Flowability of particulate solids of the powder was measured using the angle of repose (Kanha and Laokuldilok 2014). It is used as a predictor of possible flow difficulties that might arise in industrial applications and also used as a rough flowability indicator. The angle of repose is defined as the angle of the free surface of a heap of powder formed to the horizontal pane. In addition, if material is non-cohesive, the powder can flow smoothly as a low heap is formed. According to Begum et al. (2017), the dried powder’s ability to flow well and smoothly depends on various factors for instance the powder shape and size distribution, moisture content, electrostatic effects and particle size being the main factor. The smaller the particle size of the powder, the decrease in the powder flowability rate (Jinapong et al. 2008). Table 4.8 illustrates the flowability of drum dried and vacuum oven dried ambarella, Bintangor orange and Sarawak pineapple powder.
Based on Table 4.8, the vacuum oven dried powder of ambarella, Bintangor orange and Sarawak pineapple had the angle of repose of 43.01°, 42.09° and 43.91° were higher than drum dried powder of ambarella, Bintangor orange and Sarawak pineapple which had the angle repose of 37.81°, 33.27° and 37.57° respectively. Moreira et al. (2009) stated that free-flowing powder as powder having an angle repose below 45°. Therefore, all the fruit powders above have free-flowing properties which categorizes them as good flowability powder. To date, there is limited data on the flowability of drum dried and vacuum oven dried ambarella, Bintangor orange and Sarawak pineapple powder. The decrease in processing temperature decreases the powder flowability. This seems to be attributed to their higher moisture contents, since higher moisture content decreases the rate of powder flowability. This supports the results above, where the angle of repose in vacuum oven dried powder is higher than drum dried powder (Fitzpatrick 2005). The flowability of the powder can be improved by increasing the particle size of the powder, reducing the moisture of the powder and selecting the type of drying agent used (Moreira et al. 2009). For example, the drying agent of maltodextrin was replaced by cashew tree gum, which had an increase in the flowability of the powder.
4.2.8 Water Solubility Index (WSI)
Solubility is defined as reconstitution property which is used to determine the effect of process parameters in downstream processing because the physical and chemical functionality of food powder are only functional when it completely dissolves in a solution (Santhalakshmy et al. 2015). The water solubility index measures the ability of the powder to dissolve in water and express in terms of percentage by the difference between the volume of the reconstituted liquid and the volume of sediment (Schuck et al. 2012). The table 4.9 shows the water solubility index (WSI) of drum dried and vacuum oven dried ambarella, Bintangor orange and Sarawak pineapple powder.
Table 4.9 Water solubility index (WSI) of drum dried and vacuum oven dried of ambarella, Bintangor orange and Sarawak pineapple powder
Drum drying (%) Vacuum oven drying (%)
Ambarella 22.79±0.10a 19.51±0.11b
Bintangor orange 16.50±0.64b 23.25±0.16a
Sarawak pineapple 16.33±0.86b 23.64±0.24a
Data expressed in mean ± standard deviation based on triplicate readings (n=3). a-b Values in each row with different superscripts are significantly different at (p 0.05).
For ambarella, both drum dried and vacuum oven dried powder of the b* values had no significant difference at (p; 0.05) however, the vacuum oven dried Bintangor orange and Sarawak pineapple powder with values of 37.87 and 11.55 tend to appear more yellowish compared to the drum dried Bintangor orange and Sarawak pineapple powder with values of 28.90 and -0.43 may be due to the longer drying time (Zielinska and Markowski 2012). Generally, the lightness in the Bintangor orange and Sarawak pineapple powder indicates the colour is affected by the addition of drying aid such as maltodextrin while the processing temperature of drying used affect both the a* and b* colour parameters (Fegus et al. 2015). An increase in the drying temperature during processing, decreases the value of redness and yellowness in the powder (Zielinska and Markowski 2012).