Chemistry of Crops

1. CHEMISTRY OF TUBER CROPS

2. Tuber crops
 Cassava (Manihot esculenta Crantz)
 Sweet potato (Ipomoea batatas L.)
 Edible aroids (Colocasia spp. and Xanthosoma
sagittifolium)
 Yam (Dioscorea spp.)

3. General nutrition
 Widely grown and consumed as subsistence staples in
many parts of Africa, Latin America, the Pacific Islands and
Asia.
 Second only in importance to cereals as source of CHO.
 Provide minerals and essential vitamins.
 Used by supplementing with groundnut, meat, vegetable
soups, grain legumes and fishes to compensate protein
deficiencies.
 It is supplemented with the tender leaves of sweet potato,
cassava and cocoyam which are rich sources of protein,
minerals and vitamins

4. Nutritional and chemical characteristics
of main edible aroids

5. General characteristics of roots & tubers
compared with cereals (FAO, 1983)
Cereals Roots and tubers
Low moisture content, typically 10% to 15% High moisture content, typically 70% to
80%
Very low respiration rate with very low
generation of heat. Heat production is
typically 0.05 mega joule/ton/day for dry
grain
High respiration rate. Heat production is
typically 0.5 to 10 megajoules/ ton/day at
0°C to 5 to 70 megajoules/ton/day at 20ºC
Hard texture Soft texture, easily bruised
Stable, natural shelf life is several years Perishable, natural shelf life is a few days to
few months
Losses usually caused by moulds, insects
and rodents
Losses usually caused by rotting (bacteria
and fungi), senescence, sprouting and
bruising

6. CASSAVA
(Manihot esculenta C.)
 Cassava, a starchy food, has high calorific value
compared to rice, wheat and maize.
 A mature cassava plant has 6% leaves, 44% stems
and 50% tubers.
 Cassava tubers - 55 to 60% water, 30 to 40%
carbohydrates, 1% protein and 1% mineral matter.
 Major carbohydrate – starch (from 1 to 6%).
 Non-reducing sugars predominate & small amounts
of glucose, fructose and maltose.

7.  Protein - low with lysine and tryptophan -major amino
acids & small amounts of methionine, cystine and
cysteine.
 Vitamins- vitamin C, lacks other vitamins.
 Minerals – Ca & P in sufficient amounts; deficient in
other minerals.
 Some products of cassava are rich in calcium and
iron and has been attributed to the absorption Ce
and Fe from water.

8. Cyanogenic glycosides in cassava
 Cassava roots and leaves contain cyanides in two
different forms:
 i) Glycosides; linamarin and lotaustralin.
 ii) Non-glycosides; hydrogen cyanide (HCN) and
cyanohydride.
 Cyanogenic glycoside HCN
linamarase

9.  Great variation in toxicity between cultivars.
 A distinction is usually made between "sweet"
cultivars with relatively low contents of cyanogenic
glycosides (below 50mg/kg on a fresh weight basis),
and "bitter" cultivars with high cyanogenic glycoside
content 400 mg/kg.
 Highest proportion of HCN is found in the peels and
the cortex layer immediately beneath the peels
 Cassava root is always peeled before being
processed or consumed.
 Peeling removes the cortex and the outer periderm
layer adhering to it.

10. Cyanogenic glycosides in cassava -
REMOVAL
 The glycosides are hydrolysed to HCN by the
endogenous enzyme linamarase, which is present in
the human digestive tract.
 All the traditional cassava processing methods
reduce or remove the toxicity by releasing HCN from
the glycosides.
 Since HCN is soluble in water it can be removed by
soaking.
 Boiling fresh cassava has little effect on its toxicity as
the glycoside linamarine is heat resistant and the
enzyme linamarase is inactivated at 75°C.

11.  Peeling
Removal of peels reduces the cyanogenic
glycosides. This can reduce the cyanide content by
at least 50% in cassava tubers.
 Grating
This process takes place after peeling. Grating also
provides a greater surface area for the
fermentation to take place.
 Soaking of cassava roots normally preceeds cooking
or fermentation. It provides a larger medium for
fermentation and allows for greater extraction of the
soluble cyanide into soaking water.

12.  Boiling/cooking
About 90% of free cyanide is removed within 15
minutes. Cooking destroys the enzyme linamarase at
about 72oC.
 Fermentation
Some cyanidrophilic/cyanide tolerant micro-organisms,
effect breakdown of the cyanogenic glycosides.
 Drying
Cassava roots contain about 61% water, coupled with the
solubility of its cyanogenic glucoside component, the
dehydration (dewatering) process results in a substantial
reduction in the content of this toxin in the pressed pulp.

13. Cassava tubers as food
 Peeled and cut into pieces and boiled in water & eaten
as such or along with side dishes.
 Consumed after toasting and baking.
 Frying of sliced cassava is a common process in India.
The fried chips are used as snack foods.
 Parboiling cassava slices is another process carried
out in Kerala. - keeping cassava slices in boiling water
for 10-15 minutes- then drying in the sun & fried and
consumed as snack food.

14. Cassava flour products
 Porridge by mixing with hot water.
 Preparing traditional South India dishes such as
chappathi, puttu, idly, dosa and uppuma.
 Prepare bread by mixing 15% cassava with wheat
flour.
 Farinha - made by cooking the cassava pulp, with
continuous stirring, on the griddle on low fire, and the
product appears granular. It has good storage life
and may be eaten dry, mixed with hot or cold water
to make a paste or gruel or mixed with other foods.

15.  Sago - obtained from cassava starch in India.
 Moist starch is made into globules by shaking and
the globules are surface gelatinized. Wholesome
infant and convalescence food. Eaten generally by
mixing with milk and sugar, as porridge.
 Macaroni is prepared by blending cassava,
groundnut flour and wheat in the ratio of 60:15:25.
The product contains a good amount of proteins and
can be used after cooking like rice.

16. Cassava Starch and Industrial uses
 Industries like textiles, food, dextrin, sweetener and
chemical industries
 Starch is mainly used in textiles for sizing, which
involves coating of the warp yarn with a smooth film
of starch withstand the abrasive and flexural stresses
which the yarn is subjected to, during weaving
operations.
 Advantages of cassava starch are
Lower price
Higher strength
Higher viscosity, Transparency.

17.  Cassava starch is also used for finishing operations in
textile industries to give brightness to the fabrics. In
this respect cassava starch is superior to other
starches.
 Paper
 Increase the tearing and bursting strength of the
finished paper, improve the retention of fibres in the
wire mesh conveyor, allow higher retention of fillers in
the final product and reduce the BOD of the effluent.
 Oxidized and cationic starches are preferably used.
Oxidized starch is a low viscosity product obtained by
treating the starch with sodium hypochlorite of low
concentration.

18.  Cationic starch - reacting starch with reagents like
diethyl-amino ethyl chloride or glycidyl trimethyl
ammonium chloride.
 Oxidized starch is used in coating operations when a
pigment coating is required for the paper.
 Starch imparts sufficient fluidity to the coating.
Glyoxal is used for imparting water resistance for
coating paper used in lithograph offset printing.
 Cationic starch - efficient surface sizing agent for
paper.

19.  Food
 As a thickener, filler, binder and stabilizer due to
desirable properties like easy and complete
gelatinization, high viscosity, high clarity, low
retrogradation tendency and neutral flavour except for
the long stringy nature of its paste.
 Best suited for preparing Instant puddings, pie fillings,
icings, instant powders, etc., in the Western countries.

20.  Bread making.
 Adhesive industry as carton sealing,laminated and
corrugated boards, pastes or glues etc.,
 Sweetener industry. Malto – dextrins, glucose syrup,
high fructose syrup.
 Ethyl alcohol from hydrolyzed starch. Fresh cassava
roots or flour by gelatinization and break down of
starch to simple sugars is known as saccharification
which can be accomplished by using saccharifying
agents like mild acids, amylase enzymes, and
substances like malt which contain amylase
enzymes.

21. SWEET POTATO
(Ipomea batatas L.)
 Moisture 50 - 80%
 Protein 0.95 - 2.4%
 Ether extractable 1.8 - 6.4%
 Starch 8 - 29%
 Reducing sugars 0.5 - 2.5%
 Pectin, non-starch carbohydrate 0.5 - 7.5%
 Mineral matter 0.88 - 1.38%

22.  Potassium is a predominant.
 mineral constituents in mg/100 g are
 Ca-30, Mg-24, K-373, Na-13, P-49, S-29 and Fe-0.8.
 Quality judged on the basis of its sugar content which
varies from 2 to 6%
 Sucrose major & minor quantities of reducing sugars.
 Maltose - traces.
 Tri-saccharides are also present in very small
amounts

23.  Boiled tubers immediately after harvest are not quite
sweet.
 During storage a part of the starch is converted into
reducing sugars and subsequently into sucrose. The
rate and extent of change in the sugar and starch
content vary with the variety, temperature and
humidity.
 During storage, starch content got reduced from 19.1
to 14.1%, while the percentage of reducing sugars (as
dextrose) and sucrose increased from 0.9 to 1.7 and
1.9 to 6.1 respectively in a sample stored for five
months.

24. Biochemical changes during processing
of sweet potato
Curing, or baking - starch content is drastically reduced
with simultaneous increase in reducing sugars, total and
dextrin contents.
Starch gets degraded into dextrins and sugar.
Sweet potato - flatulence producing agent. - fermentation
of carbohydrates by colon bacteria to yield flatus gases
namely H2 and CO2,
Oligosaccharides stachyose and verbascose.

25. Starch
 Varies from 15 to 28%.
 Size of the starch granule varies from 10 to 25 μ; they
are polygonal in shape
 Starch shows 'A' pattern in X-ray diffraction and the
properties are in between those of cassava and maize.
 Amylase content is normally between 18 and 22%.
 Starch forms clear stable gelatine with high holding
capacity and forms a useful ingredient of food production,
confectionery and baking industry.

26. Composition of sweet potato
 Colour due to carotenoid content, especially β-carotene
& accounts for approximately 90 per cent.
 Precursor of vitamin A
 Content varied from 4-12 mg/ 100g
 Vitamin C (20 - 30 mg/ 100 g)-major
 Others in mg/ 100 g of sample are:
Thiamine 0.10
Riboflavin 0.06
Nicotinic acid 0.90.

27. Composition of sweet potato vines
 4% protein
 Fair source of Ca and P and
 Good source of iron
 100 g of vine tops with leave contain
Moisture 87.1 g
Nitrogen 0.57 g
Ether extract 0.67 g
Crude fibre 1.4 g
 Calcium 81.2 mg
 Phosphorus 67.3 mg

28. Industrial uses
 Source of starch.
 A variety of products such as edible syrups, industrial
alcohol, acetone, lactic acid, vinegar and yeast can be
prepared from the tubers.
 Varieties of sweet potatoes rich in dyes - have been
made from stems.
 Pectin is obtained as a by-product from the peels and
trim wastes and also from the pulp residue after starch
recovery.
 Useful in laundry work where it imparts a cleaner colour,
greater smoothness and stiffness to fabrics treated.

29. Process for starch making
Washing of tubers
Grinding with lime water (pH 8.8-9.2)
Separation of starch from pulp
Treated with sodium hypochorite and cetrifuged
Dried to 12% moisture in vacuum drier
Pulverised and screened (Starch 85.37 %)

30. CHEMISTRY OF EDIBLE AROIDS

31. Organic acids and Oxalates in aroids

32. Chemistry of Taro
(Colocasia sp.)
 Corms and cormels contain 63 to 85 % moisture, 13
to 30% carbohydrates, 1.4 to 3.0% protein, 0.16 to
0.36% fat, 0.6 to 1.18% fibre and 0.6 % ash.
 Protein - rich in most of the essential amino acids,
but is rather low in histidine, lysine, isoleucine,
tryptophan and methionine.
 Higher score for total essential amino-acids and
sulphur bearing amino-acids than other root crops.

33. Chemical Composition
 Amylose - 3 to 43% (28%). has a chain length of 490
glucose units, while the amylopectin has 22 glucose
units per molecule.
 Starch grains - very small (diameter from 1 to 5 μ with
an average of 3.3μ).
 Dry matter as well as the starch content of the corms is
lower at the apex of the corm than at the base.
 Most of the non-starchy nutrients of the corms are
concentrated in the outer peel.
 Starch similar to cassava and gelatinization temperature
of 68 to 75OC; easily digested. Human digestibility of
raw starch was reported to be 97%.

34.  Fatty acids - linoleic 42%, palmitic 25.6%, oleic 22%,
linolenic 8%, stearic 1.6% and myristic acid 0.1%.
 Poor source of vitamin C, but carotene content is
equivalent to that of cabbage and twice that of potato,
 Ca, P and Fe - adequate. rich in potassium.
 Mucilage -negligible quantity of protein and is made of
galactose and arabinose in the ratio 6:1. The mucilage
often creates problems in extraction of starch, but is
used in sizing of paper and in pills.
 Contain anthocyanins, alkaloids and sterols. The sterols
present are cholesterol, campesterol, stigmasterol and
b-sitosterol- involved in preventing fungal attack.

35. Taro Leaves and petioles
 Used as vegetable.
 Source of vitamins A and C
 23% protein
 Rich in calcium, phosphorus,
iron, vitamin C, thiamine,
riboflavin and niacin.
 Fresh taro leaf lamina contains
80% of moisture while the
petiole has 94% of moisture.

36. Uses of Taro
 Petioles - preparation of soup.
 Young taro leaves - vegetable -boiled or covered with
coconut creams, wrapped in banana or bread fruit
leaves and cooked on hot stones.
 Problems - presence of acrid principles- cause a
burning, itching sensation in the mouth, throat and on
the skin also.
 Selection of varieties and proper cooking solve this
problem.
 Taro cormels and leaves - fed to household stocks
such as goats & as animal feed is limited due to the
principles

37.  Use of taro as animal feed some degree of processing
for neutralization of acridity, addition of supplements
and improving storability are necessary.
 Taro corms are rich in starch which can be utilized in
various industries for preparation of high fructose
syrups and alcohol.
 Small particle size of taro starch and the efficiency with
which colour can be added to the particles make the
taro starch suitable for cosmetic dusting preparations
such as face powders and other cosmetic powder.
 Taro mucilage which swells in water and becomes
highly hydrated may be used as emulsifying, thickening
and smoothening agent for creams suspensions and
other colloidal food preparations.

38. Tannia
(Xanthosoma sp.)
 Corms contain 17 - 26% carbohydrate, 1.3 - 3.7% protein,
0.2 - 0.4% fat, 0.6 -1.9% fibre and 0.6 - 1.3% ash.
 Starch 17 - 35% (20%). larger in size as compared to that
of taro and hence the digestibility is poorer.
 Pastes less viscous than cassava and yam starches and
the gelatins are similar in strength to maize starch gel.
 Young leaves - rich in vitamin A and C but cooking led to
a loss of about 30 per cent in ascorbic acid
 Tubers and petioles contain calcium oxalate and the
oxalate contents are higher than that of Colocasia sp.

39. Uses of Xanthosoma (Tannia)
 Dried tubers may be ground to produce flour, which is
considered to be as palatable as cassava flour, but more
nutritious.
 Tannias are more nutritious than taro, but because their
starch grains are large (average diameter 17 to 20
microns) they are not so easily digested.
 Young leaves of many cultivars are used but care is
required in their preparation as calcium oxalate raphides
are present.

40. Alocasia
(giant taro)
 Giant taros are used as food in India.
 Tuber is peeled and eaten as vegetable after thorough cooking
usually as curries and stews.
 Corms and leaf juice are used for medicinal purposes in India. In
Brazil, used as pig feed.
 Acid juice of A. macrorrhiza and A. denudate are Poisonous.
 Boiled petiole exudate acts as a mild laxative and also as
stimulants to the skin for removing fever blotches.

41. Elephant foot yam
(Amorphophallus carnpanulatus)
 Tuber has Carbohydrate -18.0%, Protein 1 - 5% and fat
- 2%.
 Higher fat and vitamin A than other aroids
 Starch 4-12%.
 Sufficient quantity of glucomannan, a hemicellulose
made up of glucose and mannose unit.
 Glucomannan has been found to reduce the serum and
cholesterol levels in rats and is an ingredient in various
ayurvedic medicines.
 Leaves has 2 to 3% protein, 3% carbohydrate and 4 to
7% crude fibre.

43. Acridity in aroids
 Alocasia and Amorphophallus exhibit higher percentage
compared to Colocasia and Xanthosoma.
 On chewing the corm, a stinging effect is felt on the lips
and tongue, increase in salivation occurs and the
stinging effect extends to the throat and it is
accompanied by hearing impairment and headache.
 Strong correlation between the presence raphides and
acridity has been noted and various theories have been
put forward to explain acridity.
 Raphides are crystals of calcium oxalate which may be
of various shapes.

44.  Some of the theories suggested include:
 Forceful ejection of raphides from the idioblast cell into
the mouth and throat,
 Irritation caused by toxins associated with raphides
and
 Mechanical irritation caused by the raphide crystals.

45. Chemical theory of acridity
 Cooking or alcohol treatment does not usually affect the
raphide structure, but reduces acridity. A very acrid
chemical, the glucoside of 3,4-diglycosiliobenzaldehyde
has been identified from Colocasia antiquorum.
 Protein theory - certain proteolytic enzyme is released
which causes the release of kinins and / or histamins
leading to irritation. Some proteolytic enzymes in taro
have been found and named as `taroin‘ - deactivated on
boiling for 30 minutes
 All parts acrid including leaf blade, petioles, stems, etc.
Mature leaves are found to be more acrid. Though
cooking is not completely effective in removing acridity, it
is the only easy method to reduce it.

46. Chemistry of yam (Dioscorea sps)
Yam is the common name for some plant species in the
genus Dioscorea (family Dioscoreaceae) that form edible
tubers.
These are perennial herbaceous vines cultivated for the
consumption of their starchy tubers in Africa, Asia, Latin
America, the Caribbean and Oceania.
There are many cultivars of yam.
Yam contain mucilageous substances- extraction of
starch is difficult.

47. Chemistry of yam (Dioscorea sps)
Nutritional value per 100 g
Energy 494 kJ (118 kcal)
Carbohydrates 27.9 g
Sugars 0.5 g
Dietary fiber 4.1 g
Fat 0.17 g
Protein 1.5 g
Trace metals
Calcium 17 mg
Iron 0.54 mg
Magnesium 21 mg
Manganese 0.397 mg
Phosphorus 55 mg
Potassium 816 mg
Zinc 0.24 mg
Vitamins
Vitamin A equiv. 7 μg
Thiamine (B1) 0.112 mg
Riboflavin (B2) 0.032 mg
Niacin (B3) 0.552 mg
Pantothenic acid (B5) 0.314 mg
Vitamin B6 0.293 mg
Folate (B9) 23 μg
Vitamin C 17.1 mg
Vitamin E 0.35 mg
Vitamin K 2.3 μg

48.  Yams contain toxic and pharmaceutically active
constituents. These include alkaloids, tannins and
steroidal sapogenins.
 D.alata – dioscorin alkaloid, which is toxic.
 Tannins are found in dark red fleshed varieties
D.alata.
 Saponins sapogenins- most important compound.
 Three sapogenins isolated from Dioscorea species
are diosgenin, botogenin and kryptogenin.
 Yam sapogenins used in production of steroid drugs
and diosgenin is most used.
Phytochemicals

49. Enzymes
 Polyphenol oxidase, phosphatase, phosphorylase, etc.
 Ezymes phosphorylase and phosphatase were found in
D. alata, D. esculenta and D. rotundata, while amylase
occurred only in D. esculenta stored at 22°C for five
weeks.
 A high molecular weight amylase inhibitor has been
isolated from mature tubers of D. alata. Quite stable to
heat and activate against human pancreatic and pig
pancreatic amylases. But had no action on fungal or
bacterial amylases.
 Glycoprotein contain 64% carbohydrates and the
monosaccharide present are glucose, mannose and
galactose in the ratio 5.5 : 3.8 : 1.0.

50. References
 Vincent Lebot. Tropical Root and Tuber Crops: Cassava, Sweet Potato,
Yams and Aroids
 EDIBLE AROIDS Post-harvest Operations. http://www.fao.org /fileadmin/
ser_upload/inpho/docs/Post_Harvest_Compendium_-_Edible_aroids.pdf
 http://www.fao.org/docrep/x5415e/x5415e01.htm
 http://www.itmonline.org/arts/dioscorea.htm
 http://en.wikipedia.org/wiki/Yam_%28vegetable%29