Drying (or Dehydration) Dehydration or drying is often used towards the end of CF processing to finish up with a shelf stable product. The moisture content of wet chips or mash is reduced by drying to about 10–14 % on a wet weight basis after drying. The wet chips or mash may be sun or solar dried on a small scale to deliver between 50 and 100 kg per day. Although they are cheaper alternatives, their capacity and throughput are smaller, making them only suitable for small-scale processing outfits. Smoke dryers are also used mainly at the household level to produce pupuru (Figure 10.1.3). This particular method of drying cassava mash makes the product unique among other cassava flours. The smoke drying method was originally used for drying fish. It was adopted by peasants and cottage processors for drying fermented cassava mash in the riverine areas of Nigeria and some West African countries due to non-conducive climatic situations that could support sun drying of food (Shittu et al., 2005). The wet fermented cassava mash is molded into a balls of 1–2 kg weight and laid on racks under which smoky heat is supplied over 2–7 days, depending on heating rate (Figure 10.1.3). Dried or incompletely dried balls are also retailed by the roadside. The drying rate is slow and often leads to spoilage via mold growth. Spoilage organisms associated with this product are aerobic spore-forming and non-sporing bacteria, as well as potentially toxigenic molds such as Aspergillus flavus and Penicillium sp. (Shittu et al., 2010b).
Figure 10.1.3 Typical smoke drying facility for dying fermented cassava mash in West Africa (source: Shittu et al., 2005).
The presence of these aflatoxigenic organisms in the retailed and stored pupuru balls signals some public health concern.
Kiln drying as a form of smoke drying technology was evaluated as an alternative method for making pupuru balls (Shittu et al., 2001). Kiln drying resulted in faster moisture removal from the balls. The traditionally dried pupuru gave flour with the highest setback retrogradation. Similar observation was later reported by Bindzi et al. (2014). Flour from kiln-dried balls was more acceptable than the traditional and oven dried samples. The study recommended that energy efficiency of these methods be established. Osundahunsi (2005) compared the traditional smoke drying method with oven and solar-cabinet drying of pupuru ball. The smoke dried product was more acceptable than others in terms of aroma, as similarly reported by Shittu et al. (2001). The better aroma could be due to impregnation of the ball with some volatiles from the burning wood. Further studies are required to unravel the volatile composition of the product.
Due to recent advances in cassava-processing scales, some medium- and large-scale processors have now adapted some higher capacity artificial dryers such as cabinet, rotary and flash (pneumatic) dryers (Figure 10.1.4). These dryers are still currently used for batch production of cassava flours in Nigeria. Depending on design, flash dryers are capable of delivering 1-60 metric tons of dry product per 8 h working day. To date, the maximum throughput for a Nigerian designed flash dryer is still less than 3 metric tons per day. There are, however, few imported flash dryers capable of delivering 60 tons dried products per day. Not less than 100 units of such facilities are currently operating in Nigeria, mainly for cassava products. Nigerian flash dryers ordinarily use spent automobile (black) oil or diesel as fuel. The efficiency of a flash drying facility depends on variables such as number of cyclones, fuel burner efficiency, and feed moisture content among others.
Figure 10.1.4 Flash dryer (source: Sanniet al., 2006).
Drying of cassava is a critical operation that affects the quality of the final product. Starch granular properties and by implication of the starch-based functional properties, cassava flour may be modified by the drying process (Maziya-Dixon et al., 2005; Shittu et al., 2001). However, this depends on the drying conditions such as drying temperature, drying time, drying method and so on. Higher temperatures and longer drying times lead to increased starch granular modification and change in starch-based functional properties.
Dry Milling Milling is an energy-intensive operation which leads to production of particulates size of less than 400 pm, referred as flour in many instances. Regardless of processing scales, it is carried out mechanically with the use of milling machines. The most popular types of milling machines used are attrition and hammer mills (Nwaigwe et al., 2012). A study of cassava processing machineries used in the Oyo State of Nigeria indicated that milling machines are the third-most popular after graters and pressing machines. Dry milling operations in the processing factories surveyed are manned mainly by males (Davies et al., 2008).
Few studies have been conducted on dry milling of cassava. Nwaigwe et al. (2012) designed a mechanical mill for converting cassava chips into flour due to some ineffectiveness of existing machines to produce acceptable flour grade for the bakeries. The mill was based upon both an impact and shearing milling action, with a pneumatic conveying and classifying action. The modified milling gave an efficiency of 82.3 % with fineness modulus of 0.31 and average particle size of 0.075 mm compared to 2.35 and 0.085 mm of an existing hammer mill, respectively.
The influence of moisture content on the dry milling characteristics of dried cassava chips was studied by Shittu et al. (2002). The amount of energy used in dry milling depends on the initial particle size of feed and moisture content. Increased feed size and moisture leads to higher milling energy consumption to produces flour of specific particle size.
The effect of milling was studied by Adesina and Bolaji (2013). Although the authors did not describe moisture content and size of the cassava chips milled, they reported that milling affected the flour yield and mill recovery. Moreover, the pin milling method gave complete flour yield and recovery. Similarly, differences in the dry milling procedure led to different compositional values. Defloor and Delcour (1993) reported that insignificant differences were found in the thermal and pasting properties of CF obtained from hammer, ball and roller milling of cassava chips at two moisture content levels (11.5 and 15.9 %). However, milling cassava chips at
11.5 % gave lower yield of break roll flour but higher yields of reduction roll flour than was obtained at 15.9 % moisture.
10.1.2.2 Cassava Flour Properties
The various types of cassava flour-based products are listed in Table 10.1.2. Like any other food product, CF properties can be divided into physical, chemical, functional and microbial properties. Certain properties (also known as standard quality indices) are used to provide a guide in regulating local and international trades, only while others have been used to determine potential end uses of CF.