30 November 2017

Hot food under pressure

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European researchers carried out final tests on an innovative technique to lengthen shelf life of food

A robotic arm grabs a long metallic cylindrical container and puts it smoothly into a steaming water basin. Benito de Orte, co-owner of the engineering company Metronics, observes carefully how the arm lifts a second container out of the water and places it on a conveyor belt, which transports it to the pressurisation unit. His eyes relax as the container enters the pressure chamber. A group of food technologists have come to his workshop in the outskirts of Pamplona, Spain, to watch a presentation of the first prototype of an HPT machine.

HPT stands for High Pressure and Temperature and is a new food processing concept that involves three steps. First, the food samples are pre-heated, then very high pressure is applied, and finally, the pressure is released and the food is cooled down again.

Until recently, scientists were not 100% sure whether this method would be able to eliminate the risks of food contamination by microbiological pathogens, nor was it clear what kind of food products would be suitable for this treatment. Therefore, the European Commission supported a research project called HIPSTER, where food technologists, microbiological experts and engineers joined forces to investigate the pro and cons of HPT. Part of this project was to build this prototype machine.

During the presentation, Benito de Orte shares some insider information on his machine: “We had suffered a number of set-backs. For example, the first version of the pressure chamber did not perform as planned.” Only six months before delivery, a patent issue forced him to change the alignment of the pressure chamber. But finally – with a delay of a few months – Benito de Orte was able to present a machine that is working and performing.

The core and at the same time Achilles’ heel of the process, is the pressure chamber. Here, the 30-litre cylinder with the preheated food is inserted, filled with water and pressure applied. In less than a minute, the pressure rises to 100, 250 and eventually 600 MPa, which is equivalent to six times the pressure at the bottom of the oceans.

Pa stands for Pascal a unit of pressure. It is named after French scientist Blaise Pascal, who in the 17th century studied the effects of pressure on fluids. The term pascalisation is the method used to preserve and pasteurise food by high pressure. But how successful is the pasteurisation, when you preheat a product and then apply high pressure to it?

High pressure alone can inactivate vegetative bacteria on the food, but not the spores that could make it unsafe or lead to spoilage. Therefore, Silvia Garcia de la Torre and her team from the Spanish National Centre for Technology and Food Safety CNTA, partner of the project, have investigated whether pre-heating the food to around 80°C could help to inactivate the spores. A temperature of 80°C alone would not be sufficient, but due to the thermal law of physics, the food heats up even more as the pressure is applied.

As the pressure rises to 600 MPa, the food heats up to around 120°C by this phenomena. In a series of experimental tests, the researchers were able to show that the most critical microbial pathogens in food products were inactivated by this combination of heat and high pressure. This is good news for the food industry interested in adapting the HPT treatment in the future.

The rise in temperature due to the increase in hydrostatic pressure has an important side effect for the food processing in general. The researchers are able to heat a product very quickly as they apply pressure to their samples, but they can also cool it down at the same speed as the pressure is released again. This way, it is possible to regulate the heat applied to their products very precisely – something that is not really possible with other methods of thermally treated food. “We are able to heat our food samples for a much shorter time than you would be able to do in a thermal process,” summarises Silvia Garcia de la Torre, “which is the key advantage of this process.”

The scientists have not only investigated the food safety issues, like the inactivation of microbiological pathogens, but also the effect the treatment had on the food quality, like colour, taste or firmness. It turns out that the effects of the HPT treatment did vary greatly, depending on the particular food products that were tested.

“We have noticed that green vegetables like peas made a real improvement in the quality of the food,” says de la Torre. “The green colour became even more intense than in the untreated food. When you compare it to pre-cooked or tinned peas, our HPT-treated peas look much more appealing.” Similarly, when the researcher treated chicken filet, salmon or shrimps with HPT, the food testers often preferred them to other food processing methods. But meatballs, for example, did not pass the quality test, as the combination of heat and pressure seemed to have a negative effect on their appearance.

In principal, this new food processing technique seems promising, but there are still some critical issues to resolve. The most important one is scalability. “The capacity of the cylinders in our machine is currently 30 litres. This is too small for the food industry,” admits Benito de Orte.

But he has a plan. As the audience watches the cylinders of the machine move from the hot basin to the pressure unit, and then to the cold basin, Benito de Orte explains his next steps: “We have started to develop a larger 300 litre HPP machine, and we hope to have the next one up and running by the end of 2018, and then hopefully we will make the due adaptations to be ready to work with temperature.” If all goes well, this could then mark the beginning of a new era in food processing. 

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