Foam Spray Drying

Parks, O. W., Wong, N. P, Allen, C. A. and Schwartz, D. P. 1969. 6-frarcs-Nonenal An off-flavor component of foam spray-dried milks. J. Dairy Sci. 52, 953-956. 

Tamsma, A., Kurtz, F. E. and Pallansch, M. J. 1967. Effect of oxygen removal technique on flavor stability of low-heat foam spray dried whole milk. J. Dairy Sci. 50, 1562— 1565. 

Hanrahan, F. P. and Webb, B. H. 1961B. U.S. Department of Agriculture develops foam-spray drying. Food Eng. 55(8), 37-38. 

Foams can also be involved in the drying of food products. A wide variety of foods, including fruits and vegetables, can be foamed and then spread out in thin layers on a support for foam-mat drying [633,845], Other foods, such as milk, cream, and cheese can be dried by foaming them, then injecting the foam into a spray-drier. This process is called foam spray drying [633],... 

Creamy flavors in butter have been associated with 4-cis heptenal produced for autoxidation of isolinoleic acid (Begeman and Koster, 1964). Drier flavor in foam spray-dried milk has been associated with 6-rra x-nonenal, which has a flavor threshold in fresh milk of 0.07 pg/kg (Parks et al., 1969). Bassette and Keeney (1960) implicated a homologous series of autoxidation-derived saturated aldehydes, together with products of Maillard browning, in cereal-type off-flavors in powdered skim milk. Staleness in dry whole milk may be associated with saturated and unsaturated aldehydes (Parks and Patton, 1961). 2,4-Decadienal has been reported to be the principal compound responsible for the off-flavor associated with spontaneously oxidized milk (Parks et al., 1963). Oxidized flavors in sunlight-exposed milk are commonly related to C6 to Cn alk-2-enals... 

Aside from aforementioned hybrid technologies, there are two basic techniques for convective drying of foamed materials, namely, drying of bulk foams (foam-mat drying) and drying of dispersed foams (spray drying). 

In summary of foam-spray drying, all available literature results confirm a strong effect of feed foaming on final product properties of porous and skin-forming materials, for example, an increase of Sauter mean diameter and porosity of the products decrease of bulk, tap density, and apparent density and effect on particle morphology. Foam-spray drying enhances retention of highly volatile substances and... 

Zbicinski, I. and Rabaeva, J. 2010. Analysis of gas-admixing foam spray drying process. Drying TechrwL, 28(1) 103-110. 

Rabaeva, J. 2012. Kinetics of foam-spray drying process. PhD thesis. Technical University of Lodz, Poland. 

Lewandowski, A., Czyzewski, M., and Zbicinski, I. 2012. Morphology and microencapsulation efficiency of foamed spray-dried sunflower oU. J. Chem. Process Eng., 33(1) 95-102. 

The foam drying process is limited to specific products, such as fruit powders, for preparation of instant drinks. Techniques like vacuum puff drying, foam mat drying, microflake dehydration, and foam spray drying have been described elsewhere in this book. Among these, the foam mat drying process has received considerable attention. 

An overvie v of the principles and applications of foam-mat and foam spray drying processes is provided in the follo ving subsections. 

The drying mechanism of a droplet depends on the droplet structure. Abdul-Rahman et al. (1971) and Crosby and Weyl (1977) estimated that the heat and mass transfer rates for foamed droplets (Fig. 6.1b and c) were faster than for non-foamed droplets (Fig. 6.1a), due to an accelerated transport of liquid water to the evaporation front (Ratti and Kudra, 2006) and an increase in the droplet surface area caused by gas expansion during drying. During the foam spray drying process, particles with all of the above structures are formed simultaneously (Fig. 6.1a-c). 

Figure 6.2a and b show the structure of foamed and non-foamed spray-dried maltodextrin powder at drying temperature 200 °C, respectively (Rabaeva, 2012). On analyzing Fig. 6.2b, particle structures can be observed with voids and inner bridges, which are similar to the foamed droplet structure shown in Fig. 6.1c. 

The foam spray drying process was developed for dairy industry at the start of the twentieth century. The spray drying of foamed materials was patented in 1917 (Campbell, 1917) to dry foamed milk and egg albumin (Rath and Kudra, 2006), but some 40 years later Sinnamon et al. (1957) found that the vacuum drying of concentrated milk foam would produce a whole-milk powder of good flavor and dispersibility. Morgan et al. (1959) showed that the dispersibility of whole-milk powder could also be maintained in foamed products during drying at atmospheric pressure. 

Although, since that time, very few studies on foam spray drying have been reported, this technology has more recently attracted attention because of the possibilities of processing hard-to-dry food materials and to control the final product properties such as bulk density, porosity, solubility, and wettability. 

A systematic analysis of the foam spray drying process, including investigations of spray hydrodynamics (particle size distribution, particle velocity, centricity), heat and mass transfer between the phases, drying kinetics and the effects of feed foaming on final product properties, was carried out by Zbicinski and Rabaeva (2010) and Rabaeva (2012). 

Fig. 6.8 Retention ofn-propyl acetate during non-foamed and gas-desorption foam spray drying of 60% sucrose solution (200°C, 70 bar atomization pressure). Adapted from Frey and King (1986).

Figures 6.9 and 6.10 show changes in the moisture content of foamed and non-foamed feed as a function of the distance from the atomizer for two drying air temperatures (175 and 150 °C) for the drying of maltodextrin. The drying process proceeded faster for foamed feed (feed rate 9kgh , foaming gas (N2) rate 40 gh ) at both air temperatures. The foam spray drying process of maltodextrin was complete at about 0.4 m (1.2 m for non-foamed feed) from the nozzle at a...

Figures 6.12-6.15 show example electron microscopy images of foamed and non-foamed detergent and maltodextrin powders for different drying temperatures. In these images, a significant difference can be observed in the structure of foamed and non-foamed spray-dried products, with foaming leading to an increase in particle diameter and the formation of a porous shell. Under certain drying...

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