Lock In Flavours And Extend Shelf-Life With High Pressure Processing

An understanding of High Pressure Processing (HPP) will potentially help the local consumer market receive high/better quality products. By Dr Matthew Zhao, Packaging Technologist, Food Innovation & Resource Centre, Singapore Polytechnic.

The trend towards minimally processed foods that retain food quality in terms of flavour and nutrients has driven food manufacturers to explore non-thermal processing as a means to extend the shelf-life of these foods. Food waste is a significant problem in food supply chains as spoilage at any stage of the supply chain would mean economic loss to the food manufacturer. Hence, it is important that any non-thermal processes have to not only extend the shelf-life of the products but also improve their quality and nutritional benefits.

In the food supply chain, food products are constantly exposed to fluctuating environmental conditions and the uncertainty of food deterioration. Thus, it is important to conduct shelf-life evaluation to better understand the product’s stability. Food deterioration consists of development of off-flavour, loss of texture, loss of colour, loss of nutrients, and microbial spoilage. During storage, food deterioration can occur, which results in consumer rejection, and may subsequently lead to food wastage. There are many different approaches to extend shelf-life, including product reformulation, the adoption of processing techniques, and using packaging materials and technology.

There are many different processing methods to extend the shelf-life of food products. The products can be subjected to either pasteurisation or sterilisation. The processing methods can be categorised as either thermal or non-thermal techniques. In certain occasions, thermal processing methods may impart undesirable modification, such as an obvious colour change (Figure 1).

A classic example for a thermal processing technique is retort process. During retorting, the food products are exposed to high temperature conditions whereby microorganisms are killed, thus achieving a sterilised product. Another example is Ultra High Temperature (UHT), which is a process that is suitable for liquid products. Similarly, the product will be exposed to high temperature but with a shorter holding time compared to retort.

Figure 1. After applying thermal treatment to a beverage containing fresh fruits and herbs.

What Is High Pressure Processing?

High pressure processing is a non-thermal processing method that uses high pressure to extend the shelf-life of the product. The packaged product is placed in a vessel filled with cold water and subsequently exposed to high pressure conditions.

The selection of packaging material and the adoption of packaging techniques are critical. In order to prevent the packaged product from bursting during high pressure processing, the use of a flexible packaging material, with minimal headspace within the pouch or tray is essential. For example, the vacuum skin pack technique was adopted to pack ready-to-eat meals in a tray format, which is suitable for high pressure processing.

Apart from the packaging, it is also important to consider the food content as well. In particular, food products with sharp and hard edges might puncture the package during processing, thus rendering the sterilisation process ineffective (Figure 2).

Figure 2. A well-sealed package product vs. a punctured package.

High pressure processing is well-known to have a minimal effect on nutritional and aroma compounds, preserving the freshness and quality of food products. Therefore, high pressure processing is often adopted by juice companies to extend the shelf-life of juices without compromising on flavour and nutrients.

Due to the low compressibility of covalent bonds, high pressure processing is found to have no impact on the low molecular weight molecules such as vitamins, minerals and aroma compounds. However, high pressure treatment has a similar effect as thermal treatment on macromolecules, including protein and starch, results in a change in their native structure. Pressure modifies the protein structure, observing the coagulation of egg white proteins when the egg white is exposed to pressure above 300 MPa. Similarly, the viscosity of the egg yolk was found to increase significantly when exposed to high pressure conditions above 400 MPa.

Besides retaining the flavour and nutrient content, high pressure processing is able to inactivate microorganisms effectively through disrupting the microorganism’s cell membrane, denaturing its proteins, and modifying the enzymatic activity and configuration of ribosome. A typical fruit juice has a relatively short shelf-life of less than a week. By applying high pressure processing to fruit juices, the shelf-life is extended to at least 2 months when stored at chilled conditions, retaining the freshness and flavour of the juice. The microbial load presented initially in the fruit juice was significantly reduced (approximately 5-log reduction) after going through high pressure processing, and thus achieving a longer shelf-life. Similarly, after a week of storage, the avocado that undergoes high pressure processing did not develop any foul odour that was produced by the growth of microorganisms. However, the foul odour was detected in the unprocessed avocado.

Herbal egg requires a long processing time to impart the herbal flavour to the egg. As a result, high pressure processing is applied to herbal eggs to explore the possibility to reduce the processing time and, at the same time, retain the strong herbal flavour. A partially cooked egg was immersed in the herbal mixture and subsequently exposed to high pressure processing with a fixed holding time. The colour and herbal flavour intensity of the egg was found to be enhanced after being exposed to high pressure for 10 minutes (Figure 3).

After high pressure processing, the liquid egg yolk observed in the unprocessed egg had a modified creamy texture. This creamy egg yolk texture was not observed in a traditional thermal treated herbal egg, where the egg yolk is solidified and dry. Therefore, this case study demonstrates the possibility to adopt high pressure processing to reduce the processing time from a few hours to minutes, and simultaneously producing herbal eggs with a creamy egg yolk and enhanced herbal flavour.

Figure 3. Herbal egg exposed to high pressure processing (HPP).

Although post-high pressure processed products will require cold chain, the numerous benefits such as shelf-life extension, uncompromised flavour and nutrients retention, has attracted the attention from the food industry worldwide. In recent years, the consumers are mindful on their food choices, selecting products with added benefits to health and minimally processed. As a result, companies can explore the likelihood to take up high pressure processing to manufacture food products that retain more nutrients and freshness with an extended shelf-life.

New In Singapore

The well-established high pressure processing technique has recently been introduced to Singapore by Food Innovation & Resource Centre (FIRC). Since then, FIRC has gained insights on this process through in-depth studies on several food products. The understanding gained will potentially help local companies more readily adopt high pressure processing and offer the consumer market with high/better quality products. To facilitate its adoption, toll services for high pressure processing is available for companies to adopt within Singapore (Figure 4).

Figure 4. The high pressure processing unit that is available in Singapore.

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