Comparison of the effect of freezing on the quality of ‘nam dokmai” mango fruit

This study evaluated effect of freezing methods on freezing rate and physical qualities of mango flesh before and after thawing. The method of freezing included still air freezing (at -20oC), air blast freezing (at -35oC, velocity 3 m/sec) and Air Blast Combined Vacuum Freezing (at -40oC, were configured core temperature 2 level is -5oC and -10oC) until the temperature reaches -20oC. The frozen flesh were thawed and evaluated for drip loss, pH, total soluble solid, colour, texture (firmness and %firmness decrease). The results showed that Air Blast Combined Vacuum Freezing (-5oC) Quality of mango frozen after thawing did not show significant effect on pH and total soluble solid of fresh mango. The frozen mango had highest lightness (L*) and the total colour differences (ΔE) had lowest and can maintain the maximum. Therefore, it's possible to introduce air blast combined vacuum freezing techniques applied to frozen mango industry as a result of significant increases freezing rate.


Introduction
Mango (Mangifera indica Linn.) is an economic tropical fruit of Thailand. Ripe mango is high nutrient fruit, consisted of vitamin, carotenoids and beta carotene. As referred to year 2016, Both mango fruits and mango products were exported to other countries created value more than 521,472 million baht. Frozen mangos were exported around 2,409 tons which created value 365 million baht [8]. In present, air freight is selected to be the best way of ripe mango exportation because mango was categorized as short shelf life fruit. However, air freight caused higher cost than other shipping. Nowadays, freezing is the most selected process for ripe mango. The freezing is the method of reducing temperature until lower than -18ºC which inhibits microorganism growth or deterioration of food. Water activity was reducing by freezing process to restrain the growth of bacteria and reduce enzyme activity. [1]; [2]; [5].
The quality of freezing fruit is depended on many components which are fruit types, variety, maturity stages or shelf life. However, the problem of producing freezing mangos were occurred from high water volume, changing texture, such as soften, changing of color and changing of important nutrient, such as vitamin C, carotenoids and microstructure, were occurred from freezing mango but all changing was high or low depended on the drip loss while freezing. [1]; [4][5]) Karl and Da-Wen (2000) studied about cooling by vacuum chilling which was used with product contained water volume than 90% such as mushroom and lettuce. Vacuum chilling can be used with freezing mango. Another advantage of vacuum chilling is to specify temperature and degree. Moreover, the study of vacuum chilling and mushroom browning by Polyphenol oxidase (PPO) were shown the mushrooms frosted by vacuum chilling were browned lower than other process. The study of Cheng and Lin (2007) showed the liquid state was changed faster when reducing pressure. Therefore, there are possibility to compile with reducing pressure and freezing to keep quality of mango.
In the past, the problems between different freezing was not studied and there was no the study about the using of vacuum system in freezing. Then, Still Air Freezing and Air Blast Freezing, which were most selected in freezing industry and Air Blast Combined Vacuum Freezing were compared for the advantages of freezing industry.

Materials
Mango (cv. Namdokmai) the variety of mango used in this study, No.4 were purchased from Chachoengsao province, Thailand, the fruits were selected for the pH value and total soluble solids were 4.12±0.22 and 14.78±0.98ºBrix, amount 60 kg.

Sample preparation
The sample were washed in chlorine solution (concentration 200 ppm) for 2 minutes, air dried and peeled. Only the central segments of the fruit sample were cut into size 2 x 2 x 1.5 cm and soaked in solution (1% ascorbic acid :1% calcium chloride (w/w)) for 10 minutes. Place them apart in a stainless-steel tray.

Freezing experimental
Mango were divided into 4 sections for four freezing methods by Still Air Freezing (at -20ºC) was carried out in a still air freezer (SF-PC997, Panasonic Co., Ltd., Japan), Air Blast Freezing (at -35ºC velocity 3 m/sec) was performed in an air blast freezer (Compact freeze, PATKOL Co., Ltd., Thailand) and Air Blast Combined Vacuum Freezing (ABCVF) (at -40ºC, were configured core temperature 2 level is -5ºC and -10ºC the system will stop the air blast freezing process and was used a vacuum system to reduce the pressure at 40 Pa until the core temperature of sample reaches -20°C) was executed in a Kryo 'D' Freezer (ITC Co., Ltd. Thailand), Four freezing process were used thermocouples K type, (TREX-8, LogTag Recorder, USA) insert into the core temperature of the sample, Record temperature every 30 seconds since the core temperature of 24ºC, the temperature reached to -20ºC. The freezing rate of the samples the freezing rate was calculated by modifying the equation: [4] freezing rate (cm/hr.) = Distantce/ Freezing time (1) Where Distance from the surface to the thermal center of fruits per freezing times of temperature decrease from the initial temperature (24 ºC) to -20ºC).

Thawing processes
The frozen sample were placed on napkin on plastic grate and thawed in control temperature and control relative humidity room (at temperature 25 ºC, 50% relative humidity). Were used thermocouples K type insert into the core temperature of the sample until the temperature reaches 0 -4ºC and quality was then analysed.

Drip loss
Drip loss of the frozen samples were sampling ten sample drip loss was defined as the percentage and was calculated following equation: Were W0 is weight of frozen mango (g) and W1 is weight of tawed mango (g). Drip loss was expressed as percent (%).

pH and Total Soluble Solid (TSS)
Frozen mango sampled after thawed was blended with a blender (HR2115, Philips Electronics Co., Ltd., Nederland), measurement of pH Volume 10 ml with pH meter (Lab855,Sl Analytics, Germany) and total soluble solid was determined by refractometer (HI96800, Hanna, Romania) and repeat three repetitions.

Colour and Colour difference
The colour of the fresh and frozen-thawed sample was measured through a Hunter Lab Colorimeter (ColorFlex EZ, Hunter Associates Laboratory, Inc., UK). In this system, L* is presented to lightness (0-100 scale, black to white); a* relate to red colour (+) and to green colour (-). and b* corresponds to yellow colour (+) and blue colour (-). Compare colour difference (ΔE) of fresh and frozen was calculated following equation Where L0*, a0* and b0* are the colour of fresh mango sample, and L1*, a1* and b1* are the colour of frozen-thawed mango sample.

Texture analysis
The texture of the sample both fresh and frozen samples was determined a Texture Analyzer (TA.XT2i Stable Micro Systems, Surrey, UK) with a 100 mm cylinder probe (P/100), using a compression of 50% strain Where Fi is the firmness of fresh sample, and Ff is the firmness of frozen-thawed sample.

Statistical analysis
The experiment was completely Randomize Design (CRD), the data were analysed by analysis of variance (ANOVA) according to the Duncan's Multiple Range Test (DMRT) at 95 percentage confidence intervals. Statistical analysis was presented by SPSS Statistics Software 20.0 for Windows (SPSS, Inc., Chicago, Illinois)

Effect of freezing time and freezing rate
The freezing times of each freezing methods were present in Table 1. The ABCVF(-5ºC) method had the fastest time 30 minutes, freezing rate was 2.00 cm/hr. Each of these freezing methods has a different rate depending on the method of heat transfer, can be divided into freezing rates according to the efficiency of the equipment. The freezing rate it is classified the ABCVF and air blast freezing method was quick-freezing. In addition, found that still air freezing spends more time in the critical zone than other freezing (Fig 1.). The longer it takes for the critical zone, the greater the ice crystals [10]. On the other hand, the ABCVF and air blast freezing methods has the slope of the freezing temperature curve and passes through the critical zone. For this reason, the possibility of accelerating the freezing rate is significant of the vacuum combined air blast freezing.

Effect of freezing methods on Drip loss of mango.
Effect of difference of freezing method on drip loss of sample after frozen-thaw ( Table 4.). It was found that the samples of frozen mangoes with the ABCVF (-5ºC) has lowest value was 4.82%. The drip loss increases when the freezing rate decreases [ 10] . The results are consistent with Table 1. This results in a more complete microstructure of fruit. So, the loss of water after thawing is associated with reduced cell integrity.

Effect of freezing methods on pH and Total
Soluble Solid (TSS) of mango.
In the Table 2. pH and total soluble solids content of the frozen-thawed samples was indicated air blast freezing and ABCVF (both -10ºC and -5ºC) there was no statistically significant difference in fresh mangoes. At the same time, the frozen sample of still air freezing observed increase pH value and decrease total soluble solids content statistical significance when compared to fresh mango. Due to during frozen, the water in the sample formed ice crystal which damaged the cellular. This causes the solids inside the cell to come out of the cell with the fluid lost [1].

Effect of freezing methods on Color and Color difference of mango.
The experiment data on the color parameters (L*, a* and b*) of mango frozen-thawed by four freezing methods, and color difference. In the Table 3 show that the brightness (L *) of the frozen mangoes ABCVF (-5ºC) with the highest brightness. The red colour (a *) of frozenthawed mangoes with still air freezing has highest value and more than fresh mango. Arasorn et al. ,(2558) reported that color changes after freezing and defrosting were the main problem of enzymatic browning reactions. This is an oxidation reaction when cell injury enzyme and oxygen are in contact with monophenols, resulting in browning index in the mango. For this reason, freezing slows the freezing rate, the cells are highly destroyed, resulting in higher reaction rates. When all values were calculated for colour difference (ΔE), it was found that the samples of mangoes, colour.
3.5 Effect of freezing methods on texture of mango. The texture change of frozen foods is mainly due to the freezing rate. The experiments showed that the firmness value of fresh was 8.31 N. Frozen samples with ABCVF (-5ºC) where have significant of densely more than those at various levels was 3.68 N. (Table 4) The results of the experiment corresponded to the effect of freezing rate as shown in Table 1. In addition, different freezing methods also contributed to the firmness decrease (%) after thawed. It was found that ABCVF (-5ºC) still had the lowest percentage reduction, and same trend as the firmness. Furthermore, the less drip loss effected less destroyed cell it possible to maintain many internal structures.

Conclusion
Among the four freezing methods studied in this research, the ABCVF (-5ºC) method was found to be a greatly freezing method, with quality similar to fresh mango, whereas still air freezing method led to deterioration of fruits. The higher freezing rate and lowest drip loss of frozen-thawed mango of ABCVF (-5ºC) method likely explained the higher preservation quality in terms of reduce pH, TSS, color and color different and texture quality. Among these treatment, the high freezing rate in ABCVF (-5ºC) method the most effect to good quality during freezing because of the small size of ice crystal and reduced damage to the fruit cellular. However, since father studies the effect of long-term preservation on the quality, microstructure of frozen mango should be studied.