Effect of foaming agent concentration and drying temperature on biochemical properties of foam mat dried tomato powder

The purpose of the study was to optimize the effective drying conditions and different foaming agent concentrations on the biochemical properties of foam mat dried tomato powder. Foaming was achieved by using egg albumin as foaming agent and sodium salt of Carboxymethyl Cellulose (CMC) as foam stabilizer with different concentrations. Drying was achieved by using different drying temperatures. The changes in different physicochemical properties of foam mat dried powder viz. total soluble solid (TSS), pH, ascorbic acid, titratable acidity, β-Carotene, DPPH radical scavenging activity were observed. Foams were prepared from different concentrations of egg albumin (3-7% w/w) and sodium salt of Carboxymethyl Cellulose (1%, 0.5%). The drying temperatures were varied from 60°C to 70°C. The drying time changes with different drying temperatures. It was found that the drying time decreased with the increased foaming agent concentrations as well as with higher drying temperature. It takes almost 13 hrs for drying at 70°C. TSS and pH content was increased with the increase of foaming agent concentrations and foam stabilizer’s concentrations but decreased with the increasing temperatures. Ascorbic acid decreased with the increase of foaming agent concentrations and temperatures but increased with CMC concentration. Titratable acidity content of foam mat dried tomato powder was decreased with the increasing foaming agent concentrations but increased with the higher temperatures and decreased CMC concentration. β-Carotene contents increased with the increase of foaming agent concentrations but decreased with the increase of temperatures and foam stabilizer concentration. DPPH free radical scavenging activity increased with the increase of foaming agent, foam stabilizer and temperatures. Based on the maximum retention of physicochemical properties, the optimum treatment of foaming agent was found to be 7% egg white + 1% CMC at 60°C.


Introduction
Tomato (Solanum lycopersicum M.) is an important vegetable crop owing to its economical and nutritive significance belonging to the Solanaceae family. Tomato is taken not only the fresh state but also process it into different types of food such as juice, puree, sauce, and canned (Kadam and Balasubramanian, 2011). Tomato is a great source of essential nutrients, which increases its acceptance besides its aesthetic appeal (Abushita et al., 1997). With the increasing demand, tomato production in Bangladesh increases every year. In 2015-2016 ,Bangladesh produces almost 3,68,121 metric tons tomato and in 2016, it increases to 3, 88,725 (BBS, 2017. Moisture content in tomato is almost 93-95% which increases its perishability and makes a great difficulty in its storage and transportation (Shan et al., 2016). To overcome post-harvest losses of tomato, it is processed into different types of products to increase their shelf life and availability all the year round (Clinton, 1998;Kadam and Balasubramanian, 2011;Affandi et al., 2017). The most common method of preservation is drying tomato in the form of powder, which can be reconstituted into juice and different tomato-based products like ketchup, sauce, chutney, soups etc. can be prepared (Kadam and Balasubramanian, 2011).
Drying is a process of simultaneous heat and mass transfer, which consists removal of water or another solvent from material by evaporation. Moisture in a food converts into vapor by radiation, convection, conduction heat transfer and those vapors are removed by employing forced air. Removal of moisture brings changes in the quality of the product as well as its nutritive value (Mujumder and Devahastin, 2006;Zhang et al., 2006;Varastegani et al., 2017). Generally, drying reduces water activity in food to a certain level which makes it difficult for micro-organisms to sustain and grow and increases its shelf life (Zzaman et al., 2014;Zzaman et al., 2021). Drying also reduces the weight and volume of food by removing moisture, which reduces the cost of transportation and makes the material easy for handling (Doymaz, 2007;Kadam et al., 2009). Currently, for fruit powder production, different drying methods such as drum drying, freeze drying, spray drying and foam-mat drying are being used. The high temperature used in drum drying process causes loss of nutritional quality and creates unwanted odors in the final products (Nindo and Tang, 2007). Freeze drying and spray drying give good quality products having good rehydration ability and the color of the final products are also good. Due to high-cost production, these processes are used only for premium quality products (Ratti, 2001;Hsu et al., 2003).
Foam mat drying is a drying process where a liquid food product like fruit juices or semi-solid food products like vegetable puree, cereal pastes are whipped with an edible foaming agent with/without foam stabilizer to make stable foam. Then the stabilized foams are placed in a tray in a uniform layer followed by drying. The dried product is then scraped off and converted into fine powders by grinders and sealed immediately to stop moisture absorption (Ratti and Kudra, 2006;Rajkumer et al., 2006). Compared to other drying processes, it is less expensive, less complicated, less time consuming as well as retains better products quality than other drying techniques (Febrianto et al., 2012). Because of being the simplest forms of drying, foam-mat dried products gain consumers' attention in recent years. Foamed material takes lower drying time than non-foamed materials and in the final stages of drying. Many investigators found that the drying rate is enhanced because of the increased interfacial area of foamed materials . Foam mat dried powder is rich in biochemical composition than non-foam dried powder. Powder from foam-mat drying is almost similar to the fresh sample in color, flavor and in taste (Kandasamy, 2012). This study was carried out to select the effective foaming agent and foam stabilizer concentrations for foam-mat dried tomato powder based on biochemical properties.

Materials
Fresh ripen tomato was purchased from Modina Market, Sylhet, Bangladesh and used as raw materials. Egg white was used as a foaming agent purchased from Modina Market, Sylhet. Sodium salt of carboxymethyl cellulose (CMC) CAS No.9004-32-4 (food grade) of Dalian Chem was used as a foam stabilizer. The whole study was conducted at the laboratory of Food Engineering and Tea Technology, Shahjalal University of Science and Technology, Sylhet-3114.

Methodology
Tomato was washed with running tap water to remove unwanted external material such as dust, clay etc. Then, they were cut with a stainless steel knife and passed through a Juice Extractor (Panasonic MJ M-176P) to make tomato juice. The working flow sheet for the preparation of foam mat tomato powder is shown in Figure 1. Then the juice was pasteurized at 85°C temperatures for 3 mins. Before the formation of foam, the biochemical analysis of fresh tomato juice was performed in order to identify the relative quantity loss of total soluble solids (TSS), pH, ascorbic acid, titratable acidity, beta carotene, DPPH in tomato juice powder in contrast with the fresh juice. Foaming was achieved by adding foaming agent and foam stabilizer in different concentrations at a whipper. Egg white was used as a foaming agent and sodium salt of carboxymethyl cellulose in different concentrations. An amount of 300 mL tomato juice was taken for each sample along with the selected concentrations of egg albumin (3%, 5%, 7% v/v) and sodium salt of carboxymethyl cellulose (0.5%, 1%). Then the mixers were whipped at the whipper for 5 mins at maximum speed. The foamed tomato juice was then poured in a tray (stainless steel) and placed in an oven dryer at 60°C, 65°C , and 70°C for drying. After drying, the dried tomato juice was ground to form powder. Then the tomato powder was reconstituted. The reconstitution of powder was done by the method of Goula and Adamopoulos (2010) with few alternations. For reconstitution, 2 g of powder was mixed with 50 mL distilled water in a 100 mL glass beaker in room temperature and the mixture was agitated with the vortex at high speed. The reconstituted sample was used to determine the amount of TSS, pH, titratable acidity, ascorbic acid, beta carotene, DPPH radical scavenging assay.

Determination of biochemical properties 2.3.1 Total soluble solids (TSS)
Estimation of TSS was done with a Brix scale hand refractometer. For this, 1-2 drops of the reconstituted sample were placed between the prism and small covering plate of the refractometer. A shadow line forms between the brightened and the dark area in the refractometer shows the reading, where the shadow line crosses the scale.

Ascorbic acid
Ascorbic acid content in reconstituted foam mat dried tomato powder was determined following the method of Moazzem et al. (2019). About 5 mL sample was taken and made up to 50 mL with 3% HPO 3 . After that filtration was done using Whatman No. 2 filter paper. Approximately, 10 mL of filtrate was taken with 10 mL HPO 3 titrate it with the 2,6 dichlorophenolindophenol until it turned into pink color endpoints and the color persists in the solution for almost 15 s. Approximately, 5 mL standard ascorbic acid solution with 5 mL 3% HPO 3 was taken in a beaker and then filled the burette with dye solution and titrate the mixture solution with the dye solution until it turned into pink color which persists for almost 15 s. Dye factor in mg of ascorbic acid per mL of the dye factor is calculated by using the following formula: Dye factor = 0.5/titre.

pH determination
The pH of reconstituted foam mat tomato dried tomato was performed by a digital pH meter (HI-2211, USA). The pH meter determines the amount of hydrogen -ion in a water-based solution, which shows its acidity or alkalinity figured as pH (Encyclopedia Britannica Online, 2016).

Titratable acidity
Titratable acidity of reconstituted foam mat dried tomato powder was determined as percentage of anhydrous citric acid. For this, the aliquot of the sample was diluted with water to make a fixed volume. Then it was titrated with 0.1 N NaOH and as an indicator phenolphthalein was used. The percent acidity was calculated by using this formula:

Beta-carotene content
The β-Carotene of reconstituted foam mat dried tomato powder was determined according to the method of Biswas and Chatli (2011). In a test tube, 1 g of reconstituted tomato juice was added with 5 mL chilled acetone. Then it was shaking occasionally for 15 mins. Then it was vortexed in a vortex (Model-VM2000, Taiwan) for 15 mins at high speed. Then it was centrifuged for 10 mins with a centrifuge (Model-416G, Gyrozen, Korea) at 1370 rpm. The supernatant was then separated in a test tube. Again, this process was run once more with the remaining compounds with the addition of 5 mL chilled acetone. The supernatant was then filtered with a Whatman No. 1 filter paper. Then, 0.025 g of standard β-Carotene was mixed with 5 mL acetone and kept in a dark place for 10 mins. The absorbance of extract and standard solution of β-Carotene was measured by using UV-Vis spectrophotometer at 449 nm wavelength (Model-1800, Shimadzu).

DPPH radical scavenging activity
The DPPH radical scavenging activity of reconstituted sample was determined following the process of Kalantzakis et al. (2006). To make methanol extracts, 2.5 g tomato powder was dissolved in 10 mL of 70% methanol in a test tube. The mixture was then vortexed to mix well by a high-speed vortex meter. Then the mixture was centrifuged at 3500 rpm with a centrifuge (Model-416G, Gyrozen, Korea) for 15 mins. After that, the mixture was filtered with Whatman No. 1 filter paper and then the filtrate was shifted into a glass bottle. The extracted sample was kept in the freezer for further analysis. After that, 1 mL of extracted sample was taken in a test tube with 4 mL DPPH solution and shaken. Then the sample was let stand for 30 mins at a dark place. Then after 30 mins the absorbance was read at 515 nm at a UV-Vis spectrophotometer. DPPH solution except extract was taken as control. Percentage of DPPH radical scavenging activity was calculated from the following equation:

Total soluble solids
Total soluble solids of different samples at different drying temperatures are presented in Table 1. For fresh tomato juice, TSS was found to be 4.5°Brix and in reconstituted foam mat dried tomato powder it was varied from 3.84±0.04 to 4.10±0.03°Brix. It was found that TSS content in dried tomato powder raised with the increasing concentration of foaming agent as well as foam stabilizer concentrations because of some inherent component in egg white. TSS content was decreased with the increase of temperature because of the reduction of some heat-sensitive components presents in the powder. Similar types of results were noticed by other researchers for foam mat dried tomato powder , and alphonso mango powder (Rajkumer et al., 2007).

Ascorbic acid
Ascorbic acid (vitamin C) is a heat-sensitive watersoluble vitamin. As food processing largely depends on heat processing so the determination of ascorbic acid in processed food is a must for those who are rich in vitamin C in fresh condition. The results of this study showed that ascorbic acid degradation was largely influenced by drying temperature. The ascorbic acid content of fresh tomato juice was 13.524 mg/100 mL and in reconstituted foam mat dried tomato powder it was within the range 2.35±0.02 to 3.00±0.03 mg/100 mL. The result showed that with the rise of temperature the ascorbic acid content decreased in Table 2. This indicated heat-sensitive ascorbic acid destroyed with the rise of temperature. Demiray et al. (2013) found that hot air drying of tomato reduces the ascorbic acid content significantly at high temperature. Similar types of results were also found by other researchers for passion fruit aril (Khamjae and Rojanakorn, 2018), pulses (Mehta et al., 2007), muskmelon (Fernandez et al., 2007), foam mat dried mango powder (Kadam et al., 2010), and onion (Kadam et al., 2009) following heat treatment.

pH determination
The pH of a product usually inversely related to drying temperature. The pH of fresh tomato juice was found to be 4.59 and in reconstituted foam mat dried tomato powder pH ranges from 4.47±0.04 to 4.71±0.04. Both at very high, as well as very low concentration of foaming agent and foam stabilizer concentrations the pH, were significant at 95% confidence level. From Table 3, it is found that pH rose with the raise of foaming agent and foam stabilizer concentrations because of the alkaline behavior of egg albumin which is used as the foaming agent. The pH value in egg albumen is high (almost 9 in the alkali range) which raised the pH of the reconstituted tomato powder with the addition of albumen . A rise in temperature raised molecular vibrations and a reducing aptitude of forming hydrogen bonds raises [H + ] which reduces the pH in the reconstituted powder.

Titratable acidity
Titratable acidity of fresh tomato juice was 0.346% and in reconstituted foam mat dried tomato powder ranged from 0.37±0.01 to 0.39±0.03% (Table 4). It is found that with the raise of foaming agent concentration and temperature, titratable acidity was decreased in reconstituted powder. Statistically, the highest foaming FULL PAPER agent and foam stabilizer concentration, as well as a lowest foaming agent and foam stabilizer concentration, were significant (p < 0.05) at all temperature ranges. As low acidic egg albumin was used as a foaming agent it reduces the acidity of formed powder. The higher drying temperature raised the molecular vibrations which reduce the formation of hydrogen bonds hence increases the acidity of tomato powder. However, the changes in CMC concentration didn't substantially affect the acidity of foam mat tomato powder.

Beta-carotene content
The β-Carotene is important provitamin A which is water-soluble and heat-sensitive. The β-Carotene content of fresh tomato juice was 3.13 mg and in reconstituted foam mat dried tomato powder it ranges from 1.89±0.02 to 2.31±0.02 mg ( Table 5). The high processing temperatures significantly degrade β-Carotene in tomato powder. The foaming agent and foam stabilizer concentration exhibited statistically significant difference at different concentration among all temperature range. The β-Carotene content decreased due to its heatsensitive nature and increased with increasing protein concentration. The findings of the present study are in good harmony with Muratore et al. (2008) and Auisakchaiyoung and Rojanakorn (2015) who informed that degradation of β-carotene was attributable to drying temperature in cherry tomato and dried Gac aril, respectively. Rajkumar et al. (2006) also found similar results for foam mat dried mango pulp and reported that it may be due to increase in the surface area caused by increasing in foaming agent concentration and thus all particles are dried at low temperature.

DPPH free radical scavenging activity
The DPPH free radicals scavenging activity of fresh    (Table 6). Depending on processing conditions and product composition, they involve in various chemical pathways and form various compounds which imposed the complexion of nonenzymatic browning reactions. Also, at different temperatures, various compounds having different antioxidant activity are formed at different stages of Maillard reactions. These results also support the findings by other researchers (Giovanelli and Lavelli, 2002;Turkmen et al., 2006;Auisakchaiyoung and Rojanakorn, 2015).

Conclusion
The results of this study suggest that the use of egg white and carboxymethyl cellulose (CMC) as a foaming agent and foam stabilizer, respectively positively influence the properties of tomato powder. The relatively higher temperature has a negative impact on the quality of the prepared powder. Based on the maximum retention of physical and biochemical properties, the optimum combination of foaming agent, foam stabilizer and the temperature was found to be 7% egg white + 1% CMC at 60℃ temperature. Based on this study, we can conclude that foam mat dried tomato powder could be an excellent alternative to tomato preservation. A microbial study can be done to observe the nutritional properties changes over time.