The food trend towards healthy snacks is continuing. Snacks made from freeze-dried fruit meet consumer expectations of modern and high-quality food. However, freeze drying of whole fruits requires long drying times and substantially reduces sensorial quality, which is unappealing to consumers. Dried fruit foams offer an alternative for producing crispy fruit snacks.
To investigate this, fruits were converted into fruit pulp. The fruit pulps were subsequently converted into fruit foams by exploiting the functional properties of hydrocolloid (protein-polysaccharide) mixtures to create stable foams by facilitating the subsequent drying process. This allows a snack product in various forms and shapes and with a highly porous structure for a new and intense sensory perception to be produced from raspberries. The combination of foaming and subsequent drying is a straightforward method for obtaining crispy fruit snacks that boost taste and aroma in the mouth. In addition, foamed structures dry faster due to the increased surface area with a thin lamellae of the bubbles, reducing resistance to the removal of water vapor and therefore promoting heat and mass transfer. Furthermore, microwaves can overcome the heat transfer limitations of convective or conductive heating when applied in oxygen-free and low temperature conditions. This accelerates the drying process.
Characterization of fruit foams
Foam can be simply defined as a dispersion of gas bubbles in a liquid containing two apparent phases. A liquid continuous phase surrounds a dispersed phase of bubbles of air. Indeed, foam is formed by various bubbles surrounded by fluid films. It is of vital importance to assure proper foam formation and stabilization before, during as well as after the drying process. Whipping by means of a rotor-stator unit with or without integration of a defined amount of air or gas is a conventional and widespread unit operation in the food industry.
Fig. 1 A) Raspberry foam production, B) Raspberry foam drying, C) Quality evaluation of dried raspberry foam, modified according to 
Bubbles can be formed and stabilized by so-called foaming agents and stabilizers during the continuous phase. Therefore, the formulation applied is of crucial importance for the characteristic structure of the foam. In food systems, proteins and polysaccharides perform a techno/functional role as both are surface active components. These surface-active materials decrease the surface tension by forming a viscoelastic adsorbed layer, thus providing the intended stability [1, 2]. Proteins are widely used in the food industry, e.g. milk proteins, or egg albumen. In the context of this research, potato protein (PP) was used as the foaming agent. Maltodextrin (MD, DE 6) and citrus pectin (P) were used as foam stabilizers.
Frozen raspberries were converted to puree using a food mill. A diagram of the experimental method for preparing raspberry foam is presented in Figure 1. The foams were characterized with respect to foaming ability (foamability) refering to air holding capacity , as well as foam stability (FS) is the ability of a foam to hold air for a given period of time. Another important parameters are the bubble size and the size distribution of bubbles regards to their geometry . As foam stability is an important criterion for foam drying, the formulations were selected according to the obtained results in Dachmann et al., 2018  and Ozcelik et al., 2019a . The foams were exposed to the drying process (Fig. 1B).
Microwave freeze-foam drying
Fig. 2 Total drying time for conventional freeze-dried and microwave assisted freeze-dried raspberry foams as a function of maltodextrin concentrations at different microwave power levels. The foam samples contained 5 % (w/w) PP +2.5 % (w/w) P and varying MD concentrations. Process conditions: shelf and maximum product temperature of 30 °C and chamber pressure of 0.1 mbar 
The main advantages of foam-mat drying are lower temperatures and shorter drying times compared with non-foamed materials when using the same drying technique . This is due to the larger inner surface and the thin lamellae of the foam. These decrease the resistance to the removal of water vapor over the entire drying process . Freeze drying (FD), also known as lyophilization, is considered the most gentle dehydration process. It is based on sublimation in the absence of liquid water at a low temperature and in an oxygen-free environment. FD is often used as a reference method restricted to the food industry to produce high added-value products such as aromatic herbs, ready-to-eat products (fruit, vegetables, fish and meat), and coffee . However, FD takes a long time and is recognized as the most expensive drying method . For this reason, the process has to be accelerated while the conditions must be kept the same in order to obtain high quality heat-labile products. Put simply, FD coupled with microwave heating is called microwave-assisted FD (MWFD) . Microwaves enable selective 3D or volume heating of the material and simultaneous mass and heat transfer limitations do not occur. The 3D volumetric heating mechanism lets microwaves penetrate directly into the product, causing rapid internal heating, in particular of the contained water molecules . The samples were dried using FD (Christ Gamma 1-20, Osterode, Germany) to establish a reference. For the MWFD experiments, a pilot-scale microwave freeze dryer (Model µVac0150fd, Püschner Microwaves, Schwanewede, Germany) was used. The total drying times for the foams with varying MD concentrations at different microwave power levels are given in Figure 2. The higher MD concentrations led to faster drying. This is attributed to the faster moisture diffusion through the thin lamellae of the air bubbles.
The required time for MWFD was 3.5 to 5 minutes per gram of evaporated water, while FD required between 8.8 and 15.8 minutes per gram. MWFD shortened the total drying time by two-thirds. A 2-fold power input showed a 30 to 40 % decrease in total drying time. There was a positive correlation between drying time and MD concentration, hence with the characteristics of the fresh foam. Overall, coupling MW technology to the FD process enhanced energy efficiency by decreasing the drying time .
Fig. 3 The impact of the maltodextrin concentration and the microwave power input levels on A) ascorbic acid and B) total anthocyanin content. The foam samples contained 5% (w/w) PP +2.5% (w/w) P and varying MD concentrations .
The characteristics of foam also serve another purpose beyond the acceleration of the drying process, which is to create an aroma boost effect upon consumption. However, oxidative damage may occur during whipping with air or later during storage if the high inner surface of the foam is not protected against oxidation. We therefore determined the factors that can potentially promote the degradation of sensitive substances. The retention of ascorbic acid (AA) and anthocyanins (ACY) is depicted in Figure 3 as a function of MD concentration and specific MW power. Both drying methods resulted in a 66 % to 81 % retention of AA and a 53 % to 84 % retention of total ACY.
High retention of sensitive substances was achieved owing to the oxygen-free, low temperature environment of the FD process operated at high vacuum levels and high drying rates. The higher drying rates obtained at higher MD concentrations resulted in higher ascorbic acid and anthocyanin retention . Summing up, the MWFD process outperforms its popular alternative – the FD method – in energy efficiency while producing products of equivalent quality.
Storage stability of dried raspberry foam
Fig. 4. A) Water activity, B) Ascorbic acid, C) Anthocyanins and D) Total color difference of dried foam samples via MWFD and FD at the end of a 12-week storage period. Samples were stored in the vacuumed pack at 37 °C .
The shelf-life of a food product is an important criterion not only for consumers but also for the manufacturer. A stable food product is one that can retain acceptable quality with regards to safety and organoleptic properties. There are three main factors that can affect the storage stability of a product: formulation, processing and storage conditions. In this study, we aimed to investigate the storage behavior of raspberry fruit in different structural forms, namely foamed and non-foamed puree, with potato protein and hydrocolloids. The impact of hydrocolloids, porous structures and applied microwave energy on the stability of the dried fruit foam was compared to that of conventional freeze dried products over 12 weeks of storage at 37 °C. Dried samples were vacuum-packed. The plant bioactives, namely ACY and AA, and the color during storage were investigated. Water sorption and glass transition temperature were also measured .
The results suggest that the structure of the fruit preparation is a critical parameter and plays an important role in preserving quality. Raspberry puree without hydrocolloids was not stable during storage because of its critical glass transition temperature (Tg) level, which triggers negative changes in the AA and ACY molecules. This adversely affects the color of the samples. Figure 4 depicts the water activity, retention of AA, ACY and total color change results of FD and MWFD samples obtained at the end of the 12 weeks of storage at 37 °C.
Results show that the addition of hydrocolloids stabilized the raspberry puree when stored at high temperatures. The foam structure provided better storage stability within the range of the tested parameters. Although an open porous structure can generally be expected to lead to higher rates of deterioration reactions, the matrix composition helped here to prevent such detrimental reactions and enhanced storage stability. The results indicate that MWFD can be used to produce fruit products with a comparable or even slightly better storage stability than when using the FD method .
Microwave freeze foam drying of heat-labile fruits requires less energy than the popular alternative method of FD while yielding a product of equivalent quality. This study is the first to report on microwave freeze foam drying of fruit pulp utilizing potato protein, a process that provides an efficient way to meet the requirements for innovative products. Our results form the basis to utilize microwaves for the drying of fruit pulps and will be of value to the research environment and the food industry.
This Industrial Collective Research (IGF) Project (AiF 19015 N) of the Research Group of the German Food Industry (FEI) was supported by the Federation of Industrial Cooperative Research Associations (AiF) under the programme for promoting the IGF of the German Ministry of Economic Affairs and Energy (BMWi), based on a resolution of the German Parliament.
Category: Food Process Engineering | Freeze Drying
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