9.1.3 Processing Constraints
The shape, internal colour and changes in colour are the problems during processing of taro. Handling and processing of the taro causes itching and considerable inflammation to the tissues of consumers, which is due to the presence of acrid factors. In its raw form, the plant is toxic (Lewu et al., 2010). Taro corms and leaves should be processed and boiled before eating, as these are bitter and can cause harm to health when consumed raw. Presumably, itchiness arises when the calcium oxalate crystals are released and inflict minute punctures to the skin when in contact with it. The crystals have to interact with a certain chemical on the raphide surface before acridity is experienced (Bradbury and Holloway, 1988). The roots and leaves can impart an acrid/sharp taste due to the presence of the calcium oxalate crystals. However, the level of acridity in edible tubers is too low for them to pose any serious problems, unless very large quantities of the tuber are consumed (Ikpeme-Emmanuel et al., 2007).
Plant toxins and anti-nutrients can interfere with the bioavailability of some nutrients, or cause harm to the biological system (Whitney and Rolfes, 1999). Some of these naturally occurring toxicants are cyanogenic glycoside, phytate, oxalate, lectin, saponins, alkaloids, etc. The anti-nutritional factors in taro include oxalates, phytates and tannins (Abdulrashid and Agwunobi, 2009). The acridity and toxicity is mainly reflected from the presence of oxalates in taro, therefore the oxalate content can be adopted as an important parameter in the assessment of the nutritional status of locally grown taro in different regions. The levels of oxalate in taro leaves and corms are 278–574 mg/100 g and 65 mg/100 g (fresh weight), respectively (Holloway et al., 1989) (Table 9.1.2). However, according to Huang and Tanudjaja (1992), taro corms contain 43-156 mg oxalate/100 g fresh weight. The various anti-nutritional factors present in taro are listed in Table 9.1.2.
Table 9.1.2 Anti-nutritional factors in taro
Taro | Antinutritional factors and their levels | Reference
Taro leaves and corms | Oxalate (taro leaf): 278–574 mg/100 g; Oxalate (taro corm): 65 mg/100 g | Holloway et al. (1989)
Xanthosoma sagittifolium | Oxalate: 253.49-380.55 pg/100 g (red flesh); Oxalate: 302.19-322.82 pg/100 g (white flesh) | Afoakwa et al. (2003)
Colocasia esculenta | Oxalate: 328.41-459.85 pg/100 g | Afoakwa et al. (2003)
Raw taro | Oxalate: 156.33 mg/100 g; Phytate: 85.47 mg/100 g | Alcantara et al. (2013)
9.1.4 Solutions to Resolve Processing Constraints
Certain food processing methods need to be adopted at industrial levels to reduce the level of absorption inhibitors and improving the bioavailability of nutrients by eliminating the toxic components in roots and tubers (DeMaeyer, 1989). Food processing methods such as peeling, soaking, grating, fermentation and boiling may be effective in lowering the anti-nutritional factors. Taro is one of the tubers that are not readily adapted to modern processing facilities. Therefore, there is need to overcome these problems to utilize this tuber for further processing. The development of effective processing techniques can help to increase productivity, quality and income of farmers along with shelf life of the products. The various processing constraints along with their mode of elimination are given in Table 9.1.3.
Table 9.1.3 Various processing constraints in taro and their mode of elimination
Processing constraints | Mode of elimination/techniques used | Reference
Calcium oxalate crystals | Peeling, grating, soaking and fermentation during processing | FAO (1990)
―"― | Anaerobic fermentation in an underground pit for several weeks | Moy et al. (1979)
―"― | Boiling | Njintang and Mbofung (2003)
―"― | Boiling in tamarind solution | Soudy et al. (2010)
―"― | Fermentation, baking or extraction with Ethanol | Carpenter and Steinke (1983)
―"― | Boiling taro corms at 90 °C for 30 min and steeping in water at 30 °C for 24 h can reduce the oxalate-salt content to 32.7 % and 56.7 % of its original content | Iwuoha and Kalu (1994)
―"― | Soaking taro corm chips in 2 % w/v sodium bicarbonate solution for 20 minutes at ambient temperature | Kumoro et al. (2014)
Anti-nutritional factors (like oxalate, phytate, etc.) | Boiling | Lewu et al. (2010)
9.1.5 Taro Flour
Good-quality raw materials are needed for processing, therefore taro corms should be free from disease, infestations and damage. Proper hygienic practices should be in place to prevent contamination. Taro should be thoroughly washed and the waste generated from the processing disposed off to prevent cross-contamination. After washing the corm, trim the ends and peel away its outer skin, followed by placing its flesh in cold water to remove sticky sap (i.e. water solution rich in sugars).
Taro corms are highly perishable due to their high moisture content. Processing removes water which reduces the bulk and also increases storage life. Processing conditions for blanching and fermentation in flour may vary, depending upon the varieties of roots and tubers. The taro flour production from a fresh tuber soon after harvest serves as an effective procedure against storage loss. Converting fresh edible tubers to flours will avoid losses and ensure a supply of the products from tubers throughout the year, which may contribute to the growth of regional food industries and employability of the people.
9.1.5.1 Basic Steps in Production of Taro Flour
Taro flour processing methods involve several unit operations, including selection of raw material, cleaning, peeling, cutting, blanching, drying, milling, packaging and labelling. The specific combinations of these unit operations can provide different products from taro flour to the consumers. The general steps of preparing taro flour (Figure 9.1.2) are given as follows:
Selection of Raw Material and Grading The selection of raw materials may include simple parameters based on the physico-chemical, microbiological and sensory attributes for good yield and optimum quality of the final product. The parameters may include the various quality aspects related to wholesome taro tubers. Grading is an important step, which can help the taro producer and seller to determine its price and to widen the avenue for taro export. The quality factors such as size of taro tubers, conformity to the variety, tolerance limits for under-sized and over-sized taro tubers should be taken into consideration in this unit operation.