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The pH And The Mineral Contents Of Yam Tuber Before And After Frying




The pH and mineral composition of yam tuber before and after frying were determined. Results show that the pH values of the fresh yam and fried yam samples were (6.66+0.01) and fried yam (6.49+0.00) respectively, indicating that the pH value of the fresh yam was less acidic than the pH value of the fried yam. The Fe, Cu, Zn, Mg, Ca, K and Na of the fresh yam (mg/kg) were 9.36, 1.474, 8.48, 19.00, 1.40, 3, 188.10 and 671.17 respectively. Those of the fried yam in (mg/kg) were 13.30, 2.18, 10.29, 47.45, 2.16, 9, 996.67 and 93.71 respectively. These indicated that the mineral contents of the fried yam decreased except from potassium which increased. In conclusion, the ph and some minerals of the fresh yam were increased after frying.




Yam (Dioscorea spp.) are climbing plants with glaborous leaves and twinning stems, which coil readily around the stakes. They are perennial, through root system but are grown as annual crops. Water yam being the most economical and important part of the yam specie serves as a staple food for millions of people in tropical and sub-tropical countries (Hahn, 1995).

Yam as a monocots, are related to lilies and grass and are native to Africa and Asia. Yam tubers vary in size from that of a small potato to over 60kg (130lb). There are over six hundred (600) varieties of yam and ninety-five percent (95%) of this crop is grown in Africa. As said earlier, yams are perennial  herbaceous vines cultivated for consumption of their starchy tubers in Africa, Asia, Latin America. The Caribbean  and Oceania (Library of Congress United State of America, 2001).

However, yam tubers can grow up to 4.9ft in length and weight up to 70kg (150 pounds) and 7.6 to 15.2 (30 to 60) in high. The vegetable (yam) has a rough skin which is difficult to peel but softens after heating. The skin vary in colour from light brown to light pink, more so, the majority of the vegetable is composed of a much soften substance known as the meat. This substance ranges in colour from white or yellow  to purple or pink in mature yam (Huxley, 1992).

Generally, yams are consumed boiled roasted, fried or pounded and are eaten in association with protein rich sauces. They can also be processed into flour and reconstituted into fufu dough. However, a particular species of yam (water yam) contains less sugar and have an extended shelf life which ensures availability in periods of scarcity (Raemackers, 2001).

The project reports the pH and mineral compositions of yam tuber before and after frying.


1.2.1   Nutritional Value of Yam

Generally, the nutritional value of the vegetable yam cannot be over emphasized as one of its species (African yam) contains Thiocynate. This was suggested in 1986 paper that potentially, it is protective against sickle cell anemia (Agbai, 1986).

The protein content and quality of its tuber is lower than other food staple. However, of all roots and tubers, the protein contents of yam and that of potato are the highest, being approximately two percent (2%) on a fresh weight basis. Yam, like other root crops is not a good source of amino acid. It is rich in phenylalanine and threonine  but limiting in sulphur amino acid, cystone and methionime and tryptophan (Tropical Medicine Central Resources, 2006).

More so, yam consuming areas of Africa have a high incidence of kwashiorkor (a serious medical condition in children caused by protein deficiency). However, the need to supplement a yam-driven diet with more protein rich food in order to support active and healthy growth in infant (Home Health Hand, 2010).

Discoveries have shown that the tubers of certain wild yams, a variant of kokoro yam and other species of Dioscorea Hipponica are a source for the extraction of diosgenin, a steroid sapogenin. The extracted diosgenin is used for the commercial synthesis of cortisone, pregneolone, progesterone and other steroid products (Marker et al., 1940). However, such preparation (steroid products) are used in early combined oral contraceptive pills (Djerassi, 2010). While the unmodified steroids have estrogenic activities (Wangz et. al., 2005).

1.2.2     Economic Importance of Yam

Dioscorea rotundata, the white yam and Dioscorea cagenesi, the yellow yam are native to Africa. They are the most important cultivated yams, in the past, they were considered two separate species but most taxonomists now regard them as the same species. There are over 200 cultivated varieties between them (Schulthers et al., 1998).

White yam tuber is roughly cylindrically shaped. The skin is smooth and brown and the flesh usually white and firm, yellow yam is named after its yellow flesh, a colour caused by the presence of carotenoids, it looks similar to the white yam; in outer appearance, it tuber skin is usually a bit firmer and less extensively grooved. The yellow yam has a longer period of vegetation and shorter dormancy than white yam, the kokoro variety is important in making dried yam chips (Vernier, 2000).

They are large plants, the vines can be as long as 10 to 12m (33 to 39ft). the tubers most often weigh about 2.5 to 5kg (5.5 to 11.10lb) each, but can weigh as much as 25kg (55lb). After 7 to 12 months growth the tubers are harvested. In Africa, most are pounded into paste to make the traditional dish of pounded yam dish (Kay, 1987).

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RAW YAM (Coursey, 2009)

1.2.3     Yam Growing Cycle

Yam is a perennial plant, although, it is treated as annual one. Growth takes place in three (3) distinct places which develop over 12 months and repeats indefinitely during the life of the plant (Coursey, 2009) for this reason.

Vegetable growth phase begins with the outbreak and development of stem leaves and root of the new plant 5-10 weeks after planting. During the first two months of the growth, the stem has few or no leaves, because in this time primarily stem are roots are formed, then laves. It is important for the soil to be rich in potassium to promote the formation of tubers. In the case of D. lata and D. rotundata, you must use tutor or support (Nwosu, 2005).

Reproductive growth: The tuberous  roots and buds (tubers existing in the leaf axils) thickens, while the stem and leaves decreases their growth rate. It is the time the plant blooms and then its tubers ripen, they will be harvested in the next stage of rest, after ripening (Campbell, 1999). Yam is a dioecious that blooms irregularly, that is to say, male and female flowers open at different time, which makes the process of pollination very difficult. This is the main factors why yam does not produce seeds (Lawton, 2009).

Resting phase tuber maturity: It is the phase preceding the flowering of the plant which coincides with the dry season. The tuber materials while the aerial parts of the plant turgor, stems and leaves decays and the tuber is well separated from the stem (Njoku, 1993).

There are some techniques and theories to determine the exact point of maturity of the tuber. In some regions, the ultimate determinant of the maturity and tuber optimal state are simple organoleptic or taste criteria.

The stage ends with the sprouting of new tubers and the beginning of the growing season (Onwueme, 1995).


 1.2.4    Harvesting and Storage of Yam

The harvest of yam is characterized by wilting of the aerial parts of the plant, the stems are slightly turgid and becomes turn brownish in colour. The distant part of the tuber becomes brown. In commercial crops harvest is because mechanically of manually. The first faster but can be removed carefully with the aid of a carrier or a ‘coa’ (a sort of long handled narrow spade (Osagie, 1992).

Yam tubers are ripe for harvesting when the foliage diet harvesting takes place afterwards or tubers can simply be let in ridges. The duration of the time of storage depends on the particular variety of yam (Coursey, 1976).

Among the major roots and tubers properly tired yam is considered to be the least pin shapes. Successful storage of yam requires (Oke, 1990).

  1. Initial selection of sound and heavy yams.
  2. Proper curing if possible combined with fungicides treatment.
  3. Adequate ventilation to remove the heat generated by respiration of the tuber.
  4. Regular inspection during storage and removal of rotting tubers and any sprouts that developed.
  5. Protection from direct sunlight and rain.

Storing of yam at low temperature below 120C (540F) cause damage through chilling causing a breakdown of internal tissue, increasing water lose and yams susceptibility to decay (Onwueme, 1978).

The best temperature to store yam is between 14.160C (57.610F) with high technological controlled humidity and climate condition after a process of curing.

1.2.5     Uses of Yam

Yam is a good source of energy and each 100grams contain 118 calories. It is mainly composed of complete carbohydrates and soluble fiber. It is an excellent source of B complex vitamins like vitamin B6, B1, riboflavin, folic acid, pantothenic acid and niacin. It also contains a good amount of antioxidant and vitamin C. it provided around 20% per of the required vitamin in the body per 100grams. It also contains small amount of vitamin A and beta carotene levels. It is a rich source of minerals like copper, potassium, iron, magnesium, calcium and phosphorous 100grams of yam provides 816mg of potassium (Walsh, 2003).

1.2.6     Side Effects of Yam

Yam is not an allergenic food and does not contain too much of oxalates or purines and precautions should be exercised while consuming, speak to a doctor if one suffers from kidney or gall bladder ailments as even the small amount of oxalates  can also cause kidney damage. People who have healthy digestive tract are better able to absorb the nutrient of yam without any side effect. There may be side effects like nausea, vomiting, headache and diarrhea. Medical attention  should be sought if any of these side effects is noticed (Albihn, 2001).

1.2.7     Cooking Methods

FRYING: This is an intense process that induces a multitude of chemical reactions in the frying medium and generate a plethora of chemical compounds (Belitz et al., 2004). In frying, which ever type the oil is, it is usually heated to about 170% to 220%. However when the oil is heated to these temperature in the presence of oxygen (air). The oil undergoes thermal physical and chemical degradation by reaction (Moreira et al., 1999).

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FRIED YAM (Beliz, 2004)

Microwaving:  Microwaves cook food by heating food from the inside out, they emit radio waves the “excite” the molecules in food (makes them more all round) which generates heat cooking the food. While microwave cooking can sometimes cause food to be dry out that can be avoided by sp lashing the item with a bit of water before heating or placing a wet paper towel over the top of the dish regardless. The way that microwaves cook extra oils. The heat parts is, microwave can just do anything from veggies and rice to meat and eggs (and studies suggested it may just be one of the best ways of preserving nutrients in veggies (Liegey, 2001).

Boiling: Boiling is quick, easy and need nothing but water and a touch of salt and whenever food is being cooked, but addition to the high temperature, the large volume of water dissolves and washed away water soluble vitamins and 60 to 70 percent of foods minerals. While this method can dissolve vitamins and minerals in some foods (especially vegetables). Research actually suggests boiling could be the best way to preserve nutrients in carrots zucchinin, brocolol (when compared to steaming, frying or eating raw) (Robert et al., 2009).

Steaming: Cooking anything from fresh vegges to fish fillets allows them to cook in their own juices and retain all that natural goodness (again no need for fat-laden additions to up the moisture). It is always good to add a little seasoning whether that is a spinkle of salt or a squeeze of lemon, juice, if the caranogen-fighting glucosinotes in broccoli are important, some research suggests steaming could be the best way to cook. The little green trees in the body, glucosinolates become compound called Isothiocynates which some studies suggest inhibit the growth of cancer cells (Vallejo et al., 2003).

Poaching: The same goes for boiling, poaching –no additives, basically, poaching means cooking the given food in a small amount of hot water (just below boiling points). It takes slightly longer (which some experts believe can decrease nutrient retention) but is a great way to gently cook delicate foods like fish, eggs or fruits (plus it is just about the most delicious way to cook an egg) (Angler, 2013).

Grilling: In terms of getting maximum nutrition without sacrificing flavor, grilling is a great cooking method. It requires minimal added fats and imparts a smoky flavor while keeping meats and veggies juicy and tender, while these are definitely healthy benefits, not everything about grilling is so peachy (Sugimura et al., 2004). Some research suggests that regularly consuming charred well-done meats may increase risk of pancreatic cancer and breast cancer. Cooking at high heat can also produce a chemical reaction between the fat and protein. In meat, create toxins that are limited to the imbalance of antioxidants in the body and inflammation, which can led to an increase risk of diabetes and cardiovascular disease (Riccio, 2006).

Broilling: Broiling entails cooking food under high direct heat for a short period of time. Broiling is a great way to cook tender cut meat (remember to trim excess fat before cooking) but may not be ideal for cooking veggies as they can dry out easily.

Stir-Frying: This method does require some oil in the pan. It should only be a moderate amount. Just enough to get a nice sear on the meat and veggies. It is effective for bit sized pieces of meat grains like rice and quinoa and thin cut veggies like bell peppers, Julienned carrots and snow peas (Hotz, 2007).

No Cooking: Raw food diets have gained tons of attention recently and for good reasons, many studies suggest they are of benefit of incorporating more raw foods into the diet. Studies have shown eating the rainbow consistently reduces the risk of cancer but jurys out on whether raw or cooked is really best overall (Doll, 1987).

Since the diet is mostly plant-based, more vitamins, minerals and fiber are consumed overall with no added sugars or fats from cooking and raw items might be super healthy, studies have found that cooking can actually amplify some nutrients like; lycopene in tomatoes and antioxidants in carotenoids such as carrots spinach, sweet potatoes and peppers (Willet, 1995).

1.2.8    Historical Overview on Cooking

Cooking is the art and science of preparing food for eating by the application of heat, the term also includes the full range of culinary techniques; preparing, raw and cooked foods for the table; final dressing of meat, fish and fowl, cleaning and cutting fruits and vegetables, preparing salads, garnishing dishes, decorating desserts and planning meals (Pringle, 2012).   Earliest Types of Cooking

The origins of cooking are obscure, primitive human many first savoured roast meat by chance, when the flesh of beast killed in a forest fire was found to be more palatable and easier to chew and digest than the customary raw ,eat. They probably did not deliberately cook food. Though until long after they had learned to use fire for light and warmth, it has been speculated that peking man roasted meats, but no clear evidence supports the theory from whenever it began, however, roasting spitted meats over fires remained virtually the sole culinary technique until the Palaeolothic period when the Aurignacian people of southern France began to steam their food over hot embers by wrapping it in wet leaves. Aside from such crude procedures as toasting wild grains on flat rocks and using shells, skills or hollow stones to heat liquids. No further culinary advances were made until introduction of pottery during the Meolithic period (Rombauer et al., 1953).

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The earliest compound dish was a crude paste (the prototype of the pulmentum of the Roman Legions and the polenta of later Italians) made by mixing water with the cracked kernels of wild grasses. This paste toasted to crustiness when dropped on a hot stone made of first bread (Symons Michael, 2000).   Advances in Cooking Techniques

Culinary techniques improved with the introduction of earthware (and more or less concomitantly, the development of settled communities) domestication of livestock and the cultivation of edible plants.  A more dependable supply of foodstuffs, including milk and its derivatives was now assured. The raosting spit was Augumented a variety fired clay vessels, and the cooking techniques of boiling, stewing, braising and perhaps even Incipieng, forms of picking, frying and oven baking were added early cooks probably had already learned to preserve meats and fish by smoking, air-drying or chilling. New utensils made it possible to prepare these foods in new ways, such dishes are bacalaoa la viz caina (dried cod) and finnan haddie (smoked haddock) are still eaten (Danielle, 2012).

1.2.9   MINERALS

Minerals are essential nutrients that are needed in small amount to keep human healthy, minerals do not give energy or calories but can help with other functions in the body. The body does not make minerals, to meet daily needs, mineral must be obtained through diets (Fleisher et al., 1999).

Most people can meet their mineral needs through the following:

  • Eating well with proper food guide
  • By eating a variety of healthy foods, this means choosing foods from all four food groups; vegetable and fruit, grain products, milk and alternatives as well as meat and alternatives. (Kas, 1986).

Table 1: Minerals and their functions

Calcium: Build bones and teeth and helps keep them strong, helps muscles like the heart work properly.
Iron: Carries oxygen to all parts of the body, prevents tiredness.
Magnesium: Keeps nerves and muscles strong, helps for bones and teeth.
Potassium: Keeps fluids balanced in blood and tissue, helps in controlling to blood pressure.
Zinc: Needed for growth and development, maintains a heathy immune system.

Source: (Werk, 2004)    MINERALS IN FOODS

Minerals are substances found in food that the body needs for growth and health, these are called essential minerals. Essential minerals are sometimes divided up into major minerals (Council, 2001).

Macrominerals and Traceminerals

These two groups of mineral are equally important, but trace minerals are needed in small amount than major minerals. The amount needed in the body are not indication of their importance.

Macrominerals are the major that the body needed (Linseinsan et al., 2001).

Sodium, Chloride, Potassium, Calcium, Phosphorus, Magnesium, Sulphur

Trace Minerals: The body needs trace mineral in small amounts, but iron is considered to be a trace mineral, although the amount needed is somewhat more than for other micromineral (Bgvv, 2002).

Iron, Zinc, Iodine, Selenium, Copper, Manganese, Fluoride, Chromium, Molybdenum     Importance of Mineral

Minerals are the nutrients that exist in the body and are base essential as our need for oxygen to sustain life.

Minerals are found in organic and inorganic combination in foods. In the body, 5% of the human body weight, in mineral matter, vital to all minerals physical processes and for total well being, they are most important factor in constituents of the teeth, bone tissue blood, muscle and nerve cells.

Acting as catalyst for many biological reactions within the human body.

They are necessary for transmission of messages through the nervous system. Digestion and metabolism or utilization if all nutrients in foods, vitamins cannot be properly assimilated without the correct balance of minerals, for example:

Calcium is needed for vitamins utilization

Zinc for vitamin A, Magnesium for vitamin B, Complex vitamins, Selenium for vitamin E absorption

Minerals are very important in keeping the blood and tissue fluids from either becoming too acidic or too alkaline and they allow other nutrients to pass into the blood stream and aid in transporting nutrients to the cells (Moreias et al., 2003).

They also draw chemicals in and out of the cells, a slight change in the blood concentration of important minerals can rapidly endanger life (Dybing et al., 2002).

1.2.10   Aim of Project

The project aims of the determining the pH and mineral contents of yam tuber before and after frying using vegetable oil.

1.2.11   Specific Objectives

They include:

  1. To determine the pH and mineral contents of the intended yam tuber.
  2. To fry yam tuber using the vegetable oil.
  3. To determine the pH and mineral contents of fried yam.
  4. To compare the pH and mineral contents of the yam tuber in a and c above.

Pages:  36

Category: Project

Format:  Word & PDF        

Chapters: 1-4                     

Material contains Table of Content, Abstract and References.

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