Micronisation of pre-conditioned legumes

Get Complete Project Material File(s) Now! »

LITERATURE REVIEW

In this review, focus is placed on the effects of heat processing including micronisation on cooking quality and sensory properties of cowpeas. Phenolic compound structure and antioxidant activity relationship, as well as health promoting properties related to plant food phenolic compounds are also discussed. This review also discusses effects of heat processing on cowpea bioactive properties.

Importance and utilisation of cowpeas

Cowpeas grow well under a wide variety of soil conditions under both irrigated and nonirrigated regimes but respond more positively under irrigated conditions (Ehlers and Hall,1997). The fact that cowpeas are more drought resistant than common beans make them an important crop in many developing parts of the world where irrigation is still a problem. Nigeria is the world’s largest producer with 2.1 million tonnes, followed by Niger with 650000 tonnes and Mali with 110 000 tonnes. The total production area of cowpeas is estimated at 9.8 million hectares; about 9.3 million hectares of these in West Africa (IITA, 2012). About two-thirds of the production and more than three-quarters of the area of production is spread over the Sudan Savanna and Sahelian zones of sub-Saharan Africa (Ehlers and Hall,1997).
Cowpeas are an important source of energy and nutrients in developing countries of Africa,Latin America, and Asia. Cowpeas are a good source of dietary protein, which complements cereals, starchy roots and tubers (Phillips, McWatters, Chinnan, Hung, Beuchat, Sefa-Dedeh,Sakyi-Dawson, Ngoddy, Nnanyelugo, Enwere, Komey, Liu, Mensa-Wilmot, Nnanna, Okeke, Prinyawiwatkul and Saalia, 2003). They provide an alternative source of protein where meat and meat products are limited or expensive. The dry grain of cowpea is the principal product used for human consumption. Leaves (mostly in eastern Africa), pea seeds (the southern US and Senegal) and the green pods (humid regions of Asia and the Caribbean) are also consumed (Taiwo, Akanbi and Ajibola, 1997b). The crop is used for green manure in south eastern US and Australia (Taiwo et al., 1997b).
One of the major forms in which cowpea is utilised is as cooked whole seeds. Cooking of cowpea seeds, as with most legumes, is often achieved after boiling for up to 2 h and even more (Akinyele et al., 1986) resulting in high energy costs an increased food preparation time. The changing socio-economic conditions and demand bring the pressure on the food industries to develop convenience foods which are easy to prepare, have excellent taste and flavour, nutritious and wholesome in nature with long shelf life (Sharma, 2009). To achieve these goals, various processing operations are employed and may considerably affect the overall quality of the processed foods (Sharma, 2009). Micronisation, an infrared heat treatment applied to pre-conditioned legumes has been reported to reduce the cooking time of legumes such as cowpeas (Mwangwela et al., 2006) and lentils (Arntfield, Scanlon, Malcolmson, Watts, Ryland and Savoie, 1997).

Foods can be heat processed in any of the three ways of transfer of heat i.e. by conduction,convection and radiation. In the case of radiation, heat is directly transferred from source to the object being heated. The electromagnetic spectrum encompasses radiation from short to long wavelength, i.e. gamma rays, x-rays, ultraviolet, visible light, infra-red, microwaves, and radio waves (Sharma, 2009). According to Fasina et al. (2001), micronisation involves short time exposure of a material to electromagnetic radiation in the wavelength region of 1.8 to3.4 μm (1800 to 3400 nm). The word “micronisation” is derived from the short wavelength unit “micron” used in this processing method (Sharma, 2009). This micron size wavelength has been found to be highly efficient in achieving high temperatures (750-930˚C) in a very short time (Sharma, 2009). When infrared waves strike the food material, a part of the energy is absorbed, making the constituent molecules vibrate (Sadeghi, Nikkhah, Fattah and Chamani, 2010). During the vibration, inter-molecular friction occurs among the molecules and results in heat generation and changes in molecular structures (Sadeghi et al., 2010). For example proteins are denatured and starch is gelatinised (Mwangwela, 2006). Micronisation of moisture-conditioned seeds has generally been reported to reduce the cooking time of legumes by 50 % at most (Arntfield et al., 1997). Arntfield et al. (1997) reported that softening of lentils upon cooking increased with increase in tempering moisture. Thus the micronisation process includes moisture conditioning of legumes grains to increase moisture content (Mwangwela et al., 2006).

READ Intensification of agriculture and landscapes

Effect of micronisation of pre-conditioned of legumes on their seed components in relation to their cooking characteristics

Cooking time is one of the food quality criterions that are used to evaluate the quality of cooked whole cowpea seeds (Ehlers and Hall, 1997). Cooking time is defined as the time required for cowpeas to attain a level of softness that is acceptable for consumption (Proctor & Watts, 1987). Micronisation of moisture-conditioned seeds has been reported to reduce the cooking time of cowpeas (Mwangwela et al., 2006). Reduction in cooking time of legumes has been related to improved hydration observed in micronised legume seeds during cooking (Cenkowski and Sosulski, 1997; Arntfield, Scanlon, Malcolmson, Watts, Cenkowski, Ryland and Savoie, 2001). Arntfield et al. (2001) reported that cell walls of lentils had a more open microstructure after micronisation. In cowpeas, micronisation caused fissuring of seed coat, cotyledon, and parenchyma cell wall of micronised seeds (Mwangwela et al., 2006). These changes in physical structure improve the hydration rate which in turn resulted into cooked cowpeas with a softer texture (Mwangwela et al., 2006). Softening of legumes upon cooking has also been attributed to the disintegration of the middle lamella between cotyledon parenchyma cells, protein denaturation and starch gelatinisation within the cotyledon parenchyma cells (Sefa-Dedeh and Stanley 1979a).Solubilisation of the middle lamella is one of the factors that contribute towards the softening of texture during the cooking of dry cowpea seeds (Sefa-Dedeh and Stanley, 1979a).
Comparing scanning electron micrographs (SEM) of micronised cowpeas (41% moisture,153 ºC) with untreated samples, Mwangwela (2006) found that micronised cowpeas showed marked cell separation along the middle lamella. Similar results were reported by Arntfield et al. (2001) who found that cotyledon cells of micronised lentils (33 % moisture, 138 ºC) separated along the cell wall upon fracture during sample preparation for SEM, an indication of middle lamella disintegration. In addition, Arntfield et al. (1997) reported a significant reduction in pectic substances for micronised (29 % moisture, 88 °C) lentils. The middle lamella holds the individual parenchyma cells together giving a fixed structure to the cotyledon (Mwangwela, 2006). When the middle lamella is solubilised, parenchyma cells are separated thereby contributing to a soft texture (Mwangwela, 2006).

DECLARATION
DEDICATION
ACKNOWLEDGEMENTS
ABSTRACT
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
1. INTRODUCTION
2. LITERATURE REVIEW
2.1 Importance and utilisation of cowpeas
2.2 Micronisation of pre-conditioned legumes
2.2.1 Effect of micronisation of pre-conditioned of legumes on their seed components in relation to their cooking characteristics
2.3 Sensory quality of legumes
2.3.1 Effects of micronisation on sensory properties of legumes
2.3.2 Descriptive sensory analysis
2.4 Phenolic compounds in legumes
2.4.1 Phenolic acids
2.4.2 Flavonoids
2.4.3 Tannins
2.4.4 Structure-antioxidant activity relationships of phenolic compounds
2.5 Health-promoting properties of phenolic compounds in plant foods
2.6 Effect of heat processing on phenolic composition and antioxidant activity of legumes
2.6.1 Effect of cooking on phenolic composition and antioxidant activity of legumes
2.6.2 Effect of infrared heat processing and other dry heat processing technologies on phenolic composition and antioxidant activity of grains
2.7 Methodologies to measure health promoting properties of phenolic compounds
2.7.1 Trolox equivalence antioxidant capacity or the ABTS assay
2.7.2 The oxygen radical absorbance capacity (ORAC)
2.7.3 Inhibition of oxidative DNA damage (agarose gel electrophoresis method)
2.7.4 Inhibition of red blood cell (erythrocytes) haemolysis assay
2.7.5 Inhibition of LDL Oxidation assay
2.8 Gaps in knowledge
3. HYPOTHESES AND OBJECTIVES
3.1 Hypotheses
3.2 Objectives
4. RESEARCH
4.1 Effect of micronisation of pre-conditioned cowpeas on cooking time and sensory properties of cooked cowpeas.
4.2 Effects of micronisation of pre-conditioned cowpeas on phenolic composition of uncooked and cooked cowpeas.
4.3 Effects of micronisation of pre-conditioned cowpeas on total phenolic content, total flavonoids and in vitro free radical scavenging properties of uncooked and cooked cowpeas
4.4 Effects of micronisation on cowpea protective effects against Low Density Lipoprotein (LDL) oxidation, oxidative DNA damage and red blood cell haemolysis
5. GENERAL DISCUSION
5.1 METHODOLOGIES
5.2 RESEARCH FINDINGS
6. CONCLUSIONS AND RECOMMENDATIONS
7. REFERENCES
8. APPENDIX