To Heat or Not To Heat

With the advent of nonthermal processing methods, heat is now being portrayed as the root of all food processing evils. Numerous studies extol the virtues of nonthermal processing methods in retaining valuable quality attributes and bioactive components—and attribute all negatives to the role of heat. A trait of many such studies is a comparison between a mild nonthermal process and a needlessly severe thermal process, followed typically by a declaration of victory for the nonthermal process. Are such comparisons valid? Is heat truly the villain of food processing? We think it is time for a more detailed examination of the questions.

Although the scientific community has intrinsically assumed that heat always has a significant detrimental effect on product quality, our conceptual analysis aims to demonstrate that heat could be an effective way not only to produce safe, but also high-quality, food products. Strictly speaking, not all thermally processed foods are of low or poor quality.

A rare but interesting exception is ultra-high-temperature (UHT) processed milk. UHT milk is commonly exposed to temperatures of 145–150°C for a few seconds, resulting in a microbiologically safe product with very high quality. Experimental analysis and simple calculations show that labile vitamins, such as thiamine, retain more than 99.9% of their micronutrients, reflecting that, in this specific application, thermal processing is very effective in inactivating microorganisms and almost imperceptible in terms of quality deterioration.

Truly, temperature is very effective in inactivating microorganisms and enzymes, whereas purely nonthermal methods are unable to deliver these benefits. Indeed, the value of heat has been recognized with the advent of Pressure-Assisted Thermal Processing (PATP), where the pressure is used to manage heat input to the product, resulting in sterility with a markedly lower thermal process than conventional heat transfer-limited methods.

In our view, the real problem with conventional thermal processes is the need for improved methods of heat transfer. Thus, the aim of research studies could use creative approaches to speed up the heat transfer or, at least, heating up the food product while maintaining quality. Actual solutions include: a) for liquids, quick heating as with UHT milk, and b) for solid foods, slow heating as performed in sous vide (under vacuum) products.

Although sous vide products are high quality, they are not truly sterilized products and, also, the process is too long (sometimes more than 24 hr) and needs strict refrigeration after processing. Conceptually, sous vide processing is effective in preserving quality because the processing temperature is low (~60°C) and it avoids temperature gradients that normally cause severe surface deterioration. Computer-assisted studies reveal that sous vide processing time could be significantly reduced and its quality improved if the time-temperature of the heating bath is accurately scheduled through an optimization technique.

Improved packaging options are another example of how heat transfer efficiency is critical to improve product quality. No matter what kind of food is being sterilized, when packed in retort flexible pouches, it is of superior quality in comparison to food packaged in conventional metal cans, which must be cooked approximately twice as long.

Nevertheless, the problem persists: how to efficiently sterilize solid foods via a thermal process that achieves both safety and high quality? Most of the research done in the past 15 years has been focused on replacing heating/temperature as the direct medium to inactivate microorganisms. This has been beneficial, as we now have a better understanding of the hardiness of bacterial spore formers, and the role of various nonthermal stresses on bacteria, yeast, and molds. Still, after spending much time, research, and resources looking for alternative processes, scientists accidentally have discovered and patented PATP, which speeds up the temperature rise and fall for solid foods, resulting in a very high quality sterile end-product. Although pressure plays a significant role over certain temperature ranges, in effect, the solution is an improved method for delivery of heat.

Interestingly, sous vide processing was developed by a chef—accidentally—and now sterilization assisted by high-pressure processing has been discovered accidentally, too. In the latter, the research objective was to replace temperature as a way to inactivate microorganisms, rather than improve heat transfer. In our view, these “fortunate” accidents—at a high price—have occurred because scientists have been attracted by the Holy Grail of a true nonthermal method, while eventually stumbling on a solution—a way to improve the delivery of heat.