Innovations in spray drying process for food and pharma industries
Introduction
The first mention on SD can be found in US Patent published in 1872. Samuel Percy described the operation of SD as “an exposure of atoms to the currents of air or other gasses”. The author investigated a drying of the solutions of dextrin, starch and gelatin and claimed that this method made it possible to obtain powders of reduced chemical changes and moisture content (Percy, 1872). The industrial application of this method was first observed in 1920s, however the demands of dairy industry after the World War II caused by the excess milk production, have provoked the advancement of the technology of spray dryers (Bhandari, 2008). Nowadays, SD is applied on a large scale in various industries, such as production of food powders, nutraceuticals, aromas, probiotics, enzymes, antibiotics, blood plasma, organic and inorganic chemicals, ceramic powders, detergents and fertilizers (Celik and Wendel, 2005; Filková and Mujumdar, 1995). The primary causes of the popularity of this technology are the simplicity and effectiveness. SD enables a continuous production of free flowing powder of spherical and uniform particles, agglomerates or granules from liquid feed (Patel et al., 2009). SD is recognized as a technique that enables to produce powders of controlled physiochemical properties when process is properly manipulated. Furthermore, this technology is recommended for materials of high heat sensitivity, as atomized droplets’ exposure to hot drying medium is very short (milliseconds or seconds) due to a high surface to volume ratio, and additionally the cooling effect of evaporation is observed (Bellinghausen, 2019; Bhandari et al., 2008; Celik and Wendel, 2005; Filková and Mujumdar, 1995; Masters, 1985). The spray dried product is highly stable, because of its low moisture content and water activity (Shishir and Chen, 2017).
Despite the undoubted advantages of this method, SD also has some limitations. Widely recognized and used spray dryer configurations cannot cope with the difficulties encountered. For example, in drying particularly thermolabile materials or materials with a high sugar content, although many dryers are equipped with an intermittent sweeping facility and hammers to remove the sticky or deposited particles from the wall surfaces (Bhandari, 2008). SD can also jeopardize interface-sensitive materials such as protein. Other problems are high energy demand, low energy efficiency (Baker and McKenzie, 2005; Cheng et al., 2018), low powder recovery of traditional spray dryers, as well as often inappropriate properties of the obtained powders (Cheng et al., 2018). In response to these limitations, research on process modifications at various stages is ongoing in research centers around the world. The results of these studies, adjusting the process to changing requirements regarding its course and product properties, as well as the characteristics of the traditional SD process, are presented in the following parts of the article. The applications of the process in the food industry were also described, including the microencapsulation process of health-promoting substances, which also have additional requirements related to the course of the process and the properties of the products obtained.
Thus, the aim of this review paper is to present: shortly the background and principle of SD process; industrially implemented solutions in SD being the modifications of basic one-stage system; in the main part if the paper - the innovations in SD process, which are currently being the subject of research in the laboratories worldwide; small scale systems for screening purposes; the simulation of SD with computational fluid dynamics. Although some review papers about spray drying were published during last 3–4 years, they were not general reviews but were rather devoted for the particular groups of products, like fruit and vegetable juices (Shishir and Chen, 2017), probiotics and other food-grade bacteria (Huang et al., 2017), encapsulated oils (Mohammed et al., 2020), high-protein dairy powders (Hazlett et al., 2021) or stages of the process, like atomization technologies in the dairy industry (O'Sullivan et al., 2019).
Section snippets
Principle, stages, problems and challenges
SD process can be used for just simple pulverization of feed solutions, or, in a more complex approach, for encapsulation of several bioactive compounds (Mahdavi et al., 2014). The diagram on conventional one-stage SD system is presented in Fig. 1. The description of the basic steps of SD in the following paragraphs includes the drawbacks and challenges connected with these basic steps of SD.
Examples of current industrial spray drying systems
SD is the common drying method applied in the food and pharma industry. The capacity of industrial SD systems can vary from several dozen kg to 40 tons of evaporated water per hour, it is chosen based on process and production requirements. Grand View Research report (Grand View Research, 2019) presented that the global SD equipment market size was valued at USD 4.63 billion in 2018, while Data Bridge Market Research (Data Bridge Market Research, 2020) showed that SD equipment market is
Innovations in spray drying technology
The classic SD system possess some limitations and drawbacks (Bellinghausen, 2019). Some of them were already overcome, but some still needs attention and research, i.e. problems with highly thermosensitive materials, problems with stickiness and sugar-rich materials, problems with not proper particle size and size distribution (De Melo Ramos et al., 2019; Camino-Sánchez et al., 2020) Nevertheless, the decrease of energy consumption is an important issue (Ladha-Sabur et al., 2019; Acar et al.,
Small scale SD systems for screening purposes
Due to lacking in-dept understanding of SD process, performance assessments during pilot/commercial scale becomes a major hassle from the earliest stages of active compound development (Masters, 2007). Inappropriate carrier selection, bioactive loading, and process conditions/methods can risk phase-separation and/or active agent-recrystallization after and during processing; which may then lead to a reduction in Amorphous Solid Dispersion (ASD) performance (Poozesh and Jafari, 2019). To resolve
Simulation of SD with computational fluid dynamics
Empirical models cannot be used to describe the SD process due to the unsteady three-dimensional nature of flow in such types of facilities. Computational fluid dynamics (CFD) is a useful tool for process designing and improvement and identification of the details. In the SD, the required tools are ready to be used in all of the stages of the process to predict the hydrodynamics and also heat and mass transfer, temperature distribution, particle morphology, and drying air characteristics as
Engineering issues of SD
One of the most critical issues in CFD modeling of the spray dryers is the simulation of the turbulent airflow within the chamber using appropriate turbulence models. Other important sub-models also might be incorporated in the Lagrangian approach to capture all of the phenomena that influence the droplet/particle behavior and consequently the performance of the dryer and prediction of particle characteristics. Such sub-models transform a general CFD simulation into specified spray drying CFD
Conclusions
The literature review on the latest achievements in the field of spray drying shows that since the early development the spray drying system has evolved. This evolution is multidirectional and aims to overcome particular drawbacks of conventional spray drying system. So far, there is no universal approach, and it has to be selected based on process and final product requirements. To reduce thermal degradation, vacuum, dehumidified air, ultrasound or nano spray drying can be applied. Significant
Declaration of competing interest
None.
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