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Stability of Vitamin C in Onion During Drying and Storage-a Kinetic Approach

Md. Masud Alam
Senior scientific officer
Spices Research Center, Bangladesh Agricultural Research Institute, Shibganj, Bogura

Md. Nazrul Islam
Professor
Department of Food Technology and Rural Industries, Bangladesh Agricultural University, Mymensingh

Abstract/ Executive Summary:

The study was conducted to determine vitamin-C stability in mechanical and solar dried summer onion during storage at different temperatures. The effect was studied in terms of drying time (15, 30, 45, 60 and 75 minutes) and temperature (52⁰C, 60⁰C, 68⁰C) for mechanical drying (MD) and 45-50⁰C for solar drying (SD). Increased loss of vitamin C was observed with the increase in drying time and temperature. It was found that the reaction rate constant, K value (/min) was the highest (0.0302/min) for solar drying (45-50⁰C) and was followed by that at 68⁰C (0.029/min) and 60⁰C (0.0276/min), while the lowest k-value (0.0236/min) was given by the sample dried at 52⁰C. Vitamin C losses in onion was the highest during solar drying followed by mechanical drying at 68°C and 60⁰C, while the lowest losses of vitamin C was found for mechanical drying at 52°C. It was also seen that rate constant has an exponential type relationship with inverse absolute temperature and activation energy; Ea for degradation of vitamin C was found to be 3.20 kcal/g-mole for onion. The highest rate constant (0.0046/day) was given by dried osmosed onion stored at RT which was successively followed by dried sulphited onion stored at RT (0.004 /day), and dried sulphited onion stored at RFT (0.0033/day). On the other hand, dried osmosed onion stored at RFT gave the lowest (0.0029/day) rate constant during the period of storage (one year).

Keywords: Vitamin C, Food stability, Onion, Drying, Temperature

Location: Spices Research Centre, BARI, Bogura

Start Date: 01-07-2010

End Date: 30-06-2011

Program Area: Postharvest and Agro-processing

Commodity: Onion

Objectives:

To determine the amount of vitamin C and it’s stability in mechanical and solar dried osmosed and non osmosed summer onion during storage periods at different temperatures.

Methodology:

Ten gram sample was blended and homogenized in a blender with 3% metaphosporic acid solution. The homogenized liquid was transferred to a 100 ml volumetric flask and adjusted 100 ml with metaphos-phoric acid solution. Content of the flask was then thoroughly mixed and filtered. Then 5 ml of the ali-quot was taken in a flask and titrated with 2-6 dichlorophenol indophenol dye. The dye had been stan-dardized with vitamin C solution to find an equivalent dye factor. The ascorbic acid content of the samples was calculated from the following formula (Rangana, 1986).

mg vitamin C/ 100g = TDV₁ / V₂W ×100------------------------------------------ (i)

Where,

T= Titre

D= Dye factor

V₁ = Volume made up

V₂ = Aliquot of extract taken for estimation

W = Weight of sample taken for estimation

Analysis of experimental data

According to Heldman (1974), any reaction which is typical in nature will occur at a rate dependent upon several factors, whether the reaction is the conversion of sucrose to glucose and fructose or the rate at which some component (such as vitamin C or thiamin) of a food is reduced in concentration by heat. The rate of the reaction is indicated by a rate constant (k) and can be described by the following general equation:

dc /- dt = KC ^m.................................................. (ii)

Where C represents the component concentration at any time (t) and m represents the order of reaction. Although many reactions may be of zero order, the first order reaction is described by the following equation is common in food products:

dc /- dt=KC ............................................. (iii)

In this particular type of reaction, the reaction rate is directly proportional to the concentration of the reacting substance (c). The application of a first order reaction equation is more evident if the equation (iii) is solved and expressed in the following form (Heldman, 1974):

ln C /C_o = –Kt

ln C = ln C_o –Kt............................................................................................. (iv)

Diffusion co-efficient and activation energy for diffusion of water during drying are available for a number of products. In general, the relationship between the diffusion co-efficient and product temperature can be represented by the Arrhenius type of equation (Heldman, 1974).

Source of Information: Bull. Inst. Trop. Agr., Kyushu Univ., 38: 39-46, 2015

Article Link (Online Journal):

Funding Source:

1. Government

Budget:

Total Budget (Tk.) :

Achievement/Findings:

During mechanical, solar drying and osmosed dehydration, Vitamin C, oxidation is accelerated by air and heat. The more is the heat transfer, the more is vitamin C degradation. Furthermore, prolong drying also increases the vitamin C degradation and results in lower retention. Loss of vitamin C may vary due to their variation in composition of food products. Therefore, dehydrated onion (sulphited, osmosed and without pretreatment) have great potential to use throughout the year for several types of food preparations like canned soup, salads, hamburgers, pizzas and other fast food preparations.

Knowledge Management: Journal