Handbook of Drying of Vegetables and Vegetable Products

Min Zhang, Bhesh Bhandari, Zhongxiang Fang, Min Zhang, Bhesh Bhandari, Zhongxiang Fang

• Publisher: CRC Press
• Year: 2017
• Print ISBN: 9781498753869
• eBook ISBN: 9781351648714
• Topic: Technology & Engineering
• Subtopic: Industrial & Technical Chemistry
• Index: Technology & Engineering

This handbook provides a comprehensive overview of the processes and technologies in drying of vegetables and vegetable products. The Handbook of Drying of Vegetables and Vegetable Products discusses various technologies such as hot airflow drying, freeze drying, solar drying, microwave drying, radio frequency drying, infrared radiation drying, ultrasound assisted drying, and smart drying. The book's chapters are clustered around major themes including drying processes and technologies, drying of specific vegetable products, properties during vegetable drying, and modeling, measurements, packaging & safety.

Specifically, the book covers drying of different parts and types of vegetables such as mushrooms and herbs; changes to the properties of pigments, nutrients, and texture during drying process; dried products storage; nondestructive measurement and monitoring of moisture and morphological changes during vegetable drying; novel packaging; and computational fluid dynamics.

Section IV

Others (Modeling, Measurements, Packaging, and Safety of Dried Vegetables and Vegetable Products)

15 Vegetable Dryer Modeling

Robert H. Driscoll and George Srzednicki

The University of New South Wales

CONTENTS

1. 15.1 Introduction
   1. 15.1.1 Vegetables
   2. 15.1.2 Vegetable Preservation
   3. 15.1.3 Properties of Air
      1. 15.1.3.1 Absolute Humidity
      2. 15.1.3.2 Relative Humidity
      3. 15.1.3.3 Density
      4. 15.1.3.4 Enthalpy
2. 15.2 Thin-Layer Drying Curves
   1. 15.2.1 Important Regions in the Drying Curve
   2. 15.2.2 Falling Rate Period
   3. 15.2.3 Summary of Drying Rates
   4. 15.2.4 Pressure Drop
   5. 15.2.5 Residence Time
   6. 15.2.6 Effects of Main Parameters on Drying Kinetics
      1. 15.2.6.1 Air Temperature
      2. 15.2.6.2 Air Relative Humidity
      3. 15.2.6.3 Air Speed
      4. 15.2.6.4 Product Composition
      5. 15.2.6.5 Product Surface
      6. 15.2.6.6 Product Thickness
3. 15.3 Theoretical Predictions of Drying Behavior of Vegetables
   1. 15.3.1 Empirical Models of Product Drying
   2. 15.3.2 Modeling Drying at Surface
   3. 15.3.3 Modeling Drying within Product
   4. 15.3.4 Constant vs. Changing Conditions
      1. 15.3.4.1 Two-Layer Model
   5. 15.3.5 Finite Element/Finite Difference Models
      1. 15.3.5.1 Finite Difference Method
      2. 15.3.5.2 Finite Element Methods
      3. 15.3.5.3 Summary of Modeling Methods
4. 15.4 Modeling of Specific Dryers
   1. 15.4.1 Modeling of Dryers
      1. 15.4.1.1 The Main Equations
      2. 15.4.1.2 Direction of Air Flow
      3. 15.4.1.3 Batch Dryer or Continuous Dryer?
      4. 15.4.1.4 Type of Product Used?
   2. 15.4.2 Hot Air Drying
      1. 15.4.2.1 Kiln Dryer
      2. 15.4.2.2 Tray Dryer
      3. 15.4.2.3 Tunnel Dryers
      4. 15.4.2.4 Belt Dryers
   3. 15.4.3 Solar Dryers
   4. 15.4.4 Fluidized Bed Dryers
   5. 15.4.5 Heat Pump Dryers
   6. 15.4.6 Spray Dryers
   7. 15.4.7 Sublimation Drying
5. 15.5 Conclusions
6. References

15.1 INTRODUCTION

15.1.1 VEGETABLES

The daily availability of safe, fresh vegetables is essential to meet people’s nutritional needs for healthy living. The annual production of vegetables worldwide exceeded 10⁹ tonnes in 2010 (FAO, 2012). People who live on land where they can grow their own produce can be self-sufficient, and if their land area or farm is large enough, they may sell the produce to local people or to inhabitants of towns and cities. When produce is grown, harvested, and consumed on the day of harvesting, there is no need to apply any postharvest technologies. As farms are now located hundreds of kilometres away from the cities, it may take hours or days to transport them to the markets. As most vegetables are highly perishable, once they have been harvested, their nutritional quality and sensory characteristics may rapidly deteriorate. As opposed to grain crops, with very few exceptions, vegetables are harvested prior to the onset of senescence and dormancy, which makes them even more susceptible to deterioration.

Vegetables are mostly composed of water (generally over 80%), with carbohydrates (starch and sugars) being the second most abundant constituents, followed by dietary fibers including celluloses, hemicelluloses, pectin, and lignin. The latter group of compounds is essential for humans as it plays a significant role in the movement of food in the guts. Vegetables are also a major source of vitamins such as vitamin C and provitamin A (β-carotene) and also bioactive compounds, such as polyphenols (e.g., anthocyanins), which may act as antioxidants and thus protect the consumer from cancers or cardiovascular diseases. Last but not least, vegetables are an important source of minerals such as iron, potassium, and calcium.

Because of their importance in human nutrition, the production of vegetables on a commercial scale is constantly increasing, and thus a larger amount is to be harvested, handled, transported, and also preserved from deterioration. Because of their high moisture content after harvest, most vegetables are highly perishable. Although a large proportion of vegetables is consumed fresh, the seasonal nature of the production and the fact that large amounts are harvested at the same time result in the need to extend their shelf-life by using various methods of preservation and converting them into a more stable form. There are various ways of preserving them such as freezing, pickling, canning, or drying.

Drying belongs to the oldest ways of preserving food developed by human civilization. Its purpose is to reduce the moisture content to levels so as to prevent the dried product from deterioration due to microbial and biochemical activity. The safe levels of moisture are defined by the water activity of the dried product (a_w), which should be below 0.6 for storage of most of the dried products. The dried vegetables can be stored at ambient temperature. Other advantages of drying are the reduction in volume and weight of the dried products. This leads to the reduction in packaging, transportation, and storage cost.

Various drying techniques are used including use of high or low temperature, osmotic dehydration, low pressure, ultrasound, use of oxygen-free atmosphere, and many others. The minimization of the energy consumption in drying leads to increased use of renewable energy as a source of heat for dryers operating above ambient temperatures. The selection of the most suitable drying method depends on the type of the biological material, its price, the amount of material to be dried, and also on the capital and operating cost of the drying equipment.

Various parts of plants are consumed as vegetables.

Vegetables do not represent any particular botanical grouping; however, they can be grouped into three main categories: seeds and pods; bulb, roots, and tubers; flowers, buds, stems, and leaves. As the morphology and chemical composition of the above mentioned groups varies, the drying process will also be affected by the category to which a given vegetable belongs.

Plant parts are still alive after harvest and maintain respiration by using stored reserves of energy until, eventually, they senesce or ripen. They can be divided into vegetative and reproductive organs. The derivation of the various vegetables consumed by people is indicated in Figure 15.1.

FIGURE 15.1 Plant parts consumed as vegetables (APEC, 2008).

A. Flower bud. Example: artichoke

B. Stems. Fleshy stems are important vegetables. Examples: bamboo shoots and asparagus.

C. Seeds and seed pods.

• The seeds are small embryonic plants, usually with some stored food.
• Formation of seed starts with pollination of the flower. The embryo is developed from the zygote and the seed coat from the integuments of the ovule.
• After fertilization, the endosperm becomes the food.
• Plant seeds used for food include rice, legume seeds (peanuts, peas, and beans), corn, and coconut.
• Some seeds are consumed immature such as sweet corn and peas. Beans and peas (snow peas) are usually eaten when seed pods are fleshy.

D. Axillary buds. Example: brussels sprout.

E. Petioles. Example: celery.

F. Bulbs. Examples: onion and garlic.

G. Stem tubers and rhizome. Potato is a well-known example of a stem tuber, and ginger is an example of rhizome that is derived from swollen stem tissue.

H, I. Roots.

• The major function of roots is to anchor or support the aerial parts of the plant and to absorb water and inorganic nutrients from the soil.
• In many plant species the tap root functions as a vegetative reproductive organ that can maintain the species through winter or dry conditions until favorable growing conditions returns. These roots store large amounts of carbohydrates and usually have low rates of respiration. Typical examples are carrots and radishes (H). Sweet potatoes are swollen and...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Editors
• Contributors
• Section I Drying Processes and Technologies
• Section II Drying of Specific Vegetable Products
• Section III Changes in Properties during Vegetable Drying
• Section IV Others (Modeling, Measurements, Packaging, and Safety of Dried Vegetables and Vegetable Products)
• Index