The Effects of Processing on the Storage Stability and Functional Properties of Cowpea Flour in the Production of Moi-Moi and Akara
Chapter One
Objective of the study
The main aim of the study is to examine the effects of processing on the storage stability and functional properties of cowpea flour in the production of flour and moi moi.
The study was guided by the following minor objectives;
- To determine the particle size distribution for dry milled products
- To determine the particle size distribution for wet milled products
- To assess similarities between the functional properties of dry and wet milled products.
CHAPTER TWO
LITERATURE REVIEW
Seed and content
Cowpea (Vigna unguiculata), known as black-eyed, crowder, and field pea in the United States, is an important legume crop of East and West Africa (Prinyawiwatkul et al., 1996). It contributes a significant amount of protein and water-soluble vitamins to the African diet.
Cowpea seed is dicotyledonous, and the cotyledons form the major part of the seed. Each cotyledon contains parenchyma cells (60 to 100 m) with reserve materials in the form of elliptical starch granules (11 to 20 m). These are embedded in a proteinaceous matrix containing protein bodies (3 to 6 m). The parenchymatous cells of the cowpea cotyledon are bound by a cell wall and middle lamella. Vascular bundles containing a large number of closely packed cells are scattered throughout the cotyledon (Sefa-Dedeh and Stanley, 1979a,b).
Cowpea seed is composed of approximately 11% moisture, 23.4% protein, 56.8% carbohydrate, 3.9% fiber, 3.6% ash, 1.3% fat, and provides 343 calories/100g of seed (Deshpande and Damodaran, 1990). Cowpea also contains minerals such as potassium (1024 mg/100g), phosphorous (426 mg/100g), magnesium (230 mg/100g), calcium (74 mg/100g), sodium (35 mg/100 g) and iron (5.8 mg/100g). The nutritive value of cowpea is also enhanced by the presence of vitamins such as niacin (2.2 mg/100g), thiamin (1.05 mg/100g), riboflavin (0.21 mg/100g) and -carotene (18 g/100g) (Deshpande and Damodaran, 1990). Cowpea protein has a good quantity of the following essential amino acids: leucine (7.4 g/16gN), lysine (6.7 g/16gN), phenylalanine (5.7 g/16gN), valine (5.2 g/16gN), isoleucine (4.9 g/16gN), threonine (4.1 g/16gN), methionine (1.3 g/16gN), cysteine (1.1 g/16gN) and tryptophan (1.0 g/16gN) (Mosse and Pernollet, 1983). The low value of sulphur-containing amino acids is evident. Protein quality is synergistically improved in cereal-legume mixes. Cowpeas are rich in lysine and make good complementary food with cereals, which are rich in sulfur-containing amino acids such as methionine (Enwere and Ngoddy, 1986). As a recommendation for human diets, a weight ratio of 45 parts cereal grain to 15 parts cowpea could be used (Bressani, 1985).
The cowpea carbohydrates (56.8% content) consist of 13% total sugars, 4% crude fiber, and 48% starch (Reddy et al., 1984; Longe, 1980). About half of the total starch is in the form of amylose (Bressani, 1985). The relatively high amylose content has been shown to cause slow digestibility in vivo (Rao, 1976). The amount of amylose in starch influences starch solubility, lipid binding, and other functional properties such as swelling, solubility, water absorption, gelatinization, and pasting, all of which affect cooking quality of cowpea and its acceptability. Amylopectin is responsible for solubility of starch granules (Bresanni, 1985; Reddy et al., 1984). A wide range of processing methods such as boiling, soaking, and germination have been used to increase the utilization of cowpea. Boiling, fermentation, and germination increased carbohydrate digestibility of raw cowpea in vitro by 57.7%, 56.7%, and 51.7%, respectively, and may facilitate it in vivo to some extent. Roasting had a negative effect on carbohydrate digestibility (50.5%) in vitro (Rao, 1976; Reddy et al., 1984; Deshpande and Damodaran, 1990). Cowpea protein has a digestibility of 72%. Cooking improved protein digestibility of raw cowpeas, ranging from 87 to 92% (Khan et al., 1979). However, besides the nutritional components, cowpeas contain significant amounts of antinutritional factors that have to be eliminated to improve their nutritional quality and organoleptic acceptability. The major ones are trypsin inhibitors, tannins, and the oligosachharides, stachyose and raffinose (Wang et al., 1997).
CHAPTER THREE
Materials and Methods
Preparation of cowpea meal
All the seed used in this study was of the variety “California Cream” that was undecorticated. The “California Cream” seeds lack seed pigments and the undecorticated seeds have been reported to perform equally well as decorticated “Blackeye” seeds (McWatters et al., 1993). A small amount of seed (2g) was ground and its moisture content determined using a vacuum oven (18 h, 70 oC and 25 mm Hg). The moisture content was found to be 10.12%. The seed was milled using three different mills, namely, plate mill (PM), hammer mill (HM) and Wiley mill (WM). Wet milling was used as the control and will be described later. Screen sizes of 1.73 mm (HM-1.73) and 2.54 mm (HM-2.54) were used with the hammer mill (Champion, Model no. 6X14, Champion Products Inc., Eden Prairie, Minn., U.S.A.). The mechanics of hammer milling have been very well described by Ajayi and Clarke (1997). A screen size of 2 mm (WM) was used on the Thomas-Wiley laboratory mill (model 4, Arthur H. Thomas Co., Philadelphia, Penn., U.S.A.), and seeds were milled by the procedure described by Kerr et al. (2000).
CHAPTER FOUR
Results and Discussion
Dry milling and sieve analysis
Results from particle size distribution as determined by sieve shaking are shown in Table 2.2. Seeds milled through the plate mill with clearance of 360, 270, 180 and 90 degrees showed a high percentage of large-sized particles. Particles in these samples were mainly concentrated on the first three sieve sizes (8, 10, and 20-mesh). As the clearance decreased, particle size tended to shift towards the lower (smaller-sized) sieves. PM-360 had approximately 69.70% particles larger than 8-mesh. The next two sieve sizes had 11.87 and 12.83% of the total particles, respectively. The first three sieves held 94% of the total meal particles. Further down the sieves, the percentage of particles showed a sharp decrease.
CHAPTER FIVE
SUMMARY AND CONCLUSIONS
CONCLUSION
This study shows that milling affects the particle size distribution, which in turn is a major determinant of hydration properties of meals and flours. All milling techniques have a different mechanism by which particle size reduction is obtained. This basic principle leads to varying particle size distributions for a sample when milled through different mill screens and clearances. Material milled through a similar sized screen but by a different mill can lead to a totally different PSD. In our study, the Wiley mill used a 2.00 mm screen whereas HM-1.73 and HM-2.54 used a 1.73 mm and 2.54 mm screens, respectively. Normally it would be expected that the dgw of WM should lie between the HM samples. However, this was not true. WM produced a much greater dgw than HM samples.
Plate mill (PM) samples milled through larger clearances (PM-360, PM-270 and PM-180 degrees) produced higher SWC and WHC after blending. The higher hydration characteristics were attributed to the initial gritty size of particles. This aided in preservation of cellular structure and hence the fibrous material. The finer particle size samples such as HM-1.73, HM-2.54 and WM produced flours that had a lower SWC and WHC after whipping. Fine milling enhances protein solubility, but it adversely affects the hydration properties of paste. This also leads to lower viscosity and poor foaming properties. Due to the intensity of milling, the cellular structure and the fibers therein are destroyed. Smaller-particle size samples like HM-1.73 showed high values for SWC and WHC before blending (BB). This can be attributed to the fact that intense milling resulted in an increase in the total surface area and total pore volume of HM-1.73. This leads to an increased WHC and SWC for the sample.
This study also demonstrated the very important role of additional blending in improving hydration properties of all samples. Before the step of additional blending, the particle sizes were very widely distributed. Blending reduces the particle size considerably. In some cases it seems that particle coagulation is taking place. Previous studies (Kethireddipalli et al., 2002ab) had shown that the unblended meals and flours produced poor quality akara because of poor hydration properties. Blending improved WHC for all samples from 89.62 to 266.35%. SWC was also improved within a range of 131.82 to 214.06%.
This study showed that milling has a significant affect on hydration properties of cowpea meals and hence the quality of the end product, akara. The efficacy of PSD as a tool for product matching and development was also demonstrated.
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