It is natural that the French, with their reverential attitude to good food, should develop a food packaging technique entailing minimal processing and the maximising of taste and nutritional content.
The sous vide - it means "under vacuum" - system has been available for many years. But success in perfecting the vacuum packing technique, allied to mushrooming demand for convenience foods, has meant its adoption has become widespread only in recent years.
Sous vide processing is a multimillion-pound business in Europe and the US, though Ireland, despite its strong food processing sector in international terms, has yet to exploit its potential fully. Sous vide chestnuts, chicken dishes and fish products are, for example, only beginning to appear on Irish supermarket shelves.
The US Food and Drug Administration has expressed concern about the risks associated with the technique, should something go wrong with the process or if subsequent storage temperatures became too high. Such worries and the increasing popularity of sous vide meant that a European research programme on the microbiological safety and quality of foods processed by the system was more than timely.
Teagasc's National Food Centre in Co Dublin played a leading role in a four-year study with research centres in Northern Ireland, Britain, Belgium and Greece. Sous vide involves vacuum-packing a product, followed by gentle pasteurisation, then rapid chilling to lower than 3C.
It was, however, the notion of a "mild cook" which caused FDA alarm bells to ring, according to the NFC's Dr Declan Bolton. Many in the UK also feared the mild cook wasn't sufficient to eliminate species which could survive and grow in the particular circumstances of sous vide processing and storage.
Concern centred on the dreaded Clostridium botulinum bacterium which grows in anaerobic conditions (absence of oxygen) and produces its potentially deadly "Etype toxin" at temperatures above 3.3C. It is an insidious threat because processing kills off other microbes that spoil food, and it can get a free run to multiply in their absence if it gets an initial foothold. Moreover, when the product is then opened for final preparation/eating, there are no smell indicators of contamination (signals usually conveyed to consumers courtesy of spoilage organisms) and the food looks all right.
The NFC, however, focused on other opportunists in the form of Listeria monocytogenes and Yersinia enterocolitica, though some salmonella and E.coli 0157 types can also cause trouble with sous vide systems.
Much of its work involved determining the ability of bacteria to withstand heat, known as "D-values". This is the time required to destroy 90 per cent of the bacterial population inoculated in a test. But the NFC team also evaluated the effectiveness of current sous vide regulations and examined the role of "heat shock proteins".
These increase the resistance of bacteria to temperature elevation and are formed by cells when heated to sub-lethal levels or heated slowly, as is the case with sous vide cooking.
The NFC also helped set new "hurdles" to be built into the process as a way to restrict the survival and growth of targeted microbes. Two such hurdles involve the form of heat when pasteurising, and then rapid cooling to chilled storage levels. Their research showed how the addition of sodium lactate to a product could create an additional hurdle, notably with meat and poultry products.
Consumers can be assured "current regulations are more than sufficient to ensure the product is safe, regardless of whether or not the sous vide process induces the formation of heat shock proteins", Dr Bolton added.
Significantly, the NFC found that one commercial sous vide fish product, made outside Ireland, which did not follow the regulations, could pose a risk, i.e., an insufficient "kill" of microbes was achieved based on the level of cooking, he explained.
The overall picture is reassuring, although the safety of sous vide foods must be underpinned by use of quality raw materials and strict low-temperature storage after processing.
The National Food Centre has produced a booklet on the research, the first in a new series designed to convey the outcome of Teagasc's publicly-funded food research programme to non-specialist readers. It is available from the NFC, Dunsinea, Co Dublin.