Tomato tablet drying enhancement by intervention of infrared - A response surface strategy for experimental design and optimization

Highlights

• Compressed tablets made of tomato powder were studied.

• Effects of hot air-infrared drying method on tablet production was studied.

• Physical, mechanical, chemical and thermal properties of tablets were considered.

• Tomato tablet making from powder was optimised.

• The highest percentage of vitamin C degradation occurred during the drying process.

In this study, the effects of hot air-infrared drying methods, including inlet air temperature, air velocity, and infrared power, are investigated on physical (color indices ΔL∗, Δa∗, and Δb∗, shrinkage, and unit density), mechanical (resistance to penetration), chemical (concentration of vitamin C and lycopene content), and thermal (effective moisture diffusivity coefficient) properties of compressed tablets made of tomato powder. The drying processes of the samples were investigated at five temperature levels of 40, 50, 60, 70, and 80 °C, five infrared power levels of 0 (without infrared), 500, 1000, 1500, and 2000W, and five inlet airflow velocity levels of 0.5, 1, 1.5, 2, and 2.5 m/s. Statistical analysis of the obtained data and optimization of the drying process was performed using the response surface methodology. The results showed that in hot air-infrared drying, with increasing the inlet air temperature, the concentration of vitamin C in tomato tablets decreased, and the lycopene content of tomato tablets increased. The optimum point for drying tomato tablets was obtained at an air temperature of 40 °C, air velocity of 2.5 m/s, and infrared power of 876.5 W. The highest percentage of vitamin C degradation occurred during the drying process of tomato slices at 50 °C. The results showed that drying at lower temperatures increased the desirability index of the model obtained from the response surface method.

Introduction

Tomato (Lycopersicum esculentum) is one of the most important vegetables in Asia and Africa, with a yield of about 70%. China is the largest producer of tomatoes in the world, with a production of about 62.8 million tons (FAO, 2019). Tomato is rich in human nutrients and a good source of carotenoids (especially lycopene), vitamins (C, E, and B9), beta-carotene, flavonoids, phenolic compounds, potassium, protein, and dietary fiber. Lycopene causes the red color in fruits and vegetables, including tomatoes, and is the most abundant carotenoid in tomatoes. Regular consumption of tomatoes reduces the risk of various types of cancer (especially prostate cancer) and cardiovascular disease (Martí et al., 2018).

The high moisture content of tomatoes leads to short shelf life and high storage costs of this vegetable. Therefore, to reduce these costs and increase shelf life, providing appropriate processing methods for this product is of great commercial importance. Drying tomatoes is one of the most widely used methods to process and preserve this vegetable. Tomato powder has a special market among dried tomato products. Powder production is a good approach to increase food storage duration. The advantages of tomato powder include better management in packaging, easy transportation, easy mixing with other ingredients in food processing plants, and high commercial value (Liu et al., 2010). Tomato powder is very dry, moisture absorber, and bulky. These features lead to increased storage, packaging, and transportation costs. A unique method to overcome these problems is concentrating tomato powder in tablet form. This method reduces the sample moisture content (MC) through reducing its volume, as well as its contact surface with the surrounding air. This also reduces the chance of chemical reactions and nutrient degradation (oxidation). Powder-to-tablet conversion is widely used in the pharmaceutical industry, but its use in the food industry is less common or is limited to the early stages of development (Aziz et al., 2018). So far, research has been reported on the production of tablets from pita, mango, guava fruit powders, as well as the production of diet tablets containing various fruit powders, including dates, spirulina, and tangerines (Adiba et al., 2011; Yusof et al., 2012; Zea et al., 2013; Ong et al., 2014).

Regarding the scarcity of research conducted on the production of diet tablets, the study of parameters affecting the tablet-making process is of particular importance. The properties of the tablet are highly dependent on its production processes, including the preparatory process (peeling the fruit, pulverizing, and moistening or steaming the powder), the tablet-making process, drying, and cooling (Ghasemi et al., 2018). In the literature review, various tablet-making devices, including single-axis, multi-axis, or press simulators, have been used to perform the tablet-making process. Compression by a die and uniaxial press is widely used in tablet-making studies because of its simplicity in execution and data collection. The basic principles of all tablet presses are typically the same, i.e., filling the matrix with powder, squeezing it with a pin, releasing the pressure, and finally removing the tablet from the matrix. MC, adhesion, density, flowability, particle size, and shape of fruit powder are important parameters that strongly affect its compression behavior. For the safe storage of the tablet, it is desirable to reduce its MC to less than 12% on a wet basis (Aziz et al., 2018). This process improves the production and durability of tablets and prevents the collapse of the tablet during handling and transportation (Mahapatra et al., 2010). Drying is one of the most important post-production steps to prevent spoilage of wet compressed tablets and maintain their quality. Previous studies have shown that the drying operation of wet compressed tablets also has a significant effect on their properties (Ghasemi & Chayjan, 2019b).

Drying is an important process for transporting raw materials to prolong their lifespan by enzymatic contraction and limiting microbial growth (Chayjan et al., 2014). Unlike hot air drying, infrared (IR) radiation generates internal heat through the molecular vibration of materials and causes fast vibration and molecular absorption of specific radiation and energy wavelengths. Therefore, dried foods receive electromagnetic wave energy directly, leading to the reduction of their energy consumption. Regarding the efficiency of IR radiation in the drying of food and agricultural products (Siriamornpun et al., 2012), a combination of hot air convection and IR radiation can be a suitable approach in drying of food products. IR radiation combined with hot air is an effective way to increase the drying rate and prevent uneven heat and mass transfer compared to conventional drying methods (Chayjan et al., 2014). Simultaneous use of IR technology with fluidised bed technology (hot air) in the drying of agricultural products has many benefits such as reduced drying duration, high energy efficiency, high quality of dry products, and uniform distribution of product temperature (Nuthong et al., 2011).

Various methods, such as the response surface method (RSM), are used to obtain the optimal conditions of a process. This method examines the effect of independent variables on quality indicators and process optimization. RSM is a set of statistical techniques and methods of data collection that can optimise the factors effective in a process on response variables by mathematical and statistical modeling of the variables (Sumić et al., 2016).

Choosing the best drying approach of the tablet has the least negative effect on the physical, mechanical, and chemical properties and the active ingredients of its powder. To the best of the authors’ knowledge, no research has been reported about the optimization of hot air-IR dryer in the drying process of the tablets produced from tomato powder in terms of the physical, mechanical, thermal, and chemical parameters. Overall, the objectives of this study are two-fold. First, we investigate the effect of independent variables (inlet air temperature, inlet air velocity, and IR lamp power) on response variables (lycopene content, vitamin C concentration, color changes, shrinkage, unit density, penetration resistance, and effective moisture diffusivity coefficient (D_eff)) during the drying process of tomato tablets by the hot air-IR method and providing experimental models for predicting response variables. Second, it is tried to find the optimal drying conditions for tablets produced from tomato powder with a hot air-IR dryer using the RSM.