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Novel microwave-assisted vacuum drying for heat sensitive foods

Final Report Summary - MILD-DRY (Novel microwave-assisted vacuum drying for heat sensitive foods)

Executive Summary:
MILD-DRY was a two year applied research project, which commenced in September 2012 and finished in August 2014, and was funded under the “Research for SMEs” programme of the European Commission’s Seventh Framework Programme (FP7).
Dried food products are the foods which are preserved by removing the moisture through various methods to prevent the bacterial and fungal growth. Market demand for dried food products is fuelled by various factors, such as non-seasonal availability, increased shelf-life, and prevention of food spoilage, low price, user friendliness, wide areas of usage, and others. The demand for the convenience foods and consumer trends are the driving factors, whereas the loss of nutrients in dried foods is one of the restraints for dried food products market.
The quality of a dehydrated food product is strongly affected by the drying method, which may cause changes in colour, structure, deterioration of aroma compounds and degradation of the nutritional value, in particular if high drying temperatures are used. Thus, heat sensitive foods and products that possess excellent quality in terms of taste, aroma, texture, and appearance, pose a major challenge to food drying processors.
To this end, the MILD-DRY project aimed at developing a novel technology for the drying of heat sensitive foods based on the combined use of two effective drying methods, microwave drying (MW), in particular variable frequency microwave heating, and vacuum drying (VD), taking the most of each technique: speed in the case of MW and quality preservation in the case of VD.
Developments such as MILD-DRY offer enormous opportunity to create added value, novel offering and market differentiation, to contribute to increasing the competitiveness of dried food producing SMEs, as well as European manufacturers and suppliers of equipment.

Project Context and Objectives:
Food drying is one of the oldest methods of preserving food for later use. It can either be an alternative to canning or freezing, or it can complement these methods. Food drying represents a very good opportunity for producers and packers of fresh fruits for reducing losses and providing an export opportunity for products that otherwise would not be suitable for shipping in their fresh state. Advantages of dried food also lie in its higher nutritional value compared to food preserved using other methods, simpler and cheaper storage (no cooling needed, less storage space, longer durability), transport costs (less volume and weight) and high convenience. Therefore, food drying can provide benefits for production and can minimise losses.
Different drying processes are employed to produce dried foods in large scale, but techniques capable of producing high quality dried foods in an energy efficient manner and in short periods of times are still lacking. Conventional drying methods, such as airflow drying, vacuum drying (VD) and freeze drying (FD), result in low drying rates and consequently in long drying process and high energy consumption.
Heat sensitive foods pose a major challenge for food driers. Essential and valuable quality properties such as colour, flavour, texture, appearance and nutrient content are damaged, reduced or lost with prolonged exposure to high temperature. In order to keep the product properties like flavour, texture and ingredients etc., high temperatures have to be avoided.
In recent years, the use of microwave (MW) energy as a source of heat to process and treat food products has gained more widespread due to its unique advantages of high efficiency, environmental friendly and energy conservation. Following advances in microwave drying technology, microwave vacuum drying (MWVD) is emerging as an alternative for drying very sensitive products. MWVD combines the advantages of both vacuum drying and microwave drying. Because the boiling point of water falls under vacuum conditions, this technology provides a rapid dehydration method than can accomplish water evaporation at lower temperature, hence it is considered to be highly suitable for dehydrating heat sensitive materials.
The interaction between the MW field and the food being dried is complex in its nature and depends on state parameters such as food temperature and moisture content as well as on the frequency of the incident electromagnetic wave. Different MW frequencies are known to initiate different energy transfer mechanisms within the food being dried, with low frequencies being potentially more efficient in evaporating strong bounded water at the last stages of food drying,

The effect of different microwave frequencies on food drying raises potential applications for the optimisation of microwave drying protocols. To this end, the MILD-DRY project aimed at developing a novel technology combining the benefits of microwave and vacuum drying methods with the novel use of variable microwave frequency in order to optimize the absorption of MW energy during drying process, hence providing a more efficient MWVD technique for the gentle drying of foodstuffs while enabling increased food quality, reduced drying times and energy consumption.

The specific objectives can be summarized as follows:

• To use the results of a “bottom-up” research approach to gain understanding of the industrial needs and define the specifications of the MILD-DRY system.
• To validate the MILD-DRY technology at laboratory level in order to evaluate its effects on drying efficiency and dried product quality.
• To determine the drying kinetics of the food products and to use the results to define the parameters needed for the sale up.
• To gain an understanding of the variable frequency microwave-vacuum lab-scale system system in order to conceptualise and build the best industry applied prototype that integrates the control system based on drying curves.
• To carry out calibrations and preliminary test to ensure the proper functioning of the system and its control unit.
• To install the prototype in industry to validate its performance in the field, comparing it to conventional drying methods.
• To carry out quality analysis of samples from lab scale drying, as well as from industrial trials against food dried using conventional methods. .
• To make the necessary modifications and improvements to optimise the prototype and define future development work.
• To efficiently manage all knowledge-related matters arising from the project and to adequately protect project IPR embedded in the dissemination and use actions.
• To facilitate the uptake of the MILD-DRY results by the participating SMEs, as well as by a wider audience, by carrying out demonstration activities.
• To draft a plan for the use and dissemination of the project results, as well as to plan activities for the future exploitation of the foreground.
The overriding goal of this project is to ensure that the pre-competitive MILD-DRY prototype resulting from this project fulfils the threshold requirements to ensure its further development post-project into a fully industrial system that is taken to market, where its beneficial impact will be felt at European level.

Project Results:
The first target of the project focused on understanding the technological needs of European dried food processors, as well as market needs and perceptions in terms of product quality, new technologies, etc. To this end, a questionnaire-based on-line survey directed to food processors, and equipment providers was implemented as part of WP1. The gained in-sight was further broadened by desk research on market and socioeconomic information, literature reviews and patent searches on the technology at hand, and a number of on-site visits and in-depth consultations. With this information, both the industry and research partners worked closely to define the industrial needs of the proposed MILD-DRY system.

Early in the project different possibilities to generate variable frequency microwave were discussed. The best solution for the lab-test equipment was found to be the use of broadband microwave generator based on a discrete array of MW generators connected to a signal power amplifier. This solution was proved to be very challenging at the technical level. Basic concepts were applied to the design of the microwave generator, although the generation of a broad frequency range significantly increased the complexity of the design. The adjustable broadband response of the microwave cavity was another considerable hurdle in the design of this equipment. . Nevertheless, a complete MILD-DRY test rig was built consisting in a vacuum system; a variable frequency microwave system; and a control and logging system. .

Initial tests with the equipment for optimal matching conditions and preliminary drying trials were conducted with fresh and semi-dried apple samples at two selected frequencies (commercial frequency (2,46GHz) and an experimental lower frequency) first separately and afterwards in combination (dual frequency drying). The comparison of the drying curves for drying from fresh, indicated a trend to faster drying rates at the commercial frequency at high moisture content compared to drying at the experimental frequency. However tests conducted with semi-dried samples (about 40% water content) pointed to an opposite behaviour in relation to drying rate at the two analysed frequencies. These observations were taken as initial indication of drying rate improvements at lower microwave frequency for samples with lower moisture content.

Further experiments with both apple and onion slice, confirmed that the influence of the experimental frequency operation was particularly relevant during the last stage of drying when moisture content has decreased to a certain level.. In the case of apple slices, drying times during the second stage at the experimental frequency were 10% faster at 50% power, and still 5% faster than the commercial frequency if the applied power was 100%. This indicates that operating at a lower frequency during this last stage of drying not only favours the reduction in the overall drying time, but also, that the effect is equally present at higher and lower power levels. Similarly, it was found that the average drying rates for fresh onion slices were similar for both frequencies while the effectiveness the experimental frequency was significantly higher at lower moisture content, which validated the variable frequency microwave vacuum drying technology.
Altogether it was concluded that the drying process could be divided into two stages, where the commercial frequency should be applied at the first stage to achieve a pre-defined moisture level and the experimental frequency should be used at the second stage, in order to obtain higher drying rates and shorter overall drying times.

A switching point, dividing the 2 drying stages, in terms of moisture content for apple and onion was suggested.
The drying data of apple and onion slices in the test rig was regressed to obtain the drying kinetics under the studied drying conditions, for both single fixed frequency drying and dual frequency drying. The kinetics gave relationships between moisture ratio of the samples and the drying time, which could help to predict the drying time. The regressions result in very good fitting between experimental and predicted data, meaning success of the regression for the test rig. However,
In parallel with the lab scale experiments, the conceptualisation and building of the MILD-DRY pre-competitive prototype was carried out in order to exploit the principle of variable frequency at industrial level. However, the variable microwave generation system for the industrial prototype was restricted by ITU-R regulations, which limit the range of frequencies for industrial use. As a consequence, the technical approach was different from the one used for the lab scale test rig where such regulations do not apply. Therefore the MILD-DRY prototype was designed to integrate a discrete array of MW generators, each one emitting a specific frequency among the ones allowed by ITU-R regulations, as opposite to the test-rig equipment in which a broadband microwave power generator was used. This approach allowed for a multifrequency MILD-DRY equipment competitive in terms of cost, while keeping the technological novelty of using different frequencies during the drying process.

The final MILD-DRY pre-competitive prototype was built with the support of experts in industrial microwave and vacuum equipment and from the Consortium partners who actively supervised the construction work and provided inputs from their experience in industrial equipment and drying technologies.
Electric field simulations of the wave propagation were carried out for defining the geometry of the chamber as well as the position of the MW ports. The microwave propagation depends strongly on the vacuum chamber geometry, as the vacuum chamber is also the resonant MW cavity. Therefore the vacuum chamber was designed to work in multifrequency conditions. The MW power is generated outside the chamber by two different MW generators, whose power is transferred to the chamber through specially designed waveguides.
In addition, in order to ensure that maximal microwave energy is absorbed by the foodstuff, the matching of the cavity to the operating frequency was ensured by incorporating mobile tuning stubs.

As derived from laboratory trial with the test-rig and after deep discussions with MW experts, it was clear that the combination of vacuum, different MW frequencies and different foodstuff would be challenging in terms of performance complexity; therefore an empirical approach for establishing optimal operation parameters would be required.
Following the building of the MILD-DRY prototype, the system was tested at semi-industrial conditions according to a validation plan based on the results obtained with the laboratory test rig equipment. However, drying conditions derived from laboratory experiments could not be used directly in the prototype machine, consequently, initial trials with the prototype for knowing the particular process behaviour were undertaken. In terms of type of sample for the validation, minced fresh onions were selected as the right one because its water content and the simplicity of preparation and handling. Likewise, dried onion is an interesting product from an economical point of view and the onion behavior -under different drying processes- has been described in the literature.
The experimental part was organized and executed according to the following scheme
• Iteration 0: For knowing the particular process behaviour in order to define sampling times and detect unexpected issues, especially for Dual-MWVD.
• Fixed frequency: For evaluating a standard drying process at fixed frequency in terms of drying time, energy consumption and food quality.
• Dual frequencies: For evaluating the effect of using different frequencies during the drying process and the potential influence of different switching points in the process in terms of drying time, energy consumption and food quality.

The main conclusion that could be drawn from the industrial trials con be summarized in the following statements: The foundational concept of MILD-DRY –variable frequency microwave-assisted vacuum drying- was successfully implemented on the basis of the simplest setup -i.e. only two extreme frequencies- in a pre-competitive prototype which was also successfully validated with real samples. The MILD-DRY has proven being 10% more efficient than the standard 2.45 GHz technology in terms of energy consumption.

Potential Impact:
The Food industry sector is one of the largest and most important manufacturing sectors in Europe. It is the second largest (after metal) in the manufacturing industry, with 14.5% of total manufacturing turnover (€917bn for the EU-27). The employment in the food industry represents about 14% of the total manufacturing sector.

Dried food products are the foods which are preserved by removing the moisture through various methods to prevent the bacterial and fungal growth. Market demand for dried food products is fuelled by various factors, such as non-seasonal availability, increased shelf-life, prevention of food spoilage, low price, user friendliness, wide areas of usage, and others. The demand for the convenience foods and consumer trends are the driving factors, whereas the loss of nutrients in dried foods is one of the restraints for dried food products market.

SMEs account for more than 99% of the companies in the dried food industry, but for only 50% of the total turnover. Large companies account for the other 50%. Hence, both SMEs and multinational companies are important in the sector. There is strong competition on price and quality within the dried food market. SMEs can only survive the immense pressures on them from larger competitors through innovation. MILD-DRY could go a long way in maintaining SME production sites in Europe by giving them the competitive edge in the marketplace by equipping them to reduce their production costs, as well as safeguard the quality of the dried foods.
Although traditional drying methods and practices such as sun and air drying are still commercially employed today for many food commodities, alternative or complementary means to withdraw moisture from foods have been developed, that are more gentle, economical and efficient, and present better process conditions in order to minimise loss of important quality and nutritional properties in the dried food product. Particularly for heat sensitive materials of high commercial value, the development of alternative drying methods is needed to enable the manufacture of dried products of high nutritional content, extended shelf-life, high quality in terms of appearance, colour and shape and when applicable, excellent rehydration properties.
A key benefit derived from adopting the MILD-DRY technology will be a reduction in drying time. For most production sites, this results in a considerably larger product throughput, rather than in a strict reduction in energy consumption per se, since industrial dryers typically run their machines continuously. However, being able to dry more foods with the same energy expenditure is of key value. Moreover, energy savings resulting from reduced drying time would be a strong benefit, especially for smaller producers for whom the total product throughput often is not the crucial factor. Improved quality of high-value products, with are extremely heat-sensitive, would also be a strong selling point as these products are currently being dried with freeze drying technology, which is an expensive drying process.
Regarding the market for dried food equipment, it consists largely of SMEs and is fairly scattered around the world. The main producers of food dryers are located in either Eastern or Southern Asia (China, Korea, India) or the United States. There are several major manufacturers in Europe however, there is space for European companies to enter this market or to further expand their market share in this area.
The fact that small and medium-sized companies constitute the vast majority of firms throughout the industry, suggests that the global market has not yet consolidated. Because the market for food drying technology and equipment is relatively young, especially for advanced technology, the firms operating within this market focus on servicing the needs of the local markets. The global need for advanced drying equipment has yet to gather momentum.
One reason for this lack of concentration is the continued use of less advanced drying methods, primarily sun drying. Firms operating in warmer climates, such as those in Southern Europe, tend to take advantage of the hotter temperature to dry products without the use of technology. However, surveys of dried tropical food importers in the United States and in Europe show that they prefer machine-processed products as foreign agents are less likely to contaminate the food. As food safety concerns continue to grow, one can expect to see the market respond to an increased demand for better and more efficient technology and equipment for drying foodstuffs. This current situation opens up many possibilities for new technologies such as MILD-DRY.
Developments such as MILD-DRY offer high opportunity to create added value, novel offering and market differentiation, to contribute to increasing the competitiveness of dried food producers, as well as European manufacturers and suppliers of equipment.
Estimates on the potential market turnover, based on the experience and input of the partners, has confirmed the business opportunity for the SMEs and spurred them to draw up an initial business plan and to move forward with an exploitation agreement.
A key importance was given to the management of the intellectual properties and in agreement of the dissemination of non-confidential information throughout the project. The project developed know-how that will be exploitable by the SMEs. In this regards SMEs of the Consortium have initiated the process for IP protection of the Foreground. In parallel, they plan to undertake future development efforts to bring MILD-DRY from a pre-industrial prototype to a commercial system, and to perform practical evaluations of the technology and further demonstration work that will be bespoke to the food processing industry sector. Further funding to mitigate the cost of the additional development work will be applied for.

During the project life successful dissemination activities have been carried out to on the principles of the MILD-DRY technology in preparation for the future exploitation of the technology. These include the development of a project website, number of technical articles and attendance to conferences and trade fair both in industry and in the public domain. Post-project, MILD-DRY will continue to be disseminated, especially as a means for facilitating project-project exploitation of the results. The impact of the dissemination activities that have been carried out during the project have been measured in order to gauge their effectiveness based on quantitative and qualitative indicators.

Number of events at which the MILD-DRY technology was presented – 12,
Number of industry visitors at trade shows and exhibitions - Over 100,000 professionals at the exhibitions visited the events at which MILD-DRY was exhibited.
Number of face to face meetings - 20
Number of policy makers reached - 5
Number of press releases and articles published in the press - 24
Number of hits on the project website - 1420
Followers on social media networks – 246

List of Websites:
Dr. Imma Llop
Avda Carl Friedrich Gauss 11
08860 Castelldefels