Intelligent Control in Drying

Alex Martynenko, Andreas Bück, Alex Martynenko, Andreas Bück

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9781498732758
• eBook ISBN: 9780429811319
• Topic: Technology & Engineering
• Subtopic: Industrial & Technical Chemistry
• Index: Technology & Engineering

Despite the available general literature in intelligent control, there is a definite lack of knowledge and know-how in practical applications of intelligent control in drying. This book fills that gap. Intelligent Control in Drying serves as an innovative and practical guide for researchers and professionals in the field of drying technologies, providing an overview of control principles and systems used in drying operations, from classical to model-based to adaptive and optimal control. At the same time, it lays out approaches to synthesis of control systems, based on the objectives and control strategies, reflecting complexity of drying process and material under drying. This essential reference covers both fundamental and practical aspects of intelligent control, sensor fusion and dynamic optimization with respect to drying.

Section II

Applications of Intelligent Control in Drying

12	Automatic Control of Apple Drying with Respect to Product Temperature and Air Velocity Barbara Sturm

CONTENTS

12.1 Introduction

12.2 Heat Sensitivity of Plant Materials

12.2.1 Physical and Physico-Chemical Changes in Cell Components

12.2.2 Changes in the Nutritional Value and Associated Color Changes

12.2.2.1 Vitamins

12.2.2.2 Phenolic Compounds

12.2.2.3 Proteins

12.2.2.4 Sugars

12.2.2.5 Aromatic Components

12.2.2.6 Thermal Degradation of Pigments

12.2.3 Enzymatic and Nonenzymatic Browning

12.3 Quality Characteristics of Dried Products

12.3.1 Mechanisms of Color Changes

12.3.2 Shrinkage Behavior

12.4 Application of Product Temperature Measurement and Control

12.4.1 Drying Kinetics and Development of Temperatures

12.4.2 Impact of Process Control on Resulting Product Quality

12.4.3 Color Changes and Shrinkage

12.4.3.1 Color

12.4.3.2 Shrinkage

12.4.4 Stepwise Temperature Changes Based on Phase Transition Information Retrieved from Product Temperature

12.4.5 Technological Implementation

12.5 Conclusions

References

12.1 INTRODUCTION

Drying significantly increases the shelf life of easily perishable agricultural products. At the same time, however, it negatively impacts the content of valuable components (e.g., vitamin C, pigments, polyphenols) as well as organoleptic (e.g., mouth feel, smell, taste, and visual appearance) and structural characteristics (Crapiste 2000; Sturm et al. 2014). The extent of these changes usually correlates directly with the process and product temperatures. Fortunately, in convective drying, particularly in the first drying period, a product temperature is significantly lower than the process temperature due to the difference between dry- and wet-bulb temperatures. Critical temperature can be defined for core components of each product. To preserve these valuable components, the product temperature during drying should not exceed the critical one. The majority of convective drying applications account for this constraint by setting process temperatures that are low enough to not severely damage the products.

Equally, if the goal is to deactivate enzymes such as polyphenol oxidase (PPO), which catalyze unwanted reactions, exceeding the critical temperatures for these components can be used to minimize enzymatic browning and other unwanted reactions. Thus, product temperature is one of the most crucial factors regarding product quality and its preservation.

Commonly, product temperature during processing of foodstuffs (also technical products) is unknown and assumed to be constant after an initial heating phase. In the past, the lack of measurement of product temperature was mainly due to the high costs of noninvasive sensors for its detection. At present, affordable IR sensors with sufficient accuracy are available on the market, thus integration of product temperature into automatic control becomes possible. However, in industrial practice, and even to some extent in scientific research, the role of product temperature and dynamics of its development throughout the process has not been recognized as critical factor.

Several studies on a variety of commodities have shown that high process temperatures can be used at the beginning of the drying process without risk of damaging the product (Chua et al., 2001; Schultz et al., 2007; Sturm et al., 2014). In conventional drying applications, process settings are frequently based on experimentally found values, which were determined decades ago and tend to be a single set point or at best one-step changes, not accounting for the actual product temperature or changes in the product throughout the process. This negatively impacts product quality, processing times, and also the process energy efficiency (Mujumdar, 2007).

Over the last couple of decades, the need for the development of dynamic control systems for drying applications has been recognized (Mujumdar and Wu, 2008) and substantial research has been conducted on stepwise or periodical changes of drying conditions (Chua et al, 2001; Martynenko and Yang, 2006; Schultz et al., 2007; Cuervo Andrate, 2011) and the benefits of changing processing conditions on the product quality has been proven. However, almost exclusively, the determination of the optimum point for changing the processing conditions has been iterative rather than based on the product temperature and/or quality characteristics. Thus, active product temperature control is a promising alternative to standard procedures that could simultaneously increase product quality and process performance.

12.2 HEAT SENSITIVITY OF PLANT MATERIALS

Most biological products are sensitive to heat. Shape, size, and arrangement of macroscopic and microscopic elements are often subjected to significant changes. Further, heat can alter colloidal parts and composition of molecules. Initially, the distribution of components within the raw material is balanced. Because of water removal during drying, this balance is not maintained due to the movement of components and their concentration. In combination with high temperatures and/or long processing times, this leads to a number of biochemical, chemical, and physical changes in the product, which are detrimental to its quality, for example, loss of valuable components, cell rupture leading to degradation of pigments and other components, as well as reduced rehydration capability (Timoumi et al., 2007; Santos and Silva, 2008; Miranda et al., 2009). The degradation rate is increased by the concentration of soluble components, in addition to the influx of oxygen into the increasingly dry product.

Understanding the mechanisms and processes that lead to the product damages described earlier is essentially important for the development of drying strategies which will enable high retention of desirable components. Further, the knowledge of critical temperatures for different reactions is necessary to ensure minimal losses. Additionally, in the case of certain substances, such as enzymes involved in polyphenol oxidation, maximal destruction of the component is desirable to maintain product quality.

12.2.1 PHYSICAL AND PHYSICO-CHEMICAL CHANGES IN CELL COMPONENTS

In plant materials, most of the reactions affecting product quality occur in the cell walls and the cytoplasm (Bai et al., 2002; Lewicki and Pawlack, 2003; Mayor et al., 2005). The goal of the drying process is to preserve initial cell structure and intact cytoplasm. If the process is carried out incorrectly (e.g., product temperature is too high or the process is too long), the cell structure is destroyed during drying. As a consequence, the intercellular area is increased, which prevents the product from returning to its original structure by rehydration and swelling. The change in the swelling capacity, which does not necessarily reflect water holding capacity, predominantly impacts the sensory attributes of the product. The texture turns spongy and the taste watery. Crystallization of polysaccharides (starch, sugars) exhibits a similar impact on the water holding capacity of the cells and, thus, the sensory characteristics of the rehydrated product (Rowland, 1980).

12.2.2 CHANGES IN THE NUTRITIONAL VALUE AND ASSOCIATED ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Acknowledgments
• Introduction
• Editors
• Contributors
• SECTION I Basics of Intelligent Control
• SECTION II Applications of Intelligent Control in Drying
• Index