RISK FACTORS IN PREPARING SOUS VIDE FOODS
by O. Peter Snyder, Jr., Ph.D.
Sous vide products are foods such as meat, fish, poultry, fruits and vegetables that have been sealed under vacuum in a low-oxygen-transmission plastic film pouch, and the pouch is immersed in a constant-temperature water bath and cooked for a time that provides adequate reduction of pathogenic vegetative bacteria and a desired tenderization. Meat shrinkage in cooking meat, poultry, and fish does not take place significantly until the meat temperature is 130ºF or hotter. Typically, the tougher the meat, the richer and better the flavor – but drier – unless these tougher cuts – briskets, flanks, and ribs – are cooked sous vide in a sealed pouch, where all of the juices are retained, and the temperature is low enough so as to not cause muscle shrinkage. The foods cooked in this manner are generally thin, less than two inches thick. Cooking times in a moderate temperature water bath can be a couple of hours in order to get the food center temperature to 130ºF or above. People believe that cooking is for a desired quality. Actually, the primary purpose of cooking is to pasteurize the food to reduce vegetative pathogens to meet a Food Safety Objective (FSO) that provides an Appropriate Level of Protection (ALOP) to assure safety.
There are no significant chemical or physical hazards associated with sous vide cooking. The question is: What are the biological hazards? There are two classes of pathogens in sous vide meat, fish, and poultry products – vegetative pathogens and spore pathogens. Vegetative pathogens are destroyed as a function of time as temperatures reach 130ºF. They are killed slowly at about 130ºF, with a 6.5-log reduction of Salmonella (the target organism) at 112 minutes. At 140ºF, Salmonella dies 10 times faster, or 12 minutes. At 158ºF, it is considered to be an instant 6.5-log reduction of Salmonella.
After the food is pasteurized – received a 6.5-log reduction of Salmonella – there are still the spore pathogens, specifically, Clostridium perfringens, Bacillus cereus, and Clostridium botulinum, which are not destroyed by pasteurization, but rather, have been activated. So, when the hot food begins to cool and gets below 130ºF, spores can germinate and grow out. Cooling from 135 to 41ºF in 6 hours (FDA Food Code cooling) assures that there is no significant spore germination.
Since most sous vide products are thin, cooling is not a significant risk. Pasteurization is the critical control. The only way one can know that proper times and temperatures have been met is to measure food temperature, which is done with a needle / thin-tipped thermocouple probe that penetrates the pouch. First, a piece of sealing foam tape, specially designed to aid in the temperature measurement of sous vide products, is placed on the pouch. Then, the needle probe is directed through the tape, punctures the pouch, and is inserted into the center of the food to measure its center temperature. It is crucial to assure that consumers of the food will not become ill due to Salmonella, E. coli, or other vegetative pathogens. Therefore, the food must get an adequate dose of heat and time to reduce the vegetative pathogens by 6.5 log, which is further complicated by trying to take the center temperature of the meat, fish, or poultry.
Then why are there so few problems? First of all, meats being cooked by sous vide are usually from inside cuts of meat (e.g., steaks), fish, and poultry, which are protected during the slaughter process and do not contain 100,000 pathogens per gram, as an outside cut (e.g., brisket, flank) would contain. The surface of these inside cuts contains fewer pathogens, and their center is virtually pathogen free. These inside cuts would have, for instance, 10 pathogens per gram. Therefore, there are not many organisms to kill. In terms of safe cooking time at 130ºF, 20 minutes is probably adequate, even though the regulations require 112 minutes for a 6.5-log Salmonella reduction at 130ºF.
Also inherent to the sous vide process is the length of time (e.g., 2 to 4 hours) in order to get the desired tenderness that the cook wants to achieve. If there are foods that do contain high pathogen counts, any surface treatment would kill most of the vegetative pathogens, because the center does not have high levels of pathogens. An exception would be cooking a ground product (e.g., spiced meat pate), whereby the pathogens have been ground into the middle of the food being cooked by sous vide.
In summary, sous vide cooking must be done carefully and precisely to assure that government standards are met. Government standards, however, are very conservative, because pathogens on typical meat, fish, and poultry sous vide products are on the surface, which gets a longer cook time than the center. The sous vide process leads towards adequate times at temperatures to get sufficient reduction of pathogens to assure that no customer will become ill eating the sous vide product. Finally, the temperature of the cooked and cooled pasteurized food is critical, which should be 41ºF or less to assure that the very deadly C. botulinum does not germinate, multiply, and produce its lethal toxin.
Prepared for PolyScience – Copyright 2013