DRYING KINETICS AND QUALITY OF DRIED MORINGA LEAVES USING DIFFERENT DRYING METHODS

ABSTRACT

The drying kinetics and effect quality of dried moringa leaves using different drying methods. The methods were microwave (MW) alone, solar, sun, shade, hot air alone and blanch-assisted and MW-assisted hot air drying methods. The drying experiments were performed in the temperature range of 50 to 70 °C, MW-power of 270 to 720 W, blanching time of 1 to 3 min and sample thickness 3 to 20 mm. MW-pretreatment time, blanching time, sample thickness, microwave power and hot air temperature had significant effects on the drying time but they generally affected negatively the quality attributes (AA, TP, AOA, BI and FL) of the dried samples. Four thin layer drying models were fitted to the experimental data and the Midilli et al model most appropriately described the drying behavior of moringa leaves. Rehydration studies were carried out at the temperature of 60 ^oC and the hot air drying method best fitted Weibull equation, first order kinetic and exponential association equations. The optimal drying conditions of moringa leaves were found to be 50 ^oC at the thickness of 4.79 mm for hot air, and that for MW-alone was 501.1 W at 3 mm thickness. For blanch-assisted hot air drying, optimal conditions were 70 ^oC and blanching time of 2.58 min while for MW-assisted hot air drying, they were 70 °C at MW-power of 270 W and MW-time of 3 min. The results demonstrated that all the drying methods differentially affected the drying rates and the desirable and deleterious quality characteristics. The implications for the choice of drying method are discussed.

CHAPTER ONE

INTRODUCTION

Background of the Study

Moringa oleifera (Lam) belongs to the monogeneric family of shrubs and trees called Moringaceae (Offor, Ehiri, & Njoku, 2014). Moringa is a highly valued plant that is mostly cultivated in the tropics and subtropics (Moyo, Masika, Hugo, & Muchenje, 2013). It is considered as one of the World’s most useful trees; almost every part of the moringa tree can be used as food for human consumption as well as animal feed and other industrial uses because of its nutritional and medicinal value and water purifying characteristics (Mukunzi et al., 2011). Moringa is used as an alternative to imported food supplements to treat and combat malnutrition, especially among infants and nursing mothers because it is rich in antioxidant compounds (Iqbal & Bhanger, 2006; Santos, Argolo, Coelho, & Paiva, 2005)

The leaves of moringa are cooked and eaten like spinach or used to make soups, sauce and salads (Fuglie, 2001). It has been reported that the micro-nutrient content is more concentrated in the dried leaves: ten (10) times the vitamin A of carrots, seventeen (17) times the calcium of milk, fifteen (15) times the potassium of bananas, twenty five (25) times the iron of spinach and nine (9) times the protein of yogurt but the vitamin C drops to a half of that of oranges (Mukunzi et al., 2011).

Moringa leaves contain phytochemicals, with powerful anticancer and antihypertensive activity and are considered full of medicinal properties used for treating fevers, sore throat, bronchitis, eye and ear infections, scurvy and catarrh; leaf juice is believed to control glucose levels and is applied to reduce glandular swelling (Makonnen, Hunde, & Damecha, 1997; Morton, 1991). In the Philippines studies done by Estrella, Jacinto Bias III, David and Taup (2000) discovered that moringa leaves are ‘mothers’ best friend’ because of its use in increasing nursing mothers’ milk production and is sometimes prescribed for anemic patients.

It has been reported that moringa leaves act as a good source of natural antioxidants such as ascorbic acid, flavonoids, phenolics and carotenoids (Dillard & German, 2000). Moringa leaves are commonly dried and ground into powder and can be stored without refrigeration for months without appreciable loss of nutritional value (Fuglie, 2001).

The leaves of moringa are fragile and have high moisture, which account for their perishability after harvest. The phytochemicals in the leaves are also susceptible to losses if not dried appropriately. According to Krokida, Maroulis, & Saravacos (2001), it is important to dry moringa leaves because quality changes occur when stored in its fresh form. Such changes include, changes in optical properties (colour, appearance), sensory properties (odour, taste, flavour), structural properties (density, porosity, specific volume), textural properties, rehydration properties (rehydration rate, rehydration capacity) and nutritional characteristics (vitamins loss, proteins denaturation).

Recently, in Ghana, moringa leaf products particularly leaf powder, are becoming increasingly widespread because of its outstanding nutritional value. When the leaves are dried they can be consumed in different ways; for example as part of the main meal or in beverages. In Ghana, moringa powder is mostly used in tom brown, weanmix and cakes. Therefore, it is of importance to develop suitable drying methods that will reduce the drying time and the incidence of nutritional losses. Consequently, this study was undertaken to investigate the drying of moringa leaves using microwave alone, solar, sun, shade, hot air alone, blanch-assisted and microwave-assisted hot air drying methods.

Problem Statement

According to Premi, Sharma, Sarkar and Singh (2010), the method adopted for drying moringa leaves is mainly traditional in nature and time consuming; and thus needs systematic methodology for obtaining a good quality product. Studies done by Lakshmi and Vimala (2000) and Premi et al. (2010) on green leafy vegetable powders, traditional vegetable processing and drumsticks leaves respectively, show that drying in direct sun and under shade affect considerably the nutritional value by reducing the concentration and availability of proteins, vitamins and other essential compounds in these vegetables.

Nevertheless, drying in direct sun and under shade are the common practices used in most parts of Africa to preserve vegetables for consumption. Researches indicate losses in nutrients from vegetables during sun drying (Kendall, Safford, Flannery-Schroeder, & Webb 2004; Yadav & Sehgal, 1995). Investigators such as Khachik et al. (1992) reported cooking of vegetables as another cause of loss of nutrients in vegetables. Studies done by Kiremire, Musinguzi, Kikafunda, and Lukwago (2010) using sun drying method to dry green leafy vegetable Amaranthus dubius showed that it resulted in the greatest loss of β-carotene, 58 % and vitamin C contents (84 %).

Studies have also been done on other methods of drying such as hot air, microwave, infrared, microwave-assisted drying and freeze drying. Several researchers such as Feng, Tang, Cavalieri, and Plumb (2001) dried diced apples using microwave. Nijhuis et al. (1998) dried various fruits and vegetables using microwave to determine the quality. Torringa, Esveld, Scheewe, van den Berg and Bartels (2001) dried mushrooms using microwave-assisted drying. These researchers reported that the use of microwave can significantly reduce the drying time because of its advantages such as automatic adjustment of the energy absorption level by the wet products. The possible selective heating of the interior portions and microwave focusing effect is another advantage that the microwave methods possess. The rapid energy dissipation throughout the material and relatively minor migration of water-soluble constituents, lower product temperatures in combination with vacuum and any other drying method like hot air and more efficient drying in the falling rate period .

Numerous studies on drying kinetics have been done on different agricultural products such as pear fruit (Lahsasni, Kouhila, Mahrouz, & Jaouhari, 2004), drumstick leave (Premi et al., 2010), fever leaves (Sobukola

Dairo, 2007), red pepper (Doymaz & Pala, 2002), garlic cloves (Sharma, Prasad, & Datta, 2003) and tomato slices (Abano, Ma, & Qu, 2011). Therefore it is important to compare methods of drying such as solar, sun, shade, microwave alone, hot air, blanch-assisted and microwave-assisted drying on the drying kinetics of moringa leaves in order to determine which drying method is appropriate for preservation of phytochemicals and more cost effective for adoption by famers.

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Item Type: Ghanaian Topic  |  Size: 143 pages  |  Chapters: 1-5

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