In a recent article in Culture magazine, Dr Michelle Bull of CSIRO’s division of Food and Nutritional Sciences looks at the use of high-pressure thermal (HPT) processing techniques in the food industry. HPT processing uses a combination of elevated temperatures and high pressures to inactivate bacterial spores, thereby making some progress in eliminating the bacteria which are responsible for a range of foodborne illnesses, whilst maintaining the protective qualities of barrier packaging materials. Figure 1 shows the different high pressure processes, target microorganisms for inactivation, and example food categories: Why use HPT processing? Both heat and pressure have long been used independently as means of preserving food. For example, high pressure processing (HPP) at low or ambient temperatures can prevent the growth of bacterial spores, and is an effective means of preservation in moderate and high acid foods. However, low acid foods (LAF) remain impervious to standard pressure processing at these temperatures, which allow the Bacillus and Clostridium strains of bacteria to flourish, and have traditionally been preserved through thermal processing. But this technique requires relatively long periods of heating to ensure that all the food within the target pack has been heated to the minimum temperature – and the longer the heating period is, the greater the risk of degradation to the food quality attributes (flavour, texture, nutrient content and colour). How HPT works Food products are put through a pre-heating, holding and cooling process in order to achieve the desired result. The equipment used is designed to enable maximum compression heating which is vital for bacterial spore inactivation. It must be able to maintain temperatures of up to 90°C, at pressures of 600-800 MPa or higher (up to 1,500 MPa).
- Firstly, the food product is placed in a holding unit and heated to a set temperature.
- The holding unit is then pressurised, and the temperature is increased. The extent of these increases depends upon the composition of the food product and its reaction to heat.
- Once the required temperature has been reached it is maintained for a period of time long enough to inactivate bacterial spores.
- Decompression then takes place which cools the food product quickly, avoiding degradation in terms of colour, taste, texture and nutritional value.
The advantage of HPT processing lies in the reduced thermal load applied to products due to:
- the accelerated heating/cooling times of food products during (de)pressurisation; and potentially
- reduced processing temperatures and/or times through the synergistic effect of heat and pressure on bacterial spore inactivation.
It’s that reduced thermal load and more precise heat application that creates less heat damage to the food and its packaging. How successful is HPT? The results show that HPT processing is effective at preventing reactivation in most – but not all – types of pathogenic bacteria. Spore reaction Bacteria spores show different responses to HPT processing – even between strains of the same species. For example, proteolytic Clostridium botulinum strains were more HPT resistant compared to the nonproteolytic strains. Curiously, those bacterial spores which are heat-resistant differ in their response to the combined application of heat and pressure.
- The very heat-resistant Bacillus amyloliquifaciens strains are highly pressure HPT-sensitive.
- The very heat-sensitive Bacillus coagulans strains are the most HPT-resistant under a variety of conditions.
- While Clostridium thermosaccharolyticum strains are generally more than 10-fold more heat-resistant than C. botulinum, with the introduction of pressure they become far less HPT-resistant.
The intrinsic properties of the underlying food also play a part; research suggests that spore inactivation may be affected by the food’s water content, or its pH levels. Impact of packaging The effect of HPT processing on food products also varies according to the barrier properties used. Ideally, food packaging needs to be able to withstand changes in volume, (de)compression, and temperature without any degradation of its aesthetic appeal. However, HPT processing causes degradation to a certain extent in all forms of food packaging
- this is more visible in barrier materials such as vapour-deposited nylon and oxide film, but
- aluminium foil and PVDC-MA film are the least affected.
Further work is needed The use of HPT processing to sterilise low acid, shelf-stable foods remains a long term research challenge, although a more realistic goal is that of prolonging the shelf life of low acid chilled foods. Further work needs to be undertaken into bacterial spore inactivation in these chilled LAFs, in particular into the non-proteolytic C. botulinum strain, which can germinate and produce toxin at refrigeration temperatures. Other areas to be addressed include:
- improving pressurisation equipment to ensure optimum temperature distribution,
- developing food packaging to better withstand HPT processing, and
- better understanding the kinetics and mechanisms of spore inactivation through HPT.
To read Dr Bull’s full article, and view data and references, go to volume 33 of Thermo Scientific’s Culture magazine. Do you use HPT in your work? What are its challenges and opportunities? Let us know below.