Author: Hayley Every
Drying with supercritical CO2
Drying fruit and vegetables is a convenient way to extend shelf-life but it often comes with a loss in nutrients and structure. By using pressurized carbon dioxide (CO2) drying technology, it is no longer necessary to make such compromises.
CO2 dried products keep their colour and shape, so they look natural and fresh. As the drying process is done at relatively low temperatures in an oxygen-free environment, the vitamins, pigments and proteins are preserved, ensuring that the nutritional value is retained. Although performed under pressure, CO2 drying is a very mild process, where the water is gently removed from the product. This allows for the structure and texture to be maintained, particularly when rehydrated.
Mild and sustainable
The CO2 drying process works in a similar way to air drying, except that the drying medium consists of CO2 under pressure. When the processing temperature and pressure is above 31°C and 74 bar, the CO2 is in a supercritical state. Under these conditions, the CO2 can diffuse like a gas but has the density of a liquid. This allows the CO2 to easily permeate and readily evaporate the water from food products.
The result is faster drying times, between 2 and 24 hours, and reduced energy consumption. Once the drying process is complete, the pressure is released from the system and no CO2 remains in the product. The CO2, however, is collected and stored, to be reused during the next drying run. The CO2 drying process is therefore efficient, clean and sustainable.
More nutrients, better shelf-life
Immediately after harvesting, fruit and vegetables start to lose their nutrients. Exposure to oxygen, heat and light can accelerate this loss. As CO2 drying is conducted under mild conditions, it ensures that the nutrients are fully retained, thereby ‘locking-in’ the freshness. Removing the water also increases the shelf-life by inhibiting the growth of bacteria and mould. At the same time, CO2 also inactivates microorganisms that can lead to spoilage, thus avoiding the need for further pasteurisation. CO2 drying has been shown to be effective for a range of fruits, vegetables and herbs, including berries, apples, mangoes, broccoli, carrots, and basil.
When drying with supercritical CO2, the water is very gently removed from the fruit and vegetables. The CO2 takes up the water without interacting with the internal surfaces of the product, thereby preventing any collapse or shrinkage. The drying is also performed at a constant temperature, which avoids distortion due to thermal stress. Drying under these conditions ensures that the internal structure of the product remains intact. Plus, after rehydration, the shape and texture of the fruit or vegetables are almost identical to the original fresh product.
CO2 drying can also be used to produce powdered ingredients via a spray drying technique. The rapid evaporation of the water at relatively low temperatures is particularly beneficial for sensitive products, such as protein powders. Under these mild conditions, the protein retains its nutritional value as well as its solubility, foaming and emulsifying properties. Preserving such functionality is not only advantageous for food production, but also for the processing of therapeutic proteins for pharmaceutical applications.
Summer produce in winter
Consumers are becoming more demanding of high quality, nutritional produce. There is also the expectation that seasonal products are now available all year round. Shipping fresh produce all over the world requires effective storage and transportation, to ensure that the quality is maintained. With novel drying processes, such as CO2 drying, the shelf-life of products can be extended while also reducing the shipping weight. This not only results in lower transportation costs, but also allows for ‘fresh’ products to be transported to all corners of the globe. Overall, drying with supercritical CO2 produces longer lasting, nutritional products that retain all the naturalness of fresh products.
A 500L pilot scale CO2 drying unit has been developed by FeyeCon D&I and CO2Dry with support from two European grants – the FP7 project PRESERF and the Horizon 2020 project FUTUREFOOD. More information can be found on the FUTUREFOOD website.