Effect of solar radiation on the drying parameters of mint ( Mentha spicata L.) dried in Natural convective Solar Dryer

The paper presents a kinetic study of solar dried mint ( Mentha spicata L.) without any pre-treatment. Mint grows effortlessly in Kashmir valley but is not obtainable throughout the year because of inconsiderate weather conditions. The genus belongs to the Labiatae family and includes large varieties with different properties. A significant quality parameter is its colour. The drying kinetics of mint leaves dried in Domestic Solar Dryer in terms of colour attributes, moisture content, drying time, non enzymatic browning, water activity, rehydration ratio was studied. This study is very useful for household scale drying of mint leaves to optimize drying process and to achieve superior quality dried product at home maintaining its colour and aroma.


Introduction
Mint is an aromatic herb having many culinary and medicinal values. The leaf, fresh or dried, has medicinal value. It is used in both fresh and dried forms in different cuisines. The mint leaves are used in a variety of dishes such as chutney, fruit salads, vegetable salads, soups, Page 2 of 17 desserts, juices [1,2,3]. Dried mint is used in the traditional cooking in Kashmir valley which enhances flavor of the dish. The menthol present in it helps to diminish irritable bowel syndrome symptoms through its relaxing effects on the muscles of the digestive tract [4]. Mint may be effective in relieving other digestive problems such as upset stomach and indigestion. [ 5].
Studies have shown that the mint is used in relieving the nasal congestion [6]. It is also referred that inhaling the aroma of essential oils like mint from the plant could provide health benefits, including improved brain function [7]. This herb has numerous health benefits and grows effortlessly in Kashmir division but cannot be grown during the winter season because of the inconsiderate weather. Kashmir valley experiences more rains and lesser sunny day and experiences harsh winters from the month of November to April [8]. There is abundant growth of vegetables, fruits and herbs during summer season which need to be preserved for further use and in order to avoid the wastage.
Fresh mint is usually preferred over dried mint because storage of the mint is a problem. In order to preserve the herb it is to be dried. Usually people dry it in open sun but as observed from the open sun dried samples the discoloration of herbs and loss of aroma takes place which remains unattractive for the consumer. Moreover the insect, pest infestation, bird droppings, rodent attack etc can takes place. The drying time is very less, even if it is sunny during the day, there are chances of rain in the evening which spoils the drying vegetable. Though open sun drying is the most common practice to dry vegetables in Kashmir Valley during the summer season. Such drying conditions leads to stern losses in the quantity and quality of the dehydrated product. Hence the scientist developed different types solar dryers. The solar drying system utilizes solar energy to heat up air and to dry any food substance loaded, which is beneficial in reducing wastage of agricultural product and helps in preservation of agricultural product. The Many types of dryers have been developed on the industrial level and are being used for the commercial purpose incurring high capital and maintenance cost. Therefore there is a need to develop a domestic dryer so as to minimize the loss of surplus vegetables produced during the summer season and to preserve the herbs for winter periods with better quality of dried product.
The drying of vegetables and fruits is a complex process that is completed in two stages. In the first step evaporation takes place with high speed, eliminating free water, and then drying speed is reduced steadily in proportion to the product moisture reduction as already reported [9] The bound moisture present in the food can become the reason for the fungal growth and reduce the life span. Hence it becomes requisite condition to eliminate the moisture for longer storage of food.

Materials and Method
Drying of fresh mint is carried out to analyze the drying characteristics. Fresh mint was procured from the local kitchen garden, which is shown in figure 1. It was not de-stemmed and kept as a whole plant, then washed. It is slightly dried with blotting paper. The weight of sample on the sensitive scale was found to be 500 g. It was placed on the mesh trays of the solar dryer which is designed and fabricated from the locally available material. The dryer is natural convective in nature which does not require any external agency for the removal of moisture as it is provide with two vents for inlet and outlet of the warm air. It is constructed from locally available willow wood and is portable weighing 5 kg which can be lifted easily by everyone. The prototype of dryer can be seen in figure 2. The moisture content of the herb during the drying process is

Quality evaluation
The quality evaluation of any dried food products is determined by its color, brightness, texture changes, water activity, moisture content and stability over time. The parameters have certain standards which the prototype should adhere to. The quality analysis was done on the basis of following parameters:

Determination of Colour
Colour has an important role in food acceptability by the consumer, and it may influence taste, perception of sweetness, and pleasantness [10]. Hence, it is a very imperative quality factor in processed vegetable products to influence consumer acceptability. Numerous reactions take place during thermal processing which affect the colour of the product. The most common are chlorophyll pigment degradation and browning reactions such as Maillard reaction and oxidation of ascorbic acid [11].
Quality of dried mint was evaluated on basis of colour and the parameters of colour (lightness (L*), redness (a*), yellowness (b*) for the vegetables) were measured at the start and the end of the drying period, using colour difference meter. For reference white tile was used and values of L*, a*, b* were recorded for the same. According to the colour meter model, the L* measures the whiteness, ranges from (black at 0 to white at 100). The a* measures green when negative and The values of ∆E, Hue angle and chroma were calculated using the equations 1, 2, 3, 4, 5 given below. (1) The values of Chroma, hue Angle and ∆ E are summarized in Table 1.The results given in table indicates the better results for the samples dried in the solar dryer in terms of colour which is evident in the Figure 4 and Figure 5 also, hence more acceptance by the consumers.  vacuole of the cell changes in size. Properties of middle lamella and turgor pressure are the significant parameters to determine the texture of the tissue which is already present in the reference. [15,16] Water Activity determination Water Activity (aw) is one of the most important factor in determining quality and safety of food For all food stuffs there exists water activity limit below which microorganisms are prevented from growing [17]. It affects the shelf life, safety, texture, flavor and smell of foods. By dropping the value of water activity spoilage of herb can be reduced to huge extent. On performing the water activity analysis it is observed Solar dried sample of mint has the water activity value of 0.503 while it was 0.536 for open sun dried sample which is higher in comparison to solar dried sample.

Moisture Content
The water content in the herbs becomes one of the main causes for their spoilage during the course of storage. If the value of moisture content in food exceeds 10% it leads to growth of microorganisms in food [18]. Therefore it becomes imperative to trim down the value of

Rehydration Ratio
Rehydration test is done by dipping 5 g of sample in 60 ml of water separately in a beaker and covered with a watch glass. The sample is heated gradually and brought to boil. Boiling continued for 20 minutes which allow the sample to rehydrate and absorb water sufficiently. The sample is removed from the water and excess water is strained from the sample. The rehydrated mint is weighed again. The rehydration ratio is calculated using the formula given by expressed as equation 6. [19] Rehydration ratio = Wb/ Wa

Shrinkage
During drying process the loss of moisture from vegetable leads to structural collapse, shape deformation and capillary contraction [20]. This shrinkage of vegetables depends on the loss of the moisture from the cells. The shrinkage of food materials during drying is a common physical phenomenon which affects the textural quality and taste of the dried product. The shrinkage of food material depends on many factors like material characteristics, mechanical properties, micro-structure, and drying conditions as calculated by Mohiuddin [21].
The shrinkage is measured by the equations 7 and 8.
Page 11 of 17 (7) Where V = Initial Volume of the product Vo = Final Volume of the product The shrinkage percentage is measured in terms of equation (8) The observed value for the shrinkage were measured and it was found that the solar dried samples show better results in comparison to that of the open sun dried samples and shrinkage is related with the rehydration values, lesser shrinkage leads to better rehydration.

Determination of Ascorbic Acid (Vitamin C )
The dried samples were tested for the presence of Ascorbic acid, in order to find out how much of vitamin is vanished due to drying. The determination of Ascorbic acid in the fresh and the dried samples was done by 2,4-Dichlorophenol Indo-phenol dye method. The dried samples were soaked separately for few hours and ground to make a paste so that the content of vitamin will be determined perfectly. sensitive vitamin and its loss depends on the exposure to light and the duration for which it is exposed to light [22,23]

Browning Reactions
Browning in the eatables is high at water activity value of 0.5 -0.8 [24,25] which lead to the Milliard reactions in the food products. The value of absorbance of UV in the sample extract at 420 nm wavelengths is taken as a measure of non enzymatic browning. The samples of 5 g of dehydrated mint were soaked in 100 ml of 60% alcohol for 24 hrs and then filtered. Absorbance of the filtrate was recorded at 420 nm with the help of a UV-spectrophotometer using 60% alcohol as blank and the observation is expressed in terms of optical density given by [26] The analysis done revealed that the samples dried in open sun has more browning in comparison to the samples dried in Solar Dryer. The absorbance and transmittance of the sample is given in Table 2 and comparison can be seen in Figure 8. The change in color is linked with the changing nutritional value. The solubility cause degradation of flavor and induces textural changes.
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