Studying the drying process of vegetables by measuring the mass transfer coefficient

Dissertation for Master's degree (M.Sc.)

Treatment:

Chemical Engineering

Abstract

It has been less than two centuries that the preparation and distribution of food in a commercial way and gradually became modern today until Where any type of product or product can be purchased in any season, but since food is a perishable material, different methods have been created for long-term storage of food. According to the information and statistics given in Iran, due to the lack of sufficient facilities for storage, one third of the production of fruits and vegetables is lost. Drying agricultural products, especially vegetables, is one of the oldest methods of preserving food known to mankind. In fact, drying is the removal of moisture from products until they are removed from corruption and their nutritional value is preserved. Traditional methods of drying have negative effects on the quality of the product, such as increased tissue wrinkling, hardening of the product surface, adverse changes in color, aroma, taste, and reduction of its nutritional value. Also, in the field and outdoors, there is a possibility of products being contaminated with dust and pollution by insects and birds. Therefore, indirect drying has better results on products than the direct method, so it is necessary to develop indirect drying and use of industrial dryers. In this research, the effect of displacement heat transfer, temperature and vegetable type on the mass transfer coefficient in the process of drying potatoes, squash and onions by a laboratory dryer (oven) was investigated and the mass transfer coefficient was measured and calculated. The results showed that the increase in temperature increases the mass transfer coefficient and this coefficient is higher in forced displacement mode than normal and the type of vegetable is effective on the mass transfer coefficient only in natural displacement heat transfer mode. The increase in temperature and the type of displacement heat transfer have mutual effects and the type of displacement heat transfer is more effective than temperature in increasing mass transfer. In the current research, it can be stated that it is better to use forced movement with proper ventilation, which does not interfere with the design of the vegetable dryer and also uses less energy.

Key words: drying, mass transfer coefficient, vegetables

Introduction

Although the food industry, like many other sciences, started with traditional methods, today these industries are not only considered a complex science, but also play a very important role in the development of related sciences, and the fact becomes clear that in the food industry, you can no longer rely on traditional experiences, but you must use these traditions as much as possible to form a successful and advanced unit by using experts in various scientific fields. It has been less than two centuries that the preparation and distribution of food has been commercialized and gradually modernized to the point where any type of product or product can be purchased in any season, but since food is a perishable substance, different methods have been created for long-term storage of food. Fruits and vegetables are agricultural products that are rich in vitamins and have a high concentration of moisture and low fat. Although the production of fruits and vegetables has increased in the world, this increase in production is not proportional to the growth of the population and does not meet the needs of the population. The wastage of fruits and vegetables is due to the biological and biochemical activity of fresh products, exposure to unfavorable storage conditions, transportation and poor foreign markets. It is estimated that 25% of vegetables are wasted during the period when there is maximum production. And according to the information and statistics given in Iran, due to the lack of sufficient facilities to maintain, one third of the production of fruits and vegetables is lost. By reducing the moisture content in fruits and vegetables, which reduces the activity of microorganisms and fungi, their spoilage can be prevented. Microbial activities are at their maximum when the moisture content of the product is above 10%, so that the moisture content above (db) 4.13% or (wb) 4% for vegetables can cause spoilage.For this purpose, in order to preserve agricultural products and store them, we must reduce the humidity and actually dry them (up to a humidity of about 5%) to prevent attack and spoilage by the activities of microorganisms and fungi. Drying agricultural products, especially vegetables, is one of the oldest methods of preserving food known to mankind. In fact, drying is the removal of moisture from products until they are removed from spoilage and their nutritional value is preserved. One of the most important physical changes that occur in food during the drying process is the reduction of its volume. So, in most cases, the change in shape and volume is considered one of the negative features in the opinion of the consumer. Foods that are dried under optimal conditions suffer less damage and absorb water faster and more completely. Drying changes the surface properties of food. As mentioned, most agricultural products are perishable, so they must be consumed immediately or processed and stored for later use under controlled conditions, and drying is one of the preferred methods of preservation. so that each species of produce can be bought in each season. but since food decays, different methods exist to keep long the material of nutritious. on the basis of date and statistics given in Iran by reason of lack of sufficient possibilities for keeping, loss 1/3 of fruits and vegetables. Dry of produce of agriculture especially vegetables is one of the oldest methods that human know for keeping foods. In fact drying and separate humor of produce as long as is kept from decay and value of nutritious them so keep produce dry to direct methods and or traditional (by energy of solar) and indirect methods (methods of industrial and machinery). Methods of traditional of dry to quality of produce have negative effects, for example: increase of fade of tissue, get hard of surface of product, undesirable changes in color, perfume, flavor and reduction of value of nutritious its so in the farm and in open space exist probability of being contaminated of produce to bust and contamination by insect and birds so dry to indirect method than to direct method have more desirable results in produce. So it is necessary to develop dry by indirect method and use of drying of industry. in this search considered. Effect of heat transfer of dislocated, temperature and species of vegetable in mass transfer coefficient in process of drying potato, marrow, onion by drying of laboratory (Aven). And mass transfer coefficient measurement and calculation. results showed that the coefficient is increased by temperature increases, and in forced convection, it is more than natural convection. The vegetable type has no effect on coefficient in forced convection and effect on natural convection. Temperature and convection type have interaction effect and convection type has more effect on mass transfer coefficient changes than temperature. In this search can express that better is used force convection by suitable ventilation that in design drying vegetable type has not influence and so use of little energy.