Pretreatments, dehydration methods and packaging materials: effects on the nutritional quality of tomato powder: a review

Pretreatments and drying are commonly used before drying tomatoes to inactivate enzymes, improve the drying process, and improve the quality of dried tomato powders. In this review, the effects of different pretreatments (osmotic solutions), dehydration methods, and packaging materials on the quality attributes of tomato powder were summarized. These include pretreatments and osmotic agent solutions (potassium metabisulfite, calcium chloride, sodium metabisulphite, ascorbic acid, citric acid, sodium chloride, and sodium benzoate), thermal blanching (steam blanching and hot water) and non-thermal-processes-like-freezing, sulfuring, etc. and drying methods (oven, sun, and indirect solar dryer). The tomato powders were dried to preserve, store, and transport them. Drying implies not only physical changes, which the consumer can easily detect through visual inspection but also chemical modifications. These are responsible for alterations in color, flavor, and nutritional value, which compromise the overall quality of the final tomato powder. Maximum lycopene, vitamin A, and C contents were found in freeze-dried and direct sundried than samples dried using other methods at low drying temperatures. Freeze driers showed in keeping the nutritional quality of tomato powder with a combination of different pretreatments. Different pretreatments including osmotic agent solutions have their own merits and demerits for the final tomato powder. To overcome the drawbacks of nutritional quality, non-thermal pretreatment categories may be a better alternative to thermal blanching, and more fundamental research is required for better design and scale-up.

Tomato (Lycopersicun esculentum L.) is in the family Solanaceae. After carrots, lettuce, and onions, tomatoes are the world’s 4th most popular fresh vegetable. The production of tomatoes is growing dramatically in the world its consumption. According to the data provided by FAOSTAT [1], world produced 182,301,395 tons of tomatoes in 2017. To achieve this production, almost 5 million hectares were used. However, China, India, and the USA are the top countries dominating the production of tomatoes. Tomato is considered one of the most important vegetables produced in commercial agriculture because of the income generated from export. Moreover, tomatoes contribute to a healthy, well-balanced diet and are rich in carotene, vitamin B, ascorbic acid (vitamin C), and other nutrients that are valuable for human growth and health.

Domestic production of tomato concentrate in Ethiopia offers attractive investment opportunities, with the existing producers unable to meet the ever-growing local demand. Urban population growth in Ethiopia is about 4% while GDP has been growing by more than 7% per annum for the last few years. Ethiopia’s tomato processing sector represents the untapped market potential for export to regional, European, and Middle Eastern Markets. Regionally, tomato is one of the commodities with the most potential, especially as tomato concentrate is the most commonly used ingredient in African cooking, Europe is facing change in the tomato industry with the decoupling of subsidies in European countries, resulting in increased costs for domestic production of tomatoes. Europe’s are number one importer of tomato concentrate, and Italy imports the majority from the USA, Spain, and China. Ethiopia has an advantage over the USA and China due to its geographic proximity, availability of land, and low labor costs. UAE imports $USD 41.6 m worth of processed tomatoes, of which $USD17.9 m is sourced from China alone. This is almost double the value of Ethiopia’s export of raw tomatoes. There is potential for Ethiopia to capture some of this market share.

Tomato is highly perishable in its natural state after harvest due to its high moisture content and high rate of metabolic activities; hence, it is prone to high postharvest losses. Fresh tomatoes are difficult to preserve due to their high moisture content leading to wastages and losses during harvesting and storage, especially in sub-Saharan Africa. Losses in tomato production are also accrued to poor postharvest handling practices. Therefore, the prevention of these losses and wastage is paramount, especially in developing countries like Ethiopia whose populace is all year-round heavy tomato consumers and there is a subsequent imbalance in demand and supply at the harvesting off-seasons. The term drying usually refers to the operation by which the moisture present in a material evaporates because of heat and matter exchange between the product and the working medium. Drying is one of the most common preservation methods for extending the shelf life of tomatoes by reducing the water content to a level so as to prevent the growth and reproduction of microorganisms and to inactivate many of the moisture-mediated deteriorative reactions [2]. Tomatoes are usually subjected to physical or chemical pretreatment before drying to shorten the drying time, reduce energy consumption and preserve the quality of products [3]. The drying rate and quality of products do largely relate to the pretreatments carried out before the drying process [4].

The most antique and traditional consists of placing agricultural products on beaten earth, floor covering, or floor exposed to the sun. Although sun energy-based methods present economic advantages, being for this reason largely used in tropical countries, the product quality parameters and food safety-related issues become often difficult to monitor and control. Osmotic, convective, fluidized bed, ohmic, microwave, vacuum, or freeze-drying techniques have been applied for tomatoes dehydration. The foremost used drying techniques promote water vaporization from a food product by using heat through conduction, convection, and radiation, being the formed vapor subsequently removed through forced air.

The demand for dried tomatoes is increasing rapidly both in domestic and international markets with a major portion being used for the preparation of convenience food. And the reason for preparing dehydrated tomato powder also concerns the ease of transportation handling and storage without extra care. If powder can be prepared then it will help to reduce wastage, and price and increase the availability of powdered tomatoes throughout the year [5]. The dehydrated tomato powder can also be used as a substitute for a raw tomato to develop new food recipes. The quality of dehydrated tomato powder was influenced by storage conditions including packaging material during the storage period, and subsequent storage of product in metalized poly-ester bags is suggested to protect the product against light, oxygen, and humidity and retard the quality changes of tomato powder during the storage period [6].

The drying process has also a crucial role in the chemical composition and nutritional value of final dried tomatoes. Chemical modifications subjacent to drying include Maillard reactions, vitamin degradation, lipids oxidation, color changes, and flavor losses. To prevent or minimize these alterations and maintain as high as possible the nutritional similarities with the fresh product, the tomatoes are often submitted to treatments before the drying process. Tomatoes are commonly subjected to various chemical and/or physical pretreatments prior to thermal drying to shorten the drying time, reduce energy consumption and preserve the quality of products. In this review paper, the authors try to provide an overview of the effects of different pre-treatments, dehydration methods, and storage materials and conditions on the physicochemical, sensory, and storage stability of tomato powder.

Tomato productions

The tomato is a warm-season crop. Temperatures of 20 °C - 25 °C are considered ideal for tomato cultivation, and tomatoes develop an excellent quality red color at temperatures of 21°C - 24 °C. Due to intense heat (temperature above 43 °C), the plants get burnt, and flowers and small fruits also fall, whereas less than 13 °C and greater than 35 °C decreases the fruits and the red color production ratio.

The tomato plant is a vine that grows approxi-mately 180 cm above the ground. The plant is a dicot that grows in the form of a series of branching stems. The terminal bud is responsible for the actual growth. The vines are covered with short, fine hairs that turn into roots on coming in contact with the ground. Most of the plants have compound leaves while some have simple ones. The fruit of the plant is classified as a berry and is the part that is consumed. The fruit bears hollow spaces that are laden with seeds and moisture.

Tomato has been consumed since ancient times. The Aztecs of South America used the fruit in their dishes as per evidence. By about 500 BC, the tomato was already being cultivated in southern Mexico and a few other areas. The tomato plant was probably first introduced to Europe by the Spanish conquistador Hernan Cortes. Soon, it became a popularly cultivated crop across Europe and was also introduced to other parts of the world by European explorers and colonists.

Tomato is grown practically in every country of the world in outdoor fields, greenhouses, and net houses. China, India, the United States, Turkey, Egypt, Iran, Italy, Spain, and Brazil are the world’s leading tomato producers. China, the leading producer of tomatoes, accounted for 31% of the total production. In China, tomatoes are widely cultivated in open fields or plastic tunnels. In 2014, tomatoes accounted for 23% of total fresh vegetable output in the European Union. Of this, more than half was produced in Spain, Italy, and Poland. It covers approximately 4.73 million hectares and produces 163.96 million tons globally [7]. After potatoes and onions, it is the world’s third-largest vegetable crop. Tomatoes are a vital vegetable crop in terms of both income and nutrition. Its fruit contains vitamins like A and C and antioxidants in abundance quantity. Tomato demand remains nearly constant throughout the year due to the unique properties contained in its fruit.

Nutritional and health importance of tomato

Tomatoes contain numerous phytochemicals, the most well-known of which is lycopene. In addition, other carotenoids (e.g., β-carotene, phytoene, phytofluene), phenolics (e.g., coumaric and chlorogenic acids, quercetin, rutin, and naringenin), moderate amounts of the antioxidant vitamins and trace elements selenium and zinc, some sulfur compounds and other individual substances are present (Table 1). Carotenoids are found in a wide variety of vegetables and fruits, but lycopene is more concentrated in tomatoes, guava, rosehip, watermelon, and pink grapefruit. Lycopene is a carotenoid pigment that is primarily responsible for the deep red color of ripe tomato fruits and tomato products. It is absorbed in the human body and is one of the most common circulating carotenoids. Other tomato carotenoids may also be bioavailable for our bodies. Many factors influence the bioavailability of lycopene and other carotenoids, including the nature of the food matrix, thermal processing, and the presence of fat. Of the phenolics, naringenin from tomatoes has been shown to be bioavailable, but data on other phenolics are lacking. Tomatoes are high in vitamins such as vitamin C and vitamin A equivalents (in the form of -carotene), as well as vitamin E, folic acid, potassium, and other trace elements.

Globally, considerable research is being conducted into the health benefits of lycopene. It is a strong antioxidant; antioxidants neutralize free radicals, which can harm cell components (e.g., DNA, protein, lipids). It could also have a variety of other modes of action. The strongest scientific evidence is for the role of lycopene in reducing the incidence of prostate cancer. Lycopene may also aid in the prevention of other cancers and cardiovascular diseases, as well as play a role in eye health. There has been less study of the role of other tomato phytochemicals. β- Carotene is an important precursor of vitamin A and, like lycopene, may play a role in cancer prevention. Flavonoids also have anti-allergic, anti-inflammatory, antimicrobial, and anti-cancer properties. The yellow jelly surrounding tomato seeds may help prevent heart attacks, strokes, and blood vessel problems by preventing platelet aggregation.

Osmotic dehydration of tomato

Concerns about the prevention or minimization of tomato quality degradation during the drying process have received increased attention in recent years. Tomatoes contain a diverse range of phytochemicals, including vitamins, minerals, antioxidants, pigments, and other bioactive compounds that are thought to protect against cardiovascular disease, cancer, and age-related degenerative changes. However, some nutrients are degraded by heat during drying, affecting the quality and acceptance of the final tomato powder product [8]. To improve the retention of these important antioxidant compounds, pretreatments such as osmotic dehydration prior to drying are desired.

Osmotic dehydration involves the immersion of material in a hypertonic solution (mainly sugar or salt) for several hours. It has been used as a pre-treatment for tomato drying because it reduces drying time, resulting in cost savings and improved sensorial properties of the final product. During osmotic pretreatment, the plant’s cellular structure acts as a semi-permeable membrane, allowing for countercurrent mass transfer: the solute flows into the products while moisture is transferred from the interior to the hypertonic solution [9]. The osmotic pressure difference between the food material and the hypertonic solution is the driving force of water removal from the food material to the osmotic solution [10]. Osmotic dehydration of foods is the partial removal of water caused by the pressure created when the product comes into contact with a hypertonic solution of solutes (sugar, salt, or both), resulting in a decrease in food water activity (Figure 1). Osmotic dehydration removes 10% – 70% of the water from fruits and vegetables at room temperature without causing phase changes, providing an alternative method for reducing drying time and mitigating the thermal effects of drying on bioactive compounds [9,11].

Variables such as variety, maturity, pretreatments, osmotic agent temperature and concentration, the geometry of the material, agitation, food pieces to osmotic solution ratio, additives, physicochemical properties, and structure all influence mass transfer kinetics during osmotic dehydration [12].

The concentration of osmotic agents plays an important role in osmotic dehydration. Increased solution concentration resulted in an increase in the osmotic pressure gradients and higher water loss. The increase in solute concentrations during extended osmotic treatment causes an increase in water loss and solid gain rates [13]. The solution-to-sample ratio is another important parameter that affects osmosis. The change in ratio affects the mass transfer during osmosis up to a certain limit. The solution-to-sample ratio should be chosen wisely so that the driving force for the removal of the moisture exists till the end of the process. The driving force decreased to the release of water when osmotic solutions become dilute. As dehydration progresses, the osmotic solution becomes increasingly dilute, acting as a driving force for further water drop release [14].

Because of the increase in cell permeability with respect to process temperature, the temperature is the most important variable influencing the kinetics of mass transfer during osmotic dehydration [15]. The effect of temperature is more pronounced between 30 °C to 60 °C for fruits and vegetables on the kinetic rate of moisture loss without affecting solid gain. Initially, the water loss and solid gain increase temperature increase up to 50 °C depending upon the fruit and variety and later on falls sharply becoming nearly constant at 60 °C which indicated a negligible increase in the rate of sucrose diffusion above 60 °C. Since water loss is higher at a higher temperature, the osmotic equilibrium is achieved by the flow of water from the cell rather than by solid diffusion.

The duration of osmotic dehydration also affects the dehydration process of fruits and vegetable drying processes. Increased immersion time results in greater moisture loss during osmotic dehydration [16]. In general, weight loss increases with treatment duration, but the rate at which it occurs decreases. The treatment time can be selected in such a way that the amount of water removal is maximum with no appreciable uptake of solids. The sample weight-to-solution ratio is critical during the osmotic treatment of fruits and vegetables, and it influences mass transfer kinetics to some extent. Many researchers worked on the influence of different sample-to-solution ratios (1:1 to 1:5) on mass transfer kinetics. A higher ratio of 1:10 to 1:60 was used to avoid medium dilution caused by water gain and solute loss. As a result, the osmotic drying force decreases [12].

Because the use of highly concentrated viscous sugar solutions causes major problems such as floating food pieces hindering contact between food material and the osmotic solution, causing a reduction in mass transfer rates, agitation or stirring can be used to enhance mass transfer during osmotic dehydration [13]. Different chemical treatments (Osmotic dehydration methods) have been applied before the drying process of tomato, in order to minimize nutrient losses and thus improve the nutritional quality of dried tomato powder (Table 2). The most popular/repeated pretreatment chemicals (osmotic solutions) used for tomatoes by different researchers are: Potassium Metabisulphite (KMS), Calcium chloride (CaCl₂), Sodium metabisulphite (SMS), Ascorbic Acid (C₆H₈O₆), Citric Acid (C₆H₈O₇), Sodium chloride (NaCl) and Sodium Benzoate, are individually or by mixing them in different ratio example ascorbic acid with citric acid, KMS with CaCl₂ and with different concentrations, etc.

Effects of different pretreatments on tomato powder quality

To minimize adverse changes during drying and subsequent storage, tomatoes were pre-treated with chemicals before drying. Some quality attributes of tomatoes were affected by pretreatment, including total solids, lycopene, dehydration ratio, rehydration ratio, and color. Here below (Table 3) tried to show the effects of different chemical pretreatments on the quality of tomato powder.

Dehydration methods

Drying is the oldest method of food preservation. Throughout history, the sun, wind, and a smoldering fire have all been used to remove water from fruits and vegetables. Food dehydration is defined as the process of removing water from food by circulating hot air through it, preventing the growth of enzymes and bacteria. Although food preservation is the primary reason for dehydration, dehydrating fruits and vegetables reduce the cost of packaging, storing, and transporting the final product by reducing both its weight and volume. Given the improvement in dehydrated food quality, as well as the increased emphasis on instant and convenience foods, the potential of dehydrated fruits and vegetables is greater than ever. Dried fruits and vegetables are high in fiber and carbohydrates and low in fat, making them healthy food choices. Because dried fruit contains more carbohydrates than fresh fruit, serving sizes are typically smaller.

Dried or dehydrated fruits and vegetables can be produced by a variety of processes. These processes differ primarily by the type of drying method used, which depends on the type of food and the type of characteristics of the final product. In general, dried or dehydrated fruits and vegetables go through the following stages: pre-drying treatments like size selection, peeling, and color preservation; drying or dehydration using natural or artificial methods; and post-dehydration treatments like sweating, inspection, and packaging.

Several drying methods are commercially available and the selection of the optimal method is determined by quality requirements, raw material characteristics, and economic factors. There are three types of drying processes: sun and solar drying; atmospheric dehydration, which includes both stationary or batch processes (kiln, tower, and cabinet driers) and continuous processes (tunnel, continuous belt, belt-trough, fluidized-bed, explosion puffing, foam-mat, spray, drum, and microwave-heated driers); and sub-atmospheric dehydration (vacuum shelf, vacuum belt, vacuum drum, and freeze driers).

Sun drying refers to foods that are dried under direct sun. Sun drying is the traditional method of drying food because it makes use of direct solar radiation as well as the natural movement of air, ambient air temperature, and relative humidity. This process is slow and requires continuous care, the food must be protected from insects, covered at night, and cannot be used in rainy periods. As a result of the long drying time and the inability to control the process’s conditions and parameters, the final product’s quality is poor. However, because it is the cheapest drying method and requires no special equipment or energy consumption, it is appropriate for developing countries with suitable weather for the process. The disadvantages include total reliance on the elements and moisture levels no lower than 15% to 20%. Solar drying utilizes black-painted trays, solar trays, collectors, and mirrors to increase solar energy and accelerate drying.

Atmospheric forced-air driers artificially dry fruits and vegetables by passing heated air with controlled relative humidity over or through the food to be dried, and this is the most widely used method of fruit and vegetable dehydration. Sub-atmospheric (or vacuum) dehydration occurs at low air pressures and includes a vacuum shelf, vacuum drum, vacuum belt, and freeze driers. The main purpose of vacuum drying in ambient conditions is to eliminate moisture at temperatures below the boiling point. Vacuum driers are used for drying raw materials that may deteriorate as a consequence of oxidation or may be chemically changed as a result of exposure to air at extreme temperatures due to their high installation and operating expenses. High taste retention, maximum nutritional value retention, minimal damage to product texture and structure, little change in product form and color, and a finished product with an open structure that permits fast and thorough rehydration are all advantages of freeze drying. High capital investment, high processing costs, and the requirement for special packaging to prevent oxidation and moisture gain in the completed product are all disadvantages Table 4.

Packaging, storage stability, and sensory quality of tomato powder

Processing and preservation refer to a collection of physical, chemical, and biological procedures used to extend the shelf life of food while preserving its color, texture, flavor, and, most importantly, nutritional value. Food preservation is achieved by destroying enzymes and microorganisms using heat (blanching, pasteurization), or preventing their action by removal of water, increasing acidity, or using low temperatures. During tomato season, enormous amounts of tomatoes are condensed into tomato paste, which is then reconstituted into goods like tomato sauce, ketchup, and other value-added items [44]. Drying is also another way of extending the postharvest shelf life of tomatoes. Pizza, diverse veggies, and spicy recipes all use dried tomato products as significant ingredients [45].

Color fading and acceptability loss are common in dehydrated tomato products, owing to lycopene isomerization and oxidation. Drying processes, pre-drying treatments, and storage conditions, including packaging material, all influenced lycopene levels in dried tomato powder. Dehydrated and powdered tomatoes, in general, have low lycopene stability unless adequately processed, rapidly packed, and stored in optimal storage conditions. Various studies showed that significant oxidative damage can occur during the storage of dried tomatoes. The general result of shelf-life studies is that dried tomatoes can experience significant lycopene degradation; degradation reactions are accelerated by high temperature, oxygen, and light exposure, as well as low moisture content and water activity (aw).

Dried vacuum-packed tomatoes present the following major changes during their shelf life: a) Vitamin degradation; which occurs through a variety of mechanisms, such as hydrolysis under the action of light, heat, or acid; direct oxidation by oxygen or by participation in other oxygen reduction reactions. b) Changes in colors; which occur due to a large number of different reactions, especially oxidation of carotenoids. c) Sensory changes; In tomato-based products, color is one of the main quality parameters. The darkening of the product to a reddish-brown is due to the oxidation of carotenoid pigments and the formation of dark compounds, in addition to the browning effect of the Maillard reaction. These changes are dependent on storage temperature, oxygen availability, packaging type, pH, and product activity Table 5.

Dried tomato-based food products and market values

Tomato juice, tomato puree, tomato ketchup, tomato chutney, tomato sauces, tomato powder, tomato ready-to-eat goods, tomato paste, and instant tomato soup are all examples of tomato value-added products. Tomato soup is normally consumed for its smooth texture and provides an instant satiety effect. Tomato soups on the market are often made by dry blending of ingredients, with tomato powder and thickening agent making up the majority of the recipe. The rheological characteristics and color features of tomato soup are crucial factors in determining consumer acceptance. Color is frequently determined by the level of lycopene breakdown during processing and binding arrangements with other molecules of soup, whereas flow behaviors are usually affected by ingredients and the temperature of the soup (Barry-Ryan, 2011a, b, 2012).

Globally, tomatoes are graded as an essential agricultural crop and an indispensable part of the daily human diet. Despite the fact that tomatoes are consumed fresh, sauces, powder, juice, and ketchup account for 80% of total tomato production. Tomato powder is made by dehydrating fresh tomatoes to create a fine tomato powder. The market for tomato powder has been developing at a modest rate with considerable growth rates over the previous few years, and it is expected to rise significantly in the anticipated period 2021 to 2028. Tomato powder is one of the most prominent ingredients in fast food products. As a tastemaker and flavoring component, tomato powder is becoming more popular in these products. This will help the market expand. The worldwide tomato powder market report offers a comprehensive analysis of the industry. The research provides a thorough examination of key segments, trends, drivers, restraints, the competitive landscape, and other important market aspects.

Tomato powder is the most skillful way of storing dehydrated tomatoes. Tomato powder is a unique substitute for tomato juice; tomato sauce and paste add flavor to recipes. Tomato powder has a wide range of applications in the food and beverage industry due to its rich flavoring quality. The growing desire for healthy and natural ingredients in the food industry has led to items like tomato powder, which contains a high amount of vitamins A, C, and K, becoming more popular as an ingredient in packaged foods. Tomato powder is a dried powder made from tomatoes that are used as an ingredient in a variety of culinary and beverage products. The main element fueling the market growth of tomato powder is the growing demand for natural constituents in food products and drinks. Moreover, the increase of the application markets such as infant nutrition, bakery and confectionery, beverages, and convenience food products is also boosting the market growth. Furthermore, dried tomato powder grants a widespread shelf life as compared to fresh tomatoes; thus, tomato powder is gaining demand as a proper replacement for fresh tomatoes. These factors have positively anticipated propelling the growth of the global tomato powder market.

Bakery and confectionery, dairy and frozen sweets, beverages, newborn nutrition, sweet and savory snacks, curries, gravies, and soups are among the applications for tomato powder. The market is divided into bakery and confectionery, dairy and frozen desserts, beverages, newborn nutrition, sweet and savory snacks, curries, gravies, soups, and Others, depending on the application. The Curries, gravies, and soups segment holds the largest market share during the forecast period. The need for this segment is being fueled by the multiple properties that allow it to be used as a flavoring, coloring, and aromatic element [52].

Combining various drying technologies with improved pretreatments prior to drying yields the best results in terms of both tomato powder quality and environmental effect. Recent research has focused on the application of pretreatment technologies for drying intensification in order to improve traditional drying performance in terms of product quality and energy savings. As a result, the correct drying processes and mathematical process optimizations can cut energy usage, and operational expenses, and produce higher tomato powder quality. Thermal drying methods (direct sun dryer, indirect solar dryer, hot air) have considerable negative effects on shrinkage, color, texture, and final powder quality, but they are cost-effective. Pretreatment is a frequent operation performed on tomato powders before drying to accelerate the drying rate, preserve quality, and reduce microbial burden. Diverse pretreatment techniques reviewed here, of them, have merits and demerits. Due to migration from the tissue into an osmotic solution, osmotic agent dehydration reduced the initial water content, drying time, and energy consumption, but was averse to tomato powder quality (such as loss of minerals, vitamins, and pigment components). Pretreatments that involve dipping tomatoes in various chemicals offer the benefit of speeding up the drying process and maintaining food quality; nevertheless, residues in the food may pose a food safety risk. Future research needs on the effects of diverse pretreatments with different drying methods on the quality of tomato powder.

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