New approaches to shelf life

Food production processes are constantly evolving. That's because the efficient use of resources is more essential than ever and affects all processes in the value chain. What's more: Today, food and beverages have to meet a broad range of expectations. High-quality products that are healthy, sustainable and as natural as possible are in demand. Minimal processing is the key term here, which the development departments have taken up in order to produce food only "minimally", i.e. with fewer or shorter processing steps. This also involves new techniques to improve quality and extend shelf life. For example, the treatment of potatoes, other vegetables or fruits with pulsed electric fields (PEF). Elea Technology GmbH brought the process to market maturity together with the German Institute of Food Technologies (DIL). For the treatment, the raw materials are exposed to short high-voltage pulses. This process is now used worldwide in the production of French fries as a pre-treatment before cutting. The application makes it possible to reduce the frying time and temperature and to reduce unwanted quality losses.

Another example is the use of high pressure. Instead of heating food by boiling it for a long time, it is briefly exposed to high pressure. Experts in this field see a lot of potential in this as a gentle method of preservation. "High-pressure treatment makes it possible to preserve products directly in their final packaging," explains Boris Brockhaus. "This works for vacuum and modified atmosphere packaging (MAP) in the same way as it does for plastic bottles and offers a high level of safety, as the products cannot be contaminated afterwards," says the Uhde High Pressure Technologies (Uhde HPT) product manager. Several years ago, Thyssenkrupp specialised in the construction of lines for high-pressure pasteurisation (HPP) with its leading subsidiary in high-pressure technology. The company manufactures machines from small-scale to industrial applications, tailored to the individual requirements of the food industry. High-pressure pasteurisation has proven itself in practice for many product groups. It is used for vegetables, fruit, dairy and meat products, as well as for preserving vegetable proteins or novel foods such as algae.

Here, HPP stands for a non-thermal preservation process. The principle: The products are automatically conveyed in their final packaging into a high-pressure container filled with water. A water pressure of usually 6,000 bar acts homogeneously and evenly from all sides on the packaged product up to its core. After a few minutes, the pressure is decreased again in a controlled manner. "The high pressure eliminates numerous food-damaging organisms, which greatly slows down decay processes," explains Brockhaus. What the treatment cannot kill, however, are spores. "This is also the reason why the products have to remain in the refrigeration chain afterwards," says Brockhaus. As there is no thermal influence on the products during high-pressure pasteurisation, nutrients and vitamins remain virtually unchanged. This offers great advantages, "for example, in the application of fruit juices, which currently make up the largest share of HPP products. As high temperatures are not necessary, the products remain fresh and of high quality." What's more: Not only heat, but also the use of preservatives can be dispensed with by using the HPP process. As a result, "high-pressure processing" or "high hydrostatic pressure processing", as the process can also be called, enables a wide range of "natural" foods that have a longer shelf life and are more resource-efficient.

However, methods that remove moisture to prevent the growth of microorganisms are also at the focus of developments, as can be seen in the example of fruit juice. This is because: Although direct juices are increasingly in demand, the majority of fruit juices consumed continue to be made from juice concentrate. For good reasons: On the one hand, reducing the water content can help products maintain their functionality and make them less susceptible to damage during further processing and storage. In addition, the removal of water reduces the volume, making storage and transport more efficient and sustainable. Drying is therefore still one of the central basic operations in food production. However, established methods of concentration such as evaporation and deep-freeze concentration are fraught with disadvantages: Single-stage evaporation compromises the naturalness of the product and is energy-intensive. Deep-freeze concentration is gentler on the product, but the energy input is similar to that for single-stage evaporation. Applied research is required here: As part of an IGF project, researchers from the University of Erlangen-Nuremberg and the Technical University of Berlin have investigated gas hydrate technology as an innovative process for juice concentration as well as the concentration of water-containing products.

Background: Gas hydrates are crystalline solids of water and gas. They are made up of water molecules that form an ice-like cage structure via hydrogen bonds, which - depending on the natural occurrence or application - can trap carbon dioxide (CO2), methane or hydrogen sulphide in gaseous form. Naturally occurring hydrates in deep ice fields and permafrost regions contain mainly methane. CO2 hydrates, on the other hand, are suitable for food processing: In the presence of CO2, the ice-like cage structures that can bind large amounts of water from products containing water are formed in reactors at pressures of 30 to 80 bar and at cool temperatures of up to eight degrees Celsius. The solid hydrates formed with the bound water are subsequently separated, leaving a concentrate.

In order to be able to measure the product quality, the degree of concentration and the yield, the researchers formed the hydrates of apple, orange and sea buckthorn juice in different reactors and then characterised them together with the reference concentrates: In addition to colour measurements and particle size distributions, ingredients such as malic acid, carotenoids and polyphenols were determined, as well as other parameters such as degrees Brix, pH and aw value, density or total dry matter. It was also investigated whether value-adding ingredients such as flavours, vitamin C or colourings could be incorporated into the hydrate in the concentration step. The influence of the product composition (including sugar content and pH) on the formation of the gas hydrate and its separation was also determined. The result: The researchers could not find any significant changes in product quality before and after the process. This means that the gas hydrate technology for juice concentration is particularly gentle due to the low temperatures. Heat-sensitive ingredients such as phenols or vitamin C are retained. The process can also be used for numerous other thermally sensitive products in food processing. The product range extends from juices, fruit, tea or coffee extracts to soups and sensitive fermentation products in biotechnology.

Within the project, the researchers have worked out process requirements in order to develop three reactor concepts (spray system, bubble column and stirred tank) on this basis: Hydrate formation was successfully tested in all reactors. For example, optimal hydrate growth was achieved in a bubble column reactor at a pressure of 37.5 bar, a hydrate formation time of two hours and a reactor fill level of 35 percent. In the stirred tank reactor, juices could be concentrated to 45 degrees Brix and sugar solutions to 60 degrees Brix. It also showed that as the liquids become more concentrated, higher pressure and a lower temperature are required to form and maintain the hydrate phase.

The process also has advantages in terms of energy: Compared to evaporation, a reduction in energy demand of up to 58 percent can be achieved. Compared to freeze concentration, gas hydrate technology achieves savings of up to 66 percent. As a result, gas hydrate technology offers considerable potential - even for small and medium-sized companies.