A review of the applications of different drying methods of banana flour preparation

The perishable nature of bananas, which is highly susceptible to physical, mechanical, and physiological damage, could result in significant postharvest loss and waste. Drying is extensively used to preserve the storage life of agricultural products, thereby allowing their whole - year availability. It also assists in the development of value - added products, such as banana flour. However, despite its ability to preserve fresh materials, drying can also alter the chemical, nutritional, and functional properties of the resultant products. Hence, the selection of the most suitable method and operating parameters is critical to minimize alterations in product quality during drying. This article aimed to review and compare various drying methods applied in previous research on banana flour preparation. In general, sun - , solar - , and oven - dried banana flour often present low quality, whereas freeze - dried banana flour presents good quality. However, the results vary according to the operating parameters employed in each study.


Introduction
Banana (Musa spp.) is the second most widely grown cultivar worldwide and can be found in approximately 130 tropical countries (Falade and Oyeyinka, 2014;Fadimu et al., 2018), indeed, it is categorized as one of the most important global crops after rice, wheat, and maize (Khoozani et al., 2019).Unfortunately, the perishable nature of this climacteric fruit (Fikadu et al., 2016;Olawuni et al., 2018) could result in substantial postharvest waste and loss (Prabha and Kumar, 2013;Sunitha et al., 2017).
The effectiveness of drying treatment to prolong the stability of fruits during storage has long been recognized (Babu et al., 2018;Calín-Sánchez et al., 2020).Besides preservation, drying could also promote the development of value-added products, such as flour from dried bananas, with similar or improved quality (Berk, 2009).Banana flour is a major product that could be obtained from the drying treatment of bananas (Hasmadi et al., 2021).
Drying or dehydration refers to the process of moisture removal from food materials, thereby producing dried products with reduced water activity (Calín-Sánchez et al., 2020).According to Rahman (2007), the drying process can be grouped according to the moisture removal approach into thermal drying, osmotic dehydration, and chemical dewatering.Thermal drying, the most widely used drying method currently available, involves heat and mass transfer (Maisnam et al., 2017).During the drying process, heat transfer may occur through conduction, convection, and radiation (Sangamithra et al., 2014).Mass transfer, which refers to the removal of moisture, could reduce the availability of water for chemical, microbial, and enzymatic reactions.Inhibition of these undesirable reactions could reduce the risk of product deterioration (Ahmed et al., 2013;Guiné, 2018).Drying also improves the efficiency of product packaging, storage, and transportation (Sagar and Kumar, 2010;Olawuni et al., 2018).In this article, different drying methods applied in previous studies on banana flour are reviewed; the effects of each method on the chemical, physical, and nutritional properties of banana flour are also discussed such as the colour, bioactive compounds, and starch content.

Process of banana flour preparation
In general, cleaning, pre-treatment, drying, and size reduction are the main stages involved in banana flour preparation.Banana fruits are rinsed to remove dirt and soil by using tap water or sodium hypochlorite solution.This step is followed by skin peeling.Sodium hypochlorite is a well-known and widely used sanitizing agent for fruits and vegetables that could help control and reduce microbial growth (Sun et al., 2012;Rayo et al., 2015).
Pre-treatment is an important step that aims to prevent the enzymatic browning reaction by inactivating polyphenol oxidase (Razali et al., 2020).Two types of retreatments, namely, physical and chemical pretreatment, have been proposed.Physical pre-treatment is commonly carried out prior to slicing, the banana is blanched or steamed at approximately 80°C (Olawuni et al., 2018).Chemical pre-treatment is carried out by soaking the banana in a chemical solution after slicing.Some pre-treatment solutions include potassium metabisulfite and citric acid (Ng et al., 2014;Rayo et al., 2015).
The banana is immediately subjected to drying after pre-treatment and then size-reduced, which involves grinding and sieving to produce the fine powder of banana flour.The development of a number of drying methods that could directly produce powder as the final product (Rahman, 2007;Domínguez, 2011, Guiné, 2018;Calín-Sánchez et al., 2020) may avoid the need for grinding.Table 1 summarizes the operational parameters involved in the previous research on banana flour preparation.

Natural drying methods
Natural drying refers to a drying process that uses natural resources (e.g., the sun) as a heat source (Maisnam et al., 2017).Natural drying is economical, but the uncontrollable process resulting from the inability to control temperature and drying duration due to weather uncertainties is often caused by inconsistent quality of the dried product (Altay et al., 2019).Therefore, the uncontrollable operating parameter of natural drying has led to the development of artificial methods to overcome its weaknesses.

Sun drying
Sun drying is a traditional dehydration technique that has been used since ancient times to dry various agricultural products (Fuller, 2000).In fact, this method is still used at present because it is fairly simple, inexpensive, and highly accessible.However, this method involves long drying durations that could last for 1-3 days depending on the weather conditions (Altay et al., 2019).In general, this method is carried out by placing the food material on trays and then laying the trays out under direct sunlight (Rahman, 2007).If drying requires more than 1 day, the sample is covered or kept indoors during nighttime (Ahmed et al., 2013).
The low performance of sun drying has been reported in many research studies and attributed to various uncontrollable operating parameters, such as temperature and humidity (Altay et al., 2019), which result in inconsistent product quality (Sagar and Kumar, 2010).Product safety may also be compromised by the risk of environmental contamination resulting from exposure to dirt, dust, and pests (Fuller, 2000;Rahman, 2007).Youssef et al. (2018) revealed that banana flour prepared from sun drying has lower total phenolic, flavonoid, and tannin contents compared with oven-, vacuum-, and microwave-dried samples.Vu et al. (2017) demonstrated that sun drying results in the greatest loss of bioactive compounds.Meanwhile, Fadimu et al. (2018) reported that the lightness of banana flour prepared from sun drying is significantly lower than that of flour prepared from freeze-drying.

Solar drying
Solar drying is an improvised version of sun drying, which also utilizes the sun as a heat source (Guiné, 2018).This method is designed with equipment to exploit the heat energy from solar radiation and use it to dry samples (Akarslan, 2012).Thus, this method often employs higher temperatures and shorter durations than sun drying; ultimately, solar drying produces better final quality than sun drying (Ahmed et al., 2013).However, research by Fadimu et al. (2018) showed that solar-dried banana flour is darker in colour but has higher carbohydrate contents compared with sun-dried samples.

Artificial drying methods
Artificial drying methods employ an artificial heat source for heat and mass transfer during thermal drying.It is generally assisted by a set of electrical equipment that allows the control of the operating parameters during drying (Maisnam et al., 2017), which results in greater efficiency of moisture removal (Okoro and Madueme, 2004).The continuous advancement in drying technology has allowed the production of a dried product with desirable quality, stability and functionality (Mali and Butale, 2019).

Oven drying
Oven drying is a common and highly accessible drying method in which samples are placed in enclosed chambers and hot air is utilized as the drying medium (Rahman, 2007).This method generally involves heat, air circulation, and relative humidity (Ahmed et al., 2013).The circulation of hot air transfers heats energy to the food material, resulting in the evaporation of moisture from the surface of the material being dried  (Mazandarani et al., 2014).According to John et al. (2014), oven drying is often used in small-scale product development and laboratory operations.However, while this method is relatively inexpensive to operate (Ahmed et al., 2020), the material to be dried requires prolonged exposure to high temperatures (Ebadi et al., 2011).Thus, the risk of deterioration in the colour, flavour, texture, and nutrient content of the final product is quite high (Kumar et al., 2019).Fadimu et al. (2018) indicated that oven-dried banana flour is darker in colour and lower in protein content compared with other samples.Research by Vu et al. (2017) also demonstrated the dark brown colour of oven-dried banana flour.On the other hand, Ahmed et al. (2020) reported that oven-dried banana flour has significantly lower resistant starch contents compared with freeze-dried samples, as evidenced by the finding that the percentage of damaged starch in oven-dried samples is significantly higher than that in freeze-dried samples.

Vacuum oven drying
Vacuum oven drying is a dehydration method that takes place under vacuum conditions of reduced pressure and relatively low temperature (Rahman, 2007).This method is well-known for energy conservation because dehydration under a reduced-pressure environment does not require high heat energy (Parikh, 2015).The method is highly suitable for drying heat-sensitive food materials because it avoids exposure to high temperatures (Hubbard et al., 2021).Moreover, the absence of oxygen during the drying process prevents oxidative damage, resulting in better maintenance of colour, flavour, and texture compared with hot-air drying (Mohapatra et al., 2014).Youssef et al. (2018) found that vacuum oven-dried banana flour demonstrates higher total phenolic, flavonoid, and tannin contents compared with oven-and sun-dried samples.Moreover, the total monomeric anthocyanin content of vacuum oven-dried samples is higher than that of freeze-dried samples.Vu et al. (2017) similarly observed higher levels of total phenolic contents in vacuum oven-dried banana peel flour compared with flour samples obtained through other methods.However, the flavonoid and proanthocyanidin contents of the former were lower than those of freezedried samples.

Freeze-drying
Freeze-drying, also known as lyophilization, is a dehydration process that involves three stages, namely, freezing, sublimation, and desorption (Antal et al., 2015).In this method, the food material is frozen, and the system pressure is reduced to sublimate the frozen water within the food material directly into the gas phase (Ahmed et al., 2013).According to Sagar and Kumar (2010), freeze-drying is one of the best dehydration approaches currently available because this method is highly capable of maintaining volatile compounds, minimizing the chemical and physical changes of the product (Calín-Sánchez et al., 2020), and avoiding the loss of flavour and aroma (Altay et al., 2019).In addition, the absence of liquid-phase water and utilization of low temperatures during operation leads to products with better quality by preventing food deterioration and microbial activity (Lin et al., 2007;Gaidhani et al., 2015).Vu et al. (2017) demonstrated that freeze-dried banana peel flour maintains its bright yellow colour, whereas samples dried using other methods turn dark brown.In addition, the freeze-dried sample retained high levels of its bioactive compounds.Pacheco-Delahaye et al. (2008) showed that banana flour prepared by freezedrying has a lighter colour than flour obtained by drumand microwave-drying.Ahmed et al. (2020) showed that the browning index of freeze-dried banana flour is significantly lower than that of oven-dried samples.
Despite the superior performance of the freezedrying method, however, this method involves a slow drying process of typically 20-30 hrs (Antal et al., 2015;Calín-Sánchez et al., 2020).The long operating time involves high energy consumption, resulting in an expensive cost of operation (Domínguez, 2011;Altay et al., 2019).Hence, freeze-drying is not a common commercial method, instead, it is usually applied in the preparation of high-value products (e.g., astronaut food) (Rahman, 2006;Gaidhani et al., 2015).

Microwave drying
The principle of microwave drying involves the conversion of electrical field energy into thermal energy, during which heat is transferred through molecular interactions (Parikh, 2015).During microwave drying, electromagnetic waves cause molecular oscillations within the food material, leading to heat generation and moisture removal (Calín-Sánchez et al., 2020).According to Rahman (2007), microwave drying offers higher energy efficiency compared with hot-air drying.However, because of the absence of convection, this method is usually combined with other drying methods, such as microwave-vacuum drying (Mohapatra et al., 2014) and microwave-osmotic dehydration (Sagar and Kumar, 2010), to increase the efficiency of drying.Pacheco-Delahaye et al. (2008) showed that microwave-oven-dried banana flour has a darker colour compared with freeze-and drum-dried banana flour.Youssef et al. (2018) reported that microwave-ovendried banana flour contains higher phenolic and flavonoid contents compared with vacuum-, oven-, and sun-dried samples.Vu et al. (2017) similarly indicated that microwave-dried banana peel flour has higher phenolic contents compared to freeze-, vacuum-, oven-, and sun-dried samples.

Spouted bed drying
Spouted bed drying is the improved version of fluidized bed drying, which is used to dry granular raw materials with coarse particle sizes that are too large for fluidization (Mujumdar, 2000).Spouted bed dryers allow better heat and mass transfer as a result of the agitation of the coarse particles of the raw material.Particle agitation results in the continuous renewal of the boundary layer on particle surfaces, which increases the contact area between the raw material and the heating medium during dehydration (Kahyaoglu et al., 2012).Figure 1 briefly illustrates the general mechanism of this drying method; here, the raw material is rapidly dispersed in the heated gas within the chamber (Rahman, 2007).
Research conducted by Bezerra et al. (2013) found that spouted bed drying can retain the resistant starch content of banana flour, although drying was conducted at the temperature of starch gelatinization (80°C).The resistant starch content of raw bananas prior to drying was analyzed and compared with that of the flour obtained.Results showed no significant difference in the resistant starch contents of the samples.

Spray drying
Spray drying is an industrial-scale drying method that is widely used in the manufacture of dry powdered products originating from liquid raw materials (Guiné, 2018;Calín-Sánchez et al., 2020).Spray drying involves the conversion of slurry raw materials into the powdered form of the end product and includes four stages, namely, atomization, contact between water droplets and hot air, water evaporation, and separation of the gas and powder (Rahman, 2007;Sagar and Kumar, 2010).The working mechanism of this method is briefly illustrated in Figure 2. Banana puree is used as a raw material instead of a typical sliced banana to produce banana flour by spray drying because this method requires a liquid feed (Maisnam et al., 2017).Izidoro et al. (2011) investigated the properties of spray-dried banana starch and compared them with those of oven-dried samples.The authors' findings showed that spray-dried samples have insignificantly higher moisture and protein contents compared with oven-dried samples.By contrast, the fat and resistant starch contents of the spray-dried sample were insignificantly lower than those of the oven-dried sample.In terms of the physicochemical properties of banana starch, the solubility, swelling power, and water absorption capacity of the spray-dried sample were significantly higher than those of the oven-dried sample at 80°C.When drying was performed at 50, 60, and 70°C, these properties did not significantly differ between the spray-dried and oven -dried samples.

Drum drying
The drum drying method is commonly applied in the manufacture of dry powder originating from viscous fluid, slurry, or paste; here, the drying process takes place via the conductive heating of rotating drums (Rahman, 2007;Domínguez, 2011).As illustrated in Figure 3, the wet material is fed into continuously rotating drums to form thin layers adhered to the drum walls.The rotating drums are internally heated by condensing steam, which is responsible for the dehydration of the adhered layer of wet materials (Berk, 2009).Besides dehydration, the heat from the drum dryer also allows simultaneous cooking, which produces pregelatinized starch (Indiarto et al., 2021) as a result of  (Maisnam et al., 2017) irreversible thermochemical changes.Therefore, drum dryers are widely applied in the processing of starch powder used in the development of instant food products (Wan Daud, 2006).
This method has been applied to prepare banana flour by Pacheco-Delahaye et al. (2018).The authors' findings showed that the moisture content of drum-dried samples is significantly higher than that of freeze-dried samples and not much lower than that of microwavedried samples.In terms of starch content, the sample had significantly lower starch contents compared with the freeze-dried sample.However, the starch contents of the drum-and the microwave-dried sample did not differ significantly.

Conclusion
Previous research showed the effects of different drying methods on the quality of the resultant dried products.The final quality is often influenced by the operating parameters and the working mechanism of the method.The drawbacks observed in conventional methods have led to continuous innovations in drying technology and the development of complementary advanced methods.Banana flour should be prepared from a drying method that could preserve the starch content of green bananas while effectively producing flour of optimum quality.The selection of the appropriate processing method is critical in the food industry.Some factors that must be considered include the type of food material to be dried, the desired final quality to be achieved, and the operating cost and duration, among others.More importantly, the original properties of the product must be maintained.