Consumer demand has increased for processed products that keep more of their original characteristics. In industrial terms, this requires the development of operations that minimize the adverse effects of processing. The effect of food processing on finished product quality ultimately determines the usefulness and commercial viability of that unit process operation.
In the particular case of food drying this indicates that loss of volatiles and flavors, changes in color and texture, and a decrease in nutritional value. Furthermore, residual enzyme activity and microbial activity in dried foods are essential parameters that effect product quality and shelf life. The quality of dried foods is dependent in part on changes occurring during processing and storage. Some of these changes involve modification of the physical structure. These modifications affect texture, readability and appearance.
Other changes are due to chemical reactions, but these are also affected by physical structure, primarily due to effects on diffusivity’s of reactants and of reaction products. The most commonly examined properties of dried products can be lassie into two major categories, engineering and quality properties. The engineering properties of the dried products involve effective moisture diffusivity, effective thermal conductivity, drying kinetics, specific heat, and equilibrium moisture content. In addition there are properties related to product quality.
These properties are necessary for the determination and the characterization of the quality of dried products can be grouped into: – Thermal properties :state of product; glassy, crystalline, rubbery, – Structural properties :density, porosity, pore size, specific illume, – Textural properties :compression test, stress relaxation test, tensile test, – Optical properties :color, appearance, – Sensory properties :aroma, taste, flavor, – Nutritional characteristics : vitamins, proteins and – Reiteration properties : reiteration rate, reiteration capacity.
During the last decades, much attention is paid on the quality of dehydrated foods. The specific drying method as well as the physical-chemical changes that occur during drying seems to affect the quality of dehydrated products. More specifically, drying method and process conditions affect significantly the drying constant, color, texture, density and porosity and sorption characteristics of materials. The increasing need for producing efficiently high quality and convenient products at a competitive cost has led to the employment of several drying methods in practice.
Conventional air-drying is the most frequently used dehydration operation in food and conventional chemical industry. Dried products are characterized by low porosity and high apparent density. Significant color changes occur during air -drying, and most frequently the dried product has low sorption capacity. Microwave drying is an alternative drying method that has recently been used in food industry. Applying microwave energy under vacuum combines advantages of both vacuum drying and microwave drying as far as improved energy efficiency and product quality are concerned.
Vacuum dried materials are characterized by higher pros itty, depending on level of vacuum, and less deterioration of color and volatile aroma. Paper 31 – PAGE 1/18 enzymatic browning and thus limits the use of sulfur dioxide, increasing in this way the retention of nutrients during subsequent convective drying. Osmotic dehydration greatly affects apparent density and porosity. Freeze drying is one of the most sophisticated dehydration methods.
It provides dried products of porous structure and little or no shrinkage, superior taste and aroma retention, better reiteration properties, compared to products of alternative drying processes. However, its advantages are directly weighed against its corresponding high treatment cost. The factors that influence quality during drying have been identified briefly as in table 1 . Table 1. Factors that influence during drying (Hellman, 1992).
Chemical Physical Nutritional Browning reactions Lipid oxidation Color loss Reiteration Solubility Texture Aroma loss Vitamin loss Protein loss Microbial survival All food products deteriorate at some rate or other in a manner that depends on food type, composition, formulation, packaging and storage regime. The potential for deterioration may occur at any of the stages between the acquisition of raw materials and the eventual consumption of a finished product, and may therefore be accelerated or minimized at any of these stages also.
The complete preservation yester for food and dairy products are therefore usually multi-component in that they seldom rely on one factor alone. The major deteriorative reactions, which are the major targets for preservation, are well known and relatively few (Tablet). They include some that are essentially physical in their mode of action, some that are chemical, some that are enzymatic and some that are microbiological. When preservation fails and these reactions escape control, the consequences range broadly.
At the one extreme, these may be trivial though undesirable, such as loss of color or flavor, or texture change within a food. At the other extreme, the most serious forms of deterioration are those associated with the presence or multiplication of micro-organisms, and these range from the reactions that cause undesirable spoilage to the transmission of life-threatening diseases caused by the most hazardous of the food-poisoning, micro-organisms, such as Colostomies botulism, Salmonella, anthropogenic Escherichia coli, Listener monocotyledon. Table 2. Major Food Deterioration Reactions (Could, 1995).
Basis of reaction Physical Chemical Enzymatic Example and consequence Moisture movement, causing drying ND toughening of texture, hydration and softening of texture, aggregation Oxidation, causing oxidative rancidity, loss of color Millard reactions, causing disconsolation, change in texture Polysaccharides, causing enzymes browning Lovingness, causing oxidative rancidity Lipase, causing lipstick rancidity Protease, causing gelatin and flavor and texture changes Growth of spoilage organisms, causing quality deterioration Growth of toughening organisms, causing food poisoning Presence of infectious organisms, causing food poisoning Microbial paper 31 – PAGE 2/18 losses and other deterioration caused by browning reactions. Reactions during drying may be classified as browning reactions and nutrient losses. Moreover, there occur also structural changes, which affect quality of dried fruits and vegetables. A. CHEMICAL FACTORS 1 . BROWNING REACTIONS Browning reactions, which are some of the most important phenomena in food during processing and storage, represent an interesting research area for the implications in food stability and technology, as well as in nutrition and health. They can involve different compounds and proceed through different chemical pathways.
Browning reactions in foods are of widespread occurrence, and come evident when food materials subjected to processing or to mechanical injury. They are important in terms of the alteration of appearance, flavor, and nutritive value. Browning is considered to be desirable if it enhances the appearance and flavor of a food product in terms of tradition and consumer acceptance like in the cases of coffee, maple syrup, beer, and in toasting of bread. However, in many other instances, such as fruits, vegetables, frozen and dehydrated foods, browning is undesirable as it results in off-flavors and colors. Therefore it is important to know he mechanisms and inhibition methods of browning reactions.
Another significant adverse effect of browning is the lowering of the nutritive value of the food article. Rate of browning reactions depends on temperature of drying, pH and moisture content of the product, time of heat treatment, and the concentration and nature of the reactants. Rate increases with increasing temperature, and the increase is faster in systems high in sugar content. For moisture contents above 30 %, a decrease in reaction rate is caused by dilution, whereas below 30 %, decrease is caused by the intrinsic ability of sugars to lower water activity. Browning reactions change color, decrease nutritional value and solubility, create flavors, and induce textural changes.
There are two important forms of browning, enzymatic and non-enzymatic (Millard reactions, characterization, ascorbic acid oxidation). This color development is usually undesirable, but with knowledge of the type of reaction involved, it is easier to work out methods for controlling this change. 1 . 1. Enzymatic Browning A group of enzymes, collectively called “phenols” is responsible for browning of some fruits and vegetables, such as potatoes, apples, and banana. When the tissue is raised, cut, peeled, diseases, or exposed to any number of abnormal conditions, the color of the fruits or vegetables is changed. The injured tissue rapidly darkens on exposure to air, due to the conversion of phenol compounds to brown melanin’s.
This enzyme group includes such diverse enzymes as phonologies, casseroles, dope oxides, catecholamine, tyrosine, polysaccharides, potato oxides, sweet potato oxides, and phenols complex. Phenols is extensively distributed in plants such as roots, citrus fruits, plums, bananas, peaches, pears, melons, olives, tea, mushrooms, and others. It has a molecular weight of Paper 31 – PAGE 3/18 molecule. The freshly prepared enzyme contains cupper in the cuprous form, but it slowly oxidized to the cupric form on aging. This change thus not results in any loss of activity. Phenols in the pure form is colorless. Concentrated solutions of phenols are most stable at the neutral PH. But, heating for a short time at 60?
C inactivates enzyme. Phenols is also inhibited by substances, which form stable complexes with copper such as HAS, KEN, CO or p-monobasic acid. In plants, there are a large number of naturally occurring o -depiction compounds, which are excitable by phenols. Actually the mechanism of action of phenols on o- depiction compounds is very complicated. Since the copper is the prosthetic group of the enzyme, it has been postulated that the activity of phenols is based on the change of the copper from the cupric to the cuprous state. Simply phenols catalyst the oxidation of colorless phenol compounds into o -quinine’s, which are red to brown in color.
O-quinine’s are precursors of the brown color in cut fruits and vegetables. When they combine with amino acid derivatives, highly collared complexes forms. The initial reaction, involving the conversion of the phenol compound to the corresponding quinine, is dependent upon the presence of the phenols, its copper prosthetic group, and oxygen. Advantage may be taken of this in order to control or prevent enzymatic browning in foods. This type browning is a serious problem during the dehydration process where any injury to the plant tissue, sustained through the use of heat or through poor handling procedures, can result in phenols activation.
The enzymatic browning of foods is usually undesirable because it cuts down the acceptability of the food in question for two reasons: (1) the undesirable development of officious and (2) the formation of off-flavors. 1. 1. 1 . Control Methods of Enzymatic Browning 1. 1. 1. 1 . Heat The application of heat to the food article at a high temperature for an adequate length of time will inactivate phenols and all other enzymes present. Several problems may arise through the use of heat. The fruit or vegetable becomes cooked, and this in turn leads to unfavorable texture changes and the development of off-flavors. Such problems may occur for instance in the processing of pre-peeled potatoes, apples, pears, and peaches. There is a close relationship between imperative and time with respect to the heat-treatment of foods.
These factors themselves depend upon the amount of enzyme. It is therefore essential to control the heating time very carefully at high temperatures, so that the enzymes are inactivated w while avoiding significant changes in flavor and texture. A balance should be worked out in terms of each particular raw material and desired food product. 1. 1. 1. 2. Sulfur dioxide and culprits Sulfur dioxide and culprits, usually sodium sulfate, sodium phosphate and sodium indisputable, are the chemical inhibitors of phenols that has been used for years in the food industry. It can be applied by gaseous sulfur dioxide or dilute aqueous solutions of the culprits.
The gas will penetrate at a faster rate into the fruit or vegetable, but the culprits paper 31 – PAGE 4/18 solutions are easier to handle, as in the form of a dip in the processing plant, or as a soul pithiest. They can be used in cases where the application of heat would result in undesirable textural changes and the development of off-flavors. The internal atmosphere of the product in question must be considered when using SO 2. Apple slices, for example, have a fair amount of oxeye n in the internal tissue, which can cause browning. It is necessary, therefore, that SO 2 penetrate the entire slice, to effectively control browning. They have antimicrobial properties and also assist in preserving vitamin C.
However, their use in food material may result in an objectionable flavor and dour, or may bleach the natural color of the food. It is toxic at high levels, and can be detected organizationally. Perhaps the most serious disadvantage of using sulfur dioxide or culprits in foodstuffs is their adverse destructive effect on vitamin B or thiamine. In spite of these drawbacks, this group of phenols inhibitors is widely used in food processing, due mainly to the effectiveness and low cost of these substances. 1. 1. 1. 3. Acids This is a widely used method for controlling enzymatic browning. The acids employed are among those, which occur naturally in tissues, particularly citric, malice, phosphoric and ascorbic acids.
In general their action is to lower tissue pH and thus to decrease the rate of enzymatic browning. The optimum pH of phenols lies within the range 6 and below 3 -7, there is virtually no enzymatic activity. Citric acid, often in conjunction with ascorbic acid or sodium bushiest, has long been used as a chemical inhibitor of enzymatic browning. Cut fruit, such as peaches is often immersed in dilute solutions of these acids prior to processing. Citric acid possesses a double inhibitory effect on phenols, not only by lowering the pH of the medium, but also by chelating WI h t the copper moiety of the enzyme. A much more significant inhibitor of phenols is ascorbic acid.
It does not have a detectable flavor at the concentration used, nor does it possess a corrosive action upon metals; in addition, its vitamin value is well known. Ascorbic acid reduces the o -quinine’s formed by phenols to the original o- thyrotrophic compounds, which in turn prevents the formation of brown absences. 1. 1. 1. 4. Dehydration in sugar The fruit is partially dehydrated by reducing to 50% of its original weight by osmosis in sugar or syrup. After draining, the fruit is either frozen or dried further in an air or vacuum dryer. The sugar or syrup inhibits enzymatic browning through the complete dehydration. In addition, it has a protective effect on flavor. 1. 2.
Non-Enzymatic Browning During manufacturing process changes in the structure of derivative fruit products are produced, therefore these modify the color and final aspect of the product. Although most non-enzymatic browning in food materials is undesirable because it indicates deterioration in flavor and appearance of the product involved, the development of brown colors in some products is entirely acceptable. Examples of this are the development of brown colors in baked goods during the baking process, in beer, molasses, coffee and substitute cereal beverages, many breakfast foods, and the roasting and other forms of heat preparation of Paper 31 – PAGE 5/18 meat.
However, the brown colors developing in most other products are not main non-enzymatic reaction pathways: (I) Millard reaction, (ii) Characterization, (iii) Ascorbic acid oxidation. 1. 2. . The Millard reaction For as long as food cooked, the Millard reaction has plan yet an important role in improving the appearance and taste of foods. It has been a central and major challenge in food industry, since the Millard reaction is related to aroma, taste and color, in particularly in traditional processes such as the roasting of coffee and cacao beards, the baking of bread and cakes, the toasting of cereals and the cooking of meat. Moreover, during the Millard reaction a wide range of reaction product is formed with significant importance for the nutritional value of foods.
This can be reduced by decrease of digestibility and possibly formation of toxic and mutagen compounds, but can also be improved by the formation of antioxidant products. The chemistry underlying the Millard reaction is very complex. It encompasses not one reaction pathway but a whole network of various reactions. The Millard reaction is notoriously difficult to control. Various factors involved in food processing influence it and they can be considered as food processing variables. The Millard reaction has been named after the French chemist Louis Millard (1912) who observed the formation of brown pigments or alienation when heating a solution of glucose and glycerin. The Millard reaction is the action of amino acids and proteins on sugars.
The carbohydrate must be a reducing sugar because a free carbonyl group is necessary for such a combination. The end product is the meliorations, which are brown pigments. The mechanism of reaction has three stages: (I) Initial stage (colorless) a. Sugar-amine condensation b. Matador rearrangement (ii) Intermediate stage (colorless to yellow) c. Sugar dehydration d. Sugar fragmentation e. Amino acid degradation (iii)Final stage (highly colored) f. Aledo condensation g. Elderly-amine popularization, formation of heterocyclic nitrogen compounds. The carbonation reaction can occur in acidic or alkaline media, although it is favored under the more alkaline conditions.
A number of studies have demonstrated an increase in reaction rate with a rise in PH. The relationship between the reaction rate and pH would therefore render those foods of high acidity less susceptible to this reaction, e. G. , pickles. Formula and hydroxymethylfurfural (HIM) are the most important chemical substances produced in non-enzymatic browning processes. The HIM content is important because it indicates the degree of heating of the treated products during processing. Paper 31 – PAGE 6/18 The role of buffers in non-enzymatic browning has been shown to increase the rate of browning for sugar -amino acid systems as a result of their influence on the ionic environment in which the reaction takes place.
The temperature dependence of this reaction has been demonstrated in a number of quantitative studies, where increased rates were reported with a rise in temperature. This reaction proceeds readily in aqueous solution, although complete dehydration of the reactants results in a rapid halt in the process. Reducing sugars are essential ingredients in this reaction, providing the necessary carbonyl groups for interaction with the free x- amino groups. The reaction, itself, is not confined to inconsistencies but can also reducing sugars, however, cannot participate unless the glycoside bond is cleaved, thereby liberating its constituent reducing inconsistencies capable of entering the reaction.
The order of reactivity appears to be greater for lodestones than for lodestones, whereas reducing disaccharide exhibit considerably less activity. 1. 2. 2. Characterization This process is another example of non-enzymatic browning involving he degradation of sugars in the absence of amino acids or proteins. When sugars are treated under anhydrous conditions with heat, or at high concentration with dilute acid, characterization occurs, with the formation of anhydrous sugars. Caramels for commercial use are made from glucose syrups, but usually characterization is the result of reactions that take place when sucrose is heated. There are three stages during this process (at 200 CO), during which water is lost and first isochronal and then other anhydride are formed.
The first stage starts with the melting of sucrose, followed by foaming, which continues for 35 min. Urine this period one molecule of water, is lost from a molecule of sucrose. The foaming then stops. Shortly after this, a second stage of foaming starts which lasts 55 min. During this stage about 9% of the water is lost, and the compound formed is Carmella, a pigment with the average formula of C24H36018. Carmella melts at 138 CO, is soluble in water and ethanol, and is bitter in taste. The pigment caramel is formed during the third stage of foaming which starts after about 55 min. The formula of this pigment is C36H50025. Caramel melts at 154 CO and is soluble in water.
The main disadvantage of this reaction is the production of unpleasant, burned, and bitter products, which can arise if this process is allowed to proceed uncontrolled. This reaction may be slowed down by visibilities, which react with sugar to decrease the concentration of allowedly form. 1. 2. 3. Ascorbic acid oxidation A further mechanism appears to operate during the disconsolation of dehydrated vegetables in which ascorbic acid is involved. The formation of dehydration’s acid and technological acids from ascorbic acid is thought to occur during final stages of the drying process and is palpable of interacting with the free amino acids, enigmatically, producing the red -Tiburon disconsolation.
This reaction may involve Sticker degradation. Paper 31 – PAGE 7/18 1. 2. 4. Inhibition of non-enzymatic browning Several of factors can affect the formation of colored complexes in food products. Among these are pH, temperature, moisture content, time, concentration and nature of reactants. The rate of browning increases with rising temperature. Since these reactions have been shown to have a high temperature coefficient, lowering of the temperature during the storage of food reduces can help to minimize these processes. Reducing the moisture content through dehydrating procedures can inhibit those reactions being moisture dependent for optimum activity.
In attempting to carry out these procedures one must ensure that the dehydrated product is suitable for sale in that form, and that the product is suitably packaged so as not to permit moisture uptake during storage. Since the Millard reaction is generally favored at the more alkaline conditions, if this type of browning is involved, lowering of the pH might provide a good method of This reduces the possibility of lipid oxidation, which in turn could give rise to reducing substances capable of interacting with amino acids. While this reaction does not appear to influence the initial carbonation reaction, exclusion of oxygen is thought to effect other reactions involved in the browning process. Chemical inhibitors have been used to advantage in limiting browning reactions during the production and storage of a variety of foods.
Among those widely used are culprits, visibilities, tools, and calcium salts. Culprits proved successful in controlling a variety of browning processes. Visibilities inhibit the conversion of D-glucose to S- hydrometer- formula, as well as the conversion of ascorbic acid to formula by completing through the reducing group. Consequently the formation of formulas is blocked, thus preventing the production of the colored pigments. They can also block the carbonyl group of the reducing sugars involved in the carbonation reaction. Calcium chloride was reported to be a possible inhibitor of browning. Its inhibitory effect is due to the chelating of calcium with the amino acids.
Although the various inhibitors discussed can prevent to varying degrees of success browning room occurring, it is important to realism that the nutritional value of the foods could still have been seriously reduced. The initial stages of the Millard reaction, for example, the carbonation reaction could still have rendered the amino acids unavailable even though no browning is visible during this stage. However to be certain that this stage is the one inhibited is extremely difficult to ensure. 2. LIPID OXIDATION Lipid oxidation is responsible for rancidity, development of of -flavors, and the loss of fathomable vitamins and pigments in many foods, especially in dehydrated foods.
Factors that affect oxidation rate include moisture content, type of substrate (fatty acid), extent of reaction, oxygen content, temperature, presence of metals, presence of natural antioxidants, enzyme paper 31 – PAGE 8/18 activity, ultraviolet light, protein content, free amino acid content, other chemical reactions. Moisture plays an important part in the rate of oxidation. The elimination of oxygen from foods can reduce oxidation, but the oxygen concentration must be very low to have an effect. The effect of oxygen on lipid oxidation is also closely related to the product porosity. Freeze -dried foods are more susceptible to oxygen cause of their high porosity. Air-dried foods ten to have less surface area due to shrinkage and thus are not as affected by oxygen. Minimizing the oxygen level during processing and storage, and addition of antioxidants as well as sequestrates, have been recommended in the literature to prevent lipid oxidation. 3.
COLOR LOSS The color of foods is dependent upon the circumstances under which food is viewed, and the ability of the food to reflect, scatter, absorb, or transmit visible light. Drying changes the surface characteristics of food and hence alters the ref elasticity yellow vegetables. Chemical changes to cartooned and chlorophyll pigments are caused by heat and oxidation during drying. In general, longer drying times and h Geiger drying temperatures produce greater pigment losses. Oxidation and residual enzyme activity cause browning during storage. This is prevented by improved blanching methods and treatment of fruits with ascorbic acid or sulfur dioxide.
Many studies indicate that the bulk of carotene destruction occurs during storage rather than as a result of the dehydration process. Pigment retention in dried foods decreased as temperature and moisture increased. Thus it was found that the beet segments were most stable in the powders, then slices, and least stable in solution. The natural green pigment of all higher plants is a mixture of chlorophyll a and chlorophyll b. The retention of the natural greenness of chlorophyll is directly related to the retention of magnesium in the pigment molecules. In moist heating conditions, the chlorophyll is converted to epiphytic by losing some of its magnesium. The color then becomes an olive green rather than a grass green.
The interaction of amino acids and reducing sugars (Millard reaction) occurs during conventional dehydration of fruits. If the fruits are sculptured, enzymatic browning can be inhibited, and the Millard reaction retarded. There are certain different methodologies for analyzing the color. The most common methods are the ERG (red, green, blue), LAB (lightness, redness-greenness, wholesomeness’s) and EX. scales that analyses the color into three parameters, so that each composite color can be easily quantified by a set of three numbers. B. PHYSICAL FACTORS 1. Reiteration, Shrinkage and Food Porosity Reiteration is a complex process aimed at the restoration of raw material properties when dried material is contacted with water.
Pre-drying treatments, subsequent drying and reiteration per SE induce many changes in structure and composition of plant tissue, which result in impaired reconstitution properties. Hence, reiteration can be considered as a paper 31 – PAGE 9/18 measure of the injury to the material caused by drying and treatments preceding dehydration. Reiteration of dried plant tissues is composed of three simultaneous processes: the inhibition of water into dried material, the swelling and the leaching of soluble. It has been shown that the volume changes (swelling) of biological materials are often proportional to the amount of absorbed water. It is generally accepted that the degree of reiteration is dependent on the degree of cellular and structural disruption.
There are a large number of research reports in which authors measure the ability of dry material to reheated. The ratio between the dry material mass and water mass varies from 1 :5 to 1 :50, temperature of reiterating water is from room temperature to boiling. Time of reiteration varies from 2 min. To 24 h. The degree to which a dehydrated sample will reheated is influenced by structural and chemical changes caused by dehydration, processing conditions, sample preparation, and sample composition. Reiteration is maximized when cellular and found that freeze-drying causes fewer structural changes and fewer changes to product’s hydrophilic properties than do other drying processes.
Most of the shrinkage occurs in the early drying stages, where 40 to 50 % shrinkage may occur. To minimize shrinkage, therefore, low-temperature drying should be employed so that moisture gradients throughout the product are minimized. Many drying techniques or pre-treatments given to food before drying are aimed at making the structure more porous so as to facilitate mass transfer and thereby speed drying rate. Porous sponge-like structures are excellent insulating bodies and generally will slow down the rate of heat transfer into the food. Porosity may be developed by creating steam pressure wit hint the product and a case hardened surface through rapid drying.
Porosity also can be developed by whipping or foaming a food liquid or puree prior to drying the porous product has the advantages of quick solubility or reconstitution and greater volume appearance, but the disadvantages of increased bulk and generally shorter storage stability because of increased surface exposure to air, light, etc. 2. Solubility Many factors affect the solubility, including processing conditions, storage conditions, composition, pH, density, and particle size. It has been found that increasing product temperatures is accompanied by increasing protein denomination, which decreases solubility. A low bulk density is required for good digestibility of non-fat dry milk. It was found that particle agglomeration, which increases particle size, increased sensibility.
However, some scientists found that larger particles were less soluble. This was attributed to the longer drying time required to dry large particles. Thus more protein was denatured and solubility decreased. This shows that the heat treatments as well as the particle size must be considered when determining solubility. Paper 31 – PAGE 10/18 3. Texture Texture is one of the most important properties connected to product quality. Changes to the texture of solid foods are an important cause of quality deterioration. Factors that affect texture include moisture content, composition, variety, pH, product history (maturity), and sample dimensions.
The chemical changes associated with textural changes in fruits and vegetables include crystallization of cellulose, degradation of pecti