Based on Table 4.4, the vacuum oven dried powder of ambarella, Bintangor orange and Sarawak pineapple had water activity of 0.376, 0.489 and 0.445 were higher compared to drum dried powder of ambarella, Bintangor orange and Sarawak pineapple which had water activity of 0.307, 0.343 and 0.377 respectively. The water activity of vacuum oven dried powder were similar to the values acquired in study of vacuum oven dried powder from Jaya and Das (2009) in which mango powder had water activity of 0.44 and pineapple powder had water activity of 0.41. This study supports evidence from previous observations that drum dried powders had similar values of water activity to the drum dried tamarind powder, which is 0.326 (Jittanit et al. 2011).
Food spoilage caused by microbial activity is inhibited below water activity 0.6, whereas bacteria are inhibited at less than 0.9 and fungi are inhibited at less than 0.7 (Troller and Christian 2012). However, dried foods with water activity between 0.2 and 0.5 ensure a stable product without signs of any physical and chemical changes (Dirim and Caliskan 2012). This indicates both drum dried and vacuum oven dried powder are stable products. The differences in water activity of the drum dried and vacuum oven dried powder maybe due to the two separate drying techniques, which can be credited to the drying temperature utilized. A higher drying temperature used in drum drying resulted in lower water activity contrasted with vacuum drying (Caparino et al. 2012).
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4.2.4 Hygroscopicity
Hygroscopicity is defined as the capacity of the food powder to uptake moisture from the environment with relative humidity superior to that of equilibrium (Ribeiro et al. 2016). The measure of hygroscopicity in powdered ingredients is an important parameter because the propensity for agglomeration is an impact attributed to the absorption of water on the surface of the particles. This makes the powder particles sticky and increases its tendency to form hydrogen bridges. Thus, this causes caking problems that inhibits the flowability of the powder (Goula and Adamopoulos 2008). Table 4.5 illustrates the hygroscopicity of drum dried and vacuum oven dried ambarella, Bintangor orange and Sarawak pineapple powder after a week of storage.
As shown in Table 4.5, the hygroscopicity of drum dried powder of ambarella, Bintangor orange and Sarawak pineapple were higher 0.11%, 0.04% and 0.21% compared to vacuum oven dried powder of ambarella, Bintangor orange and Sarawak pineapple respectively. According to Costa et al. (2014) found that the hygroscopicity of powder can be grouped into five stages in which powders less than 10% (non-hygroscopic), 10.1%-15% (slightly hygroscopic), 15.1%-20% (hygroscopic), 20.1%-25% (very hygroscopic) and more than 25% as extremely hygroscopic. Therefore, these results reflect those of Costa et al. (2014) studies in which the fruit powder were considered to be non-hygroscopic powder with percentage less than 10%. However, the reason of drum dried powder having a higher percentage of hygroscopicity is because of their drying conditions, which have high processing temperature and lower moisture content in the product making it capable of absorbing moisture from the surrounding (Tonon et al. 2008).
The variation in hygroscopicity percentages acquired from two different drying methods can be attributed to the different hygroscopic constituents such as citric acid and sugar, which have different glass transition temperatures found in different fruits (Ergun et al. 2010). During the drying operation, water which act as a plasticizer was removed in order for the liquid food to transform to a glassy state. If the drying temperature is higher than the glass transition temperature of the constituents, hence the food will not be converted to the glassy state. As a result, the end product will stay in a high-energy sticky state. According to Muzaffar et al. (2015) claims that sticky products tend to be more hygroscopic. Therefore, to ensure the end product remain in a low energy non-sticky state, the drying temperature should be lower than the glass transition temperature of constituents (Nurhadi et al. 2012).
In addition, difference in hygroscopicity percentage can also be caused by their size. This is because finer particles have bigger contact surface thus resulting in a higher number of active sites. The number of active site coming into contact with air will rise in the hygroscopicity percentage due to increase in water absorption (Ribeiro et al. 2016). However, the use of drying aids such as maltodextrin helps resolve hygroscopicity problems. An increase in the percentage of maltodextrin added to the powder decreases the hygroscopic values of the powder. This occurred because maltodextrin has low hygroscopicity, which can affect the existing affinity between water and other compounds in the product (Costa et al. 2014).
4.2.5 Degree of Caking
According to Mathlouthi and Roge (2003), caking is characterized as the lumping of food powder into solid and sticky material. This is caused by the unconstrained agglomeration phenomenon caused by liquid bridges. As the powder cakes it tends to reduce in functionality and fluidity (Zafar et al. 2017). Powder containing fruit sugars and fruit acids such as fructose, glucose, sucrose, citric acid and malic acid makes the powder difficult to be dried leading to the lower glass transition temperature. However, this problem can be overcome by the addition of drying aid such as maltodextrin which is added to raise the glass transition of the mixture. The powder still has a tendency to cake even after the addition of maltodextrin (Paterson and Brockel 2015). Table 4.6 shows the degree of caking of drum dried and vacuum oven dried ambarella, Bintangor orange and Sarawak pineapple powder.
Based on Table 4.6, the degree of caking of drum dried powder of ambarella, Bintangor orange and Sarawak pineapple were higher 3.94%, 1.95% and 3.55% than vacuum oven dried powder of ambarella, Bintangor orange and Sarawak pineapple respectively. To date, there are no further studies conducted on the degree of caking of drum dried and vacuum oven dried powders. However, the effect of caking occurs on surfaces of amorphous products caused by surface plasticization induced by water sorption. In addition, caking also occurs in water soluble powder that are exposed to high relative humidity environments (Domingo et al. 2017). Commonly, hygroscopicity of the powder leads to caking of the powder.
The study of Costa et al. (2014) found that powders with the degree of caking which are more than 50% are classified as extremely caking powder, 20% to 50% are classified as caking powder, 10% to 20% are classified as slightly caking powder and below 10% are classified as non-caking powder. These results are in agreement with Costa et al. (2014) findings in which vacuum oven dried ambarella, Sarawak pineapple and drum dried Sarawak pineapple are classified as non-caking powder whereas the drum dried ambarella, Bintangor orange and vacuum oven dried Bintangor orange are classified as slightly caking powder. Anti-caking agents are used in food powder to decrease the caking of powder and ensure stability of food products such as tricalcium phosphate and silicon dioxide (Lipasek et al. 2011).
4.2.6 Wettability
Wettability of a powder is described as the ability of powder particles being penetrated by a liquid without agitation because of the capillary forces, and becomes completely wet (Hogekamp and Schubert 2003). A substance known as scum will form if the powder is not able to wet sufficiently. This layer of scum will continuously stick together and form an obvious layer on the walls of the container (Fang et al. 2007). Wettability an important factor to determine the reconstitution properties of powder. In an analysis of wettability, Schuck et al. (2012), found that powder which are high in carbohydrates (MD DE 12, sorbitol, and maltitol), have wettability index that is less than 10 seconds while powder which are high in protein (caseinates and micellar proteins) and/or fats (whey 40% fat, whole egg, egg yolk, milk 26% fat), have wettability index that is more than 120 seconds and categorised as non-wettable powder. Table 4.7 describes the wettability properties of drum dried and vacuum oven dried ambarella, Bintangor orange and Sarawak pineapple powder.
As shown in Table 4.7, the wettability of drum dried powder of ambarella, Bintangor orange and Sarawak pineapple took a longer time of 15.01s, 16.1s and 15.78s compared to vacuum oven dried powder of ambarella, Bintangor orange and Sarawak pineapple respectively. One of the reason for drum dried powder took a longer time to completely wet compared to vacuum oven dried powder may be due to the decrease in powder residual moisture content. The lower moisture content in the drum dried powder resulted in a longer time taken to wet the powder completely compared to vacuum oven dried powder (Phisut 2012). The types of carrier used, such as maltodextrin also had a significant role in determining wettability. When the De value of maltodextrin is higher, the solubility of maltodextrin in water increases (Nurhadi et al. 2012).
In addition, according to Schuck et al. (2012), standards given in a diary sector indicating a powder with wettability index below 60 seconds is considered wettable and wettability index below 30 seconds is extremely wettable. Therefore, these results reflect those of Schuck et al. (2012) studies that drum dried and vacuum oven dried powder in this study excel good instant properties accept for drum dried ambarella powder. The drum dried ambarella powder have a wettability index more than 60 seconds (64.08 seconds) because the ambarella fruit had the highest fat content of 0.62 g than the other two fruits Tiburski et al. (2011). The chemical composition of higher fat content leads to higher presence of fat (free fat) on the surface of powder. This finding limits the wettability of the powder because of the hydrophobic nature of the free fats (Fang et al. 2007).
