference 2 below.


Anonymous said...

A paper by Schuman from North Carolina State University c.1996? suggest 57Celsius, 75 minutes for commercial pasteurized whole in-shell eggs by immersion. That paper is available online.

Uncle N said...

Thanks you very much for the information. I read Dr. Schuman's paper with interest. He did do bacterial culture to prove his point but only tested 57 and 58C. My understanding is that 130F (54.4C) for prolonged time should accomplish significant reduction (may not be totally bacteriocidal) to be safe. Some also suggest to use 132-134F (56-57C) allowing the inaccuracy of the temperature measurement/control to be safe. I will add Dr. Schuman's and additional articles I found as references to my post. Thanks you again for your information.