Why thermal pasteurisation remains the benchmark

DESPITE SIGNIFICANT developments in non-thermal alternatives, heat-based pasteurisation is still the most suitable, efficient and cost-effective solution for many ingredients and products. Having a clear understanding of where non-thermal techniques excel and where they fall short is essential when selecting the right approach for a given process and product.

Non-thermal processing techniques

The most common alternatives to thermal pasteurisation are high-pressure processing (HPP) or pulsed electric field (PEF) processing, followed by ultraviolet (UV) light, ultrasound, cold plasma, and high-pressure carbon dioxide (HPCD). 

Typical industrial HPP involves underwater pressurisation of the product at ambient temperature, usually to a specific pressure between 400-600 MPa, and then holding the product at this pressure for a specific time (usually one-to-five minutes). These high pressures induce changes in the functionality of the cell membranes, resulting in inactivation of vegetative cells and some enzymes.

In contrast, PEF processing uses short bursts of high voltage electric fields (up to 30,000 pulses per second) to achieve the desired microbial inactivation or modification of the food structure. This ruptures the cell membranes, effectively deactivating the microbes.

Some PEF processes use heat exchangers to pre-heat the product before the electrical fields are applied. This is typically when less heat-sensitive products are being processed, or when the product requires relatively high specific PEF energy inputs to achieve the desired level of microbial inactivation, e.g. in the case of vegetable juices, dairy products, or products where inactivation of bacterial endospores is required.

Limitations of non-thermal options

In many cases, HPP has minimal effect on taste, texture, appearance, and nutritional value. It can also be applied to ‘solid’ products, such as sliced cooked meat and ready-to-eat meals, where it reduces contamination from the manufacturing environment. However, it requires high capital costs and is not suitable for every application. For example, any product where the air content is important (whipped creams, mousses, sauces, etc.) needs extra caution if HPP treatment is being considered, as the pressure may force the air from the product, or be compressed (and subsequently released). Both scenarios can cause catastrophic product damage.

Dry products such as powders, nuts, etc. are also unsuitable for HPP as isostatic pressure needs water to be present inside the food product to achieve uniform and efficient transmission (and therefore cause the necessary inactivation of any microorganisms present). Neither is HPP suitable for many whole fresh produce items where it is either ineffective (in the case of dense products such as fruits or tubers) or causes damage (e.g. leafy salads). 

Finally, there is uncertainty around using HPP for raw meat products as it can induce colour and texture changes. It has also been shown to cause denaturation in key proteins in certain dairy products, such as raw milk and whey.

PEF processing has a number of roles in food processing, including dehydration; but as a pasteurisation technique, it is most effective when used with liquid products (fruit juices, milk, liquid egg, etc.) and semi-solid foods (yogurts, fruit purees, salsas, sauces, etc.). However, there have been reports of a number of potential issues including corrosion and fouling of the electrodes, electrolysis of water, migration of electrode material components, and chemical changes in the food product.

Another concern is that in many locations, food safety and public health regulations have not yet caught up with these new pasteurisation techniques, meaning that further assessment may be required before they are widely accepted.

Benefits of thermal pasteurisation

In most situations, there are several key advantages to traditional thermal pasteurisation and sterilisation techniques using corrugated tube, or scraped surface, heat exchangers:

• They are proven and highly effective, killing 99.9% of pathogenic bacteria, yeasts and moulds, and deactivating enzymes which can spoil food and drink products. 
• They require lower capital costs than most non-thermal alternatives; in addition, technologies such as corrugated tube heat exchangers can offer a highly efficient process in a compact package.
• They are scalable and versatile. In commercial situations, one unit can be used for a range of temperature and time combinations, and to treat a wide range of products.
• They maintain nutritional value, particularly of many dairy and juice products. Techniques such as HTST (high-temperature short-time) pasteurisation and ohmic heating are particularly good at preserving product quality while providing effective and efficient control of harmful microorganisms.

Given the complexity and wide range of techniques available, it is important to seek expert advice, not only on thermal and non-thermal options, but also on how particular food and drink products may react to these different processes. HRS Heat Exchangers offers a variety of thermal treatment options (including corrugated tube and scraped surface heat exchangers, ohmic heating and direct steam injection), as well as experience with numerous products and access to comprehensive laboratory testing facilities, making it well placed to advise on the right solution for any application.

Francisco Hernández Ortiz is global food projects director at HRS Heat Exchangers

For more information:

www.hrs-heatexchangers.com

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