Roots and tubers

ROOT AND TUBERS CROP TECHNOLOGY

Major tropical root and tuber crops:

• Cassava: Manihot esculenta

• Sweet potato: Ipomea batatas (tropical)

• Yams: Dioscorea spp.

• Potato: Solanum tuberosum

• Taro (Cocoyams): Colocasia esculenta or Xanthosoma sagittifolium

• Arrow roots: Malanda, arandinacea: tropical Africa, South Africa, Asia and Pacific

Industrial products

· Animal feed

· Starch production

· Chips and flour production

· Chips and crisps

· Ethanol

Economic importance of root and tubers

· Critical diet components from early evolution of man-food security.

· Raw materials-industrial growth

· Employment-farming and processing

· Instruments of trade-local & int.

· Human food and animal feed.

Cassava

It is a vegetative propagated-stem cutting. Yield: 7-40 t/ha. The enlarged roots contain starch-25-40%. Starch and sugar are dominant in the dry matter; Low in protein content (2.3%) but good quality; deficient in sulfur amino acids.

Limitations:

· Short storage life-2-3 days

· Toxicity: it has high hydrocyanic glucosides (HCN)

Toxicity

There are two varieties:

· Bitter: high HCN all over roots; Longer maturity (12-18m); Not for direct consumption

· Sweet :low HCN confined to peels; matures 6-12m ;deteriorates more rapidly

Thus cyanide in cassava products exists in 3 forms:

· Glucosides: linamarin and lotaustralin (methyl-linamarin). Linamarin exists in leaves and roots of cassava. It occurs as the main toxic principle in all parts variably-80%.

· Cyanohydrins

· The free hydrocyanic acid

Most other roots have small amounts of potential toxins and anti-nutritive factors such as trypsin inhibitors. It has cyanogenic glucosides, whose lethal-Max is 50mg/kg.

Linamarin ß-linkage only broken by:

· High pressure

· High Temperature

· Mineral acids

· Enzymatic reactions: Git exposure, decomposed to hydrogen cyanide by linamarase. Linamarase, endogenous cassava enzyme breaks the ß-linkage. Enzymatic reaction occurs under optimum conditions at 25ºC, at pH 5.5 to 6.0.

Detoxication

In humans, cyanide as thiocyanate in the blood stream gets excreted in urine. Conversion catalyzed by enzyme thiosulphate cyanide sulphur transferase (rhodanase). The conversion consumes lots of sulphur and increased body demand for sulphur containing a.a.

Essential substrates for detoxication: cystine and methionine .Vitamin B12-hydroxycobalamin may also influence conversion of cyanide to thiocyanate. Low protein diet decreases detoxication.

Detoxification methods in processing

Processing reduces cyanide to harmless levels; releases linamarase by disintegrating microstructures of roots-contacts as linamarin is converted.

· Peeling: it reduces up to 50% cyanide

· Grating: promotes interaction of linamarin with linamarase

· Soaking-extracts 20% in 4hrs, if dried=90%

· Boiling: 90% soluble cyanide lost in 15min, 55% bound in 30min, at 72oc destroys linamerase

· Roasting: Whole roots buried in hot ashes or placed in front of fire till cooked or roots are soaked before roasting

· Frying: peel, wash, slicing, frying. Common for cassava crisps (coastal Kenya,

· Tanzania and West Africa)

· Steaming: Uganda-peeled roots wrapped in banana leaves and stewed in a pan.

· Sun-drying/drying and pounding or grinding: Generally peeled and dried or roots soaked in water-sundried-pounded into flour. Removes 2/3-3/4 cyanide

· Fermentation: Can be heap-peeled roots under shade 3-5d. Methods differ-rwanda 2-5d, Uganda in banana leaves, W. Kenya in gunny bags. Then dried, pounded, sieved: It removes twice as much as sun-drying. Cyanide tolerant micro-organisms may be responsible. Products obtained through fermentation:

ü Fuku

• Dried roots pounded with fermented corn

• Then roasted in flat trays-arrests fermentation

• Flour mixed with boiled water and eaten

ü Fufu

• Sticky dough or porridge from any pounded starch.

• Peeled-washed-grated/crushed-fermented 2-3d-store in cold water basins changed daily.

• Eaten several ways with stew or soup

ü Gari

• Most popular in W. Afr:

• Roots are peeled, washed, grated and fermented (3-6 days for sour flavour) then pressed to squeeze out juice.

• 50 % water left is sieved to remove fibrous materials

• Pulp cooked in shallow pans for flour or bread or sundried and eaten with stew.

• Flour made by stirring continuously in the griddle-may be eaten dry, mixed hot/cool water for paste, gruel or mixed with other foods.

• Also called farinhade, mandioca, farinha seca in S. America.

• Juice from gari could be used to prepare sauces and beverages into a syrup consistency known as cassava creep. Both alcoholic and nonalcoholic beverages are prepared.

Industrial products-cassava

Starch for processing

Chips used in animal feeds and other uses.

Cassava flour

Pellets, bread (Nigeria)

Crisps-snacks

Diseases related to cassava toxicity

1. Acute cyanide intoxication:

• Raw or insufficiently processed cassava

• Symptoms after 44 hours consumption

• Symptoms-fatigue, vomiting, collapse and death in less than 2 hours.

• Treatment: increase the detoxicating capacity of the patient

• Be treated immediately by IV of thiosulphate: more sulphur for conversion.

2. Endemic Goiter:

• Detoxication of thiocyanate -interferes with iodine intake leading to goiter by thyroid gland.

• Serum thiosulphate increases to 1-3 mg/100g from the normal to chronic loading in excess concentrations.

• Iodine excreted in the urine and reduce intake by thyroid.

• Risk of developing endemic cretinism-severe mental retardation and neurologic abnormalities.

3. Neurological disorder (Paralysis)

· Tropical ataxic neuropathy (TAN): older people who have consumed a monotonous cassava diet over years.

· Damage a sensory tract in spinal cord=uncoordinated gait=ataxia

· Progressive-causes unsteady walking, loss of sensation in hands, loss of vision, deafness and weakness,

· Requires a high protein diet

4. Epidemic spastic paraparas

• Paralysis of limbs followed by damaged spinal cord: ‘konzo’- E, S, C Africa- irreversible paralysis of the legs, particularly children and in women of child-bearing age.

Post-harvest handling

It is more perishable than other roots. Deterioration / Spoilage:

· Physiological (primary-dry)-within 24hrs after harvest.

· Blue brown discoloration on vascular bundles-streaking observed.

· Microbial (secondary-wet roots)-5-7 days post-harvest: fungi and bacteria develop and dry rots.

Ways of prolonging shelf life:

· Harvesting with less bruise injury-wet or loose soil.

· Store only roots with no injury-they cannot be effectively cured.

· Curing should be routine

· Ground storage: Cut off stems and left un-harvested, 3 weeks prior to harvest. Disadvantages-minimal land use, fibrous and woody roots develop.

· Storage under high humidity-pile roots and sprinkles them

· Harvest with stem (2-5cm)-rot that starts from the neck prevented

· Field clamp: Can store up to 8 weeks. Layer of straw and 300-500 kg heap in between, dry floor and top soil cover, ventilated openings at the bottom.

· Storage in boxes lined with moist sawdust or wood shavings. Alternative layers of sawdust and roots start and finish with sawdust.

· Plastic bags/film wraps treated with fungicide-thiabendazole, remain on thick peels. Deep roots in 0.4% w/w solution for 10 s and keep airtight in plastics bags. Active chlorine (0.95%) may also be used.

· Waxing

· Cold storage-refrigeration, deep freezing- exports in EU, America

· Controlled atmosphere