Development of methods to measure functional properties of dehydrated fruits

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(1)SIK-Report No 715 2003. Development of Methods to Measure Functional Properties of Dehydrated Fruits Emma Holtz, Eva Rud, Therese Löfstrand, Lilia Ahrné June 2003.  SIK.

(2) SIK-Report No 715 2003. Development of Methods to Measure Functional Properties of Dehydrated Fruits Emma Holtz, Eva Rud, Therese Löfstrand, Lilia Ahrné. SR 715 ISBN 91-7290-229-9.  SIK. 2 (18).

(3) Preface This report is an examination work for a Bachelor of Chemical Engineering degree at Chalmers Lindholmen University College. This work has been performed at SIK, the Swedish Institute for Food and Biotechnology, at the department of Environment and Process Engineering, Gothenburg, during ten weeks, spring 2003. The work has been performed as a part of the EU project CombiDry, ICA-4-200110047. First of all, we would like to thank our supervisors Lilia Ahrné and Emma Holtz for their encouragement, support and for all feed back. We would also like to thank: Gunvor Bäcklund, our examiner, for her support and back up. Lars-Göran Vinsmo for enduring all our questions, lending us equipment, helping us package fruit and for all his assistance. Ulla Thorsell-Nilsson for helping us in the kitchen and all her suggestions. Luigi Messner for his fast service and helping us build our equipment. Claes-Göran Andersson for practical advice and answering of our questions. Anders Pettersson for helping us package fruit. To each and everyone of the staff at SIK, thank you for being so helpful, giving an inspiring surrounding and making us feel so welcome..  SIK. 3 (18).

(4) Abstract The purpose of this examination work for a Bachelor of Chemical Engineering degree was to develop methods to measure functional properties of dehydrated fruits. Three different functional properties were selected: rehydration capacity (RC), water holding capacity (WHC) and swelling capacity (SC). The fruits that were examined were banana, cooking banana, mango, and pineapple. All methods were tested at four rehydration temperatures: 20°C, 45°C, 60°C, and 95°C. The work resulted in the following methods: To determine RC, dried fruit pieces, with known masses, were placed in distilled water and after a certain time excess water was removed from the fruit pieces and then the pieces were weighed. RC is calculated as kg moisture / kg dry matter. To determine WHC, fruit pieces were prepared by the same rehydration procedure as in RC. The samples were centrifuged at approximately 100 g for ten minutes. The fruit pieces were weighed before and after centrifugation. WHC is calculated as the ratio between mass before and mass after centrifugation. To determine SC, fruit pieces were prepared by rehydration. Dry and rehydrated fruits were weighed on a balance and then weighed immersed in water to obtain the volume of the fruit. During under-water weighing, the pieces were placed in a net basket, hanging under the balance, the basket was immersed in distilled water and the weight was read. Archimedes’s principle was subsequently utilized to establish the volume of the solid. SC is calculated as a percentage volume increase compared to dry fruit. For RC and SC, the same procedure can be performed for every kind of fruit, while WHC must be adjusted for each product that will be examined (e.g. centrifugation speed, centrifugation time etc)..  SIK. 4 (18).

(5) Sammanfattning Syftet med detta examensarbete, för högskoleingenjörsexamen i kemiteknik, var att utveckla metoder för att mäta funktionella egenskaper hos torkad frukt. Tre egenskaper valdes: rehydratiseringskapacitet (RC), vattenhållande förmåga (WHC) och svällningskapacitet (SC). Frukterna som undersöktes var: banan, matbanan, mango och ananas. Alla metoder analyserades vid fyra olika rehydratiseringstemperaturer: 20°C, 45°C, 60°C och 95°C. Resultatet blev följande metoder: RC bestämdes genom att torkade fruktbitar, med känd massa, lades i destillerat vatten. Efter bestämda tidpunkter plockades en fruktbit upp, torkades från överflödigt vatten och vägdes. RC beräknas som kg fukt / kg torr materia. För att bestämma WHC rehydratiserades frukten med samma procedur som RC. Proven centifugerades vid ca 100 g i tio minuter. Fruktbitarna vägdes både före och efter centrifugeringen. WHC beräknas som förhållandet mellan massan före och massan efter centrifugeringen. För att bestämma SC rehydratiserades frukten med samma procedur som RC. Torr och rehydratiserad frukt vägdes ovanför och under vatten för att få fram fruktens volym. Vid vägning under vatten användes en nätkorg, hängandes under vågen. Nätkorgen sänktes ned i destillerat vatten och vikten avlästes. Därefter användes Archimedes princip för att fastställa volymen på massan. SC beräknas som den procentuella volymökningen i förhållande till den torra frukten. För RC och SC kan samma procedur användas för alla sorters frukter, medan WHC måste anpassas för varje produkt som skall undersökas (t.ex. centrifugeringshastighet, centrifugeringstid etc.).  SIK. 5 (18).

(6) Table of contents 1. Introduction .................................................................................. 7 2. Theory ........................................................................................... 7 2.1. Rehydration Capacity .......................................................... 7 2.1.1. Dry Matter Loss ........................................................... 8 2.1.2. Rehydration Rate.......................................................... 8 2.2. Water Holding Capacity ...................................................... 8 2.3. Swelling Capacity................................................................ 9 3. Materials and Methods ................................................................ 10 3.1. Raw Materials...................................................................... 10 3.2. Sample Preparation.............................................................. 10 3.3. Rehydration Capacity .......................................................... 11 3.4. Water Holding Capacity ...................................................... 11 3.5. Swelling Capacity................................................................ 12 4. Results ........................................................................................... 12 5. Discussion...................................................................................... 13 5.1 Drying................................................................................... 13 5.2. Rehydration Capacity .......................................................... 13 5.2.1. Banana.......................................................................... 14 5.2.2. Cooking Banana ........................................................... 14 5.2.3. Mango .......................................................................... 15 5.2.4. Pineapple ...................................................................... 15 5.3. Water Holding Capacity ...................................................... 15 5.3.1. Banana.......................................................................... 16 5.3.2. Cooking Banana ........................................................... 16 5.3.3. Mango .......................................................................... 16 5.3.4. Pineapple ...................................................................... 16 5.4. Swelling Capacity................................................................ 16 5.4.1. Banana.......................................................................... 17 5.4.2. Cooking Banana ........................................................... 17 5.4.3. Mango .......................................................................... 17 5.4.4. Pineapple ...................................................................... 17 6. Conclusions and Outlook............................................................. 18 Appendices I – VIII.  SIK. 6 (18).

(7) 1. Introduction The purpose of this examination work was to develop methods to measure functional properties of dehydrated fruits. The fruits that were examined were banana, cooking banana, mango, and pineapple. The development of these methods is a part of an EU-project called CombiDry. The purpose of the CombiDry project is to develop new dehydrated fruits of high quality by combining osmotic and microwave drying followed by a suitable packaging. Some of the dehydrated fruits are intended for rehydration; therefore following methods for development were selected: rehydration capacity (RC), water holding capacity (WHC) and swelling capacity (SC). In this project there were some limitations. All fruits were dried in a hot air drying oven and tests were only made for one size of each piece of fruit. There was an assumption that all fruit pieces of the same fruit had similar water content. No pretreatments of the fruits were made. All methods were tested at four rehydration temperatures: 20°C, 45°C, 60°C, and 95°C to give a wide interval of temperatures. Duplicates were made for all experiments, but no standard deviations were calculated for the tests. Moreover, the field of application of the fruit was not considered. Also, there was no consideration of cell structure of the fruits or sensory tests of the dried and rehydrated fruits.. 2. Theory 2.1. Rehydration Capacity Rehydration capacity (RC) refers to the amount of water that can be absorbed by a dried product. It is usually expressed as kg moisture / kg dry matter. RC of a dried product depends on composition and size of the product, type of drying method, pre-treatment of the product and storage of the dried product. Not only the product but also the type of immersion media and temperature of the media is significant for the RC [1]. RC is calculated according to: RC =. m rh − m db. (1). m db. mrh – mass of rehydrated fruit [g] mdb – mass of fruit on dry basis [g] mdb is calculated for every rehydrated piece of fruit as: m db =. m df. (2). X +1. mdf – mass of dried fruit [g] X – water content of fruit [kg H2O/kg dry matter].  SIK. 7 (18).

(8) 2.1.1. Dry Matter Loss During rehydration, the product will concurrently absorb water and leach out substances (e.g. sugars, acids, minerals and vitamins) [1]. Therefore, the product will be gradually dissolved and it is difficult to establish the maximum amount of water that can be absorbed. The rate of dry matter loss will occur faster at high temperatures. When immersion time is extended, a high leaching will be expected [2]. Therefore, the highest loss of material is expected to be obtained at low temperatures for long times or at high temperatures for short times. 2.1.2. Rehydration Rate Rehydration rate (RR) is, as RC, dependent on temperature. Dried fruits are able to absorb more water at low temperatures than at higher temperatures, and the rate of rehydration increases with higher temperatures [2]. Rehydration rate decreases with time of soaking and it is explained in Sayar et. al. [3] that saturation of water in pores leads to a decrease of driving force between the medium and the product. This implies that leaching of solid matter will restrain the water to be absorbed by the product. RR increases with higher temperatures, because the medium (e.g. water) will show lower properties of viscosity at increasing temperatures and the diffusion of the medium will occur faster [2]. RR is calculated according to: RR =. RC. (3). RT. RC – rehydration capacity [kg H2O/kg dry matter] RT – rehydration time [min] 2.2. Water Holding Capacity A definition of water holding capacity (WHC) is described in Zamorano et. al. [4] as the ability of a product to retain water when it is exposed to an external force, for example pressing, heating or applying of a centrifugal force. A common method for measuring water holding properties of foods is to put the products that will be examined into a centrifuge and apply a centrifugal force on the products. After a certain time, and velocity, of centrifugation, the loss of water can be measured, and from this, WHC can be calculated. It is important that the centrifugation method is performed as optimal as possible because different products require different adjustments. Some products are easily damaged and do not withstand high speed as good as other, more durable, products will bear. WHC is an important factor to investigate since it can determine the quality of the product during storage, processing and preparation [5], [6]. WHC is calculated according to:.  SIK. 8 (18).

(9) WHC =. m ac m. (4). bc. mac – mass of fruit after centrifugation [g] mbc – mass of fruit before centrifugation [g] The centrifugal force, g, is calculated from the number of revolutions per minute according to:  rpm  g = r ⋅ 1,12 ⋅    1000 . 2. (5). g – number of g r – radius, [mm] rpm – revolutions per minute 2.3. Swelling Capacity Swelling capacity is the increase of volume by a known weight of fibre under the existing circumstances [7] and can be calculated as a percentage volume increase compared to dry fruit. One way to measure the volume of a solid material is to weigh the solid under liquid (in general water) and Archimedes’s principle is subsequently utilized to establish the volume of the solid. When the solid body is immersed, the Buoyant force (lifting force) is equal to the weight of the displaced water. It is not significant whether the body is flowing or if it is immersed in the liquid [8]. By knowing the density of the liquid, the volume of the displaced liquid can be calculated and from this, the volume of the solid material can be determined. SC is calculated according to:. SC =. Vrh − Vther Vther. (6). Vrh – volume of rehydrated fruit [cm3] Vther – theoretical volume of dried fruit that will be rehydrated [cm3] Since one and the same piece should not be used in different experiments, the theoretical volume of pieces that will be rehydrated, Vther, is calculated according to:. Vther =. m ther ⋅ Vdf m df. (7). Vdf – volume of dried fruit [cm3] mdf – mass of dried fruit [g].  SIK. 9 (18).

(10) mther – mass of dried fruit that will be rehydrated [g] The volume of the fruits before and after rehydration is calculated according to: Vf =. m f − m im ρ H 2O. (8). Vf – volume of fruit [cm3] mf – mass of fruit [g] mim – mass of fruit immersed in water [g] ρH2O – density of water [g/cm3]. 3. Materials and Methods 3.1. Raw Materials For information about the variety of the fruits and country of origin see table 1. Table 1. Information about the fruits. English name of fruit Banana Cooking banana Mango Pineapple. Latin name of fruit Musa paradisiaca Musa paradisiaca Mangifera indica Ananas comosus. Type of fruit Cavendish banana Plantane Tommy Atkins Sweet pineapple. Country of origin Colombia Costa Rica Brazil Ecuador. The bananas were stored at room temperature for 24 h before sample preparation and drying. The pineapples were stored at +8°C and the cooking bananas at +15°C for about a week until sample preparation and drying was carried out. Most of the mangoes were prepared directly after arrival, but some mangoes had not achieved ripeness and they were stored at +8°C for about a week. 3.2. Sample Preparation At first, degree of ripeness of the fruits was determined by taking random samples and measuring BRIX with a refractometer (DIGIT-032 ATC), see appendix I, table I1. The fruits were cut in pieces, for tools, see appendix III, figure 1. Banana and cooking banana were cut in 14 mm thick slices. The diameter of the bananas was cut to 29 mm and the diameter of the cooking bananas to 37 mm. Mangoes and pineapples were cut into cylinders with diameters of 18 mm and lengths of 14 mm. All fruits were dried in a hot air drying oven at 50°C, over night until the water activity was below 0,64 to prevent growth of microorganisms [9]. After drying, water activity and water content were determined, see appendix I, tables I2 – I3. The dried fruit were stored in a vacuum packed etylvinylalcohol (EVOH) bag in a dark cold-storage room at +8°C..  SIK. 10 (18).

(11) 3.3. Rehydration Capacity To determine water content of the dried fruit pieces, dried pieces of each fruit were put in an oven at 100°C for 16 hours. After that, the pieces were weighed and the water content of the dried fruit pieces was calculated from equation (2). All fruit was rehydrated at four temperatures: 20°C, 45°C, 60°C, and 95°C. A water bath was used to hold a constant temperature. To establish RCmax at the different temperatures, some pre-tests were made to see when the RC graph did not increase anymore. At 95°C, rehydration for just over two hours was enough, and at 60°C a rehydration time of four hours was enough. At 20°C and 45°C, rehydration experiments were made for a longer time than for the two highest temperatures. At 20°C, the fruits were immersed for 26 hours and at 45°C, immersion for 18 hours was made. Fruit pieces were weighed and placed one by one in 100 ml distilled water in beakers with lids. After a certain time, one piece of fruit was dried from excess water on a Büchner funnel. The Büchner funnel was supported by a large Erlenmeyer flask, which was equipped with a connection for vacuum suction [10]. At first, a filter paper was placed on the Büchner funnel with distilled water and the vacuum pump was started for one minute; after this the piece of fruit was placed on the filter paper and the vacuum pump was started for one minute to remove the excess water from the fruit. The fruit and the filter were weighed on a balance and then only the filter was weighed to determine the weight of the fruit piece. For equipment, see appendix II a) and appendix III, figure 2. 3.4. Water Holding Capacity The fruit was prepared by rehydration, two pieces in each beaker with 100 ml distilled water, and after a certain time, the two pieces of fruit were placed on a filter on a Büchner funnel to remove excess water in the same procedure as described in 3.3. A centrifuge (Hettich Universal 2S) was used. To receive optimal speeds for the fruits, some centrifuge tests were made. For the first experiments, a very slow speed was used, but no liquid could be obtained from the fruits at slow centrifugation speed. Gradually, the speed of centrifugation was increased until the fruits could not bear any higher speed. The optimal speed was chosen as the maximal revolutions per minute that a fruit could bear without becoming damaged. The samples were centrifuged at approximately 100 g (1000 rpm,) for ten minutes for bananas, mangoes and pineapples, while cooking bananas were centrifuged at approximately 250 g (1500 rpm) for ten minutes. For calculations of number of g, see equation (5). For the centrifugation, a net with fine mesh placed in a centrifuge tube was used. Before centrifugation, the empty net and the net with a piece of fruit were weighed on a balance, and after centrifugation the net with the fruit piece was weighed to determine the liquid loss from the fruit. An alternative method was to use centrifuge tubes filled with approximately 15 g of glass beads, diameters of 2 mm (5 – 7 mm for the banana) instead of using the net, otherwise the procedure was the same. For equipment, see appendix II b) and appendix III, figures 3 – 4..  SIK. 11 (18).

(12) 3.5. Swelling Capacity To calculate the volume of dried fruit, the dry fruit pieces were weighed on a balance. Then the dry pieces were placed in a net basket, hanging under the balance and the basket was immersed in distilled water. The weight was read within ten seconds to avoid water being absorbed by the dried fruits [10]. The empty net basket was also weighed under water and the weight was recorded. 2,5 g – 5,0 g of dry fruit pieces were prepared by rehydration (about 2 – 5 pieces in each beaker). After a certain time, the fruit was blotted with a piece of paper tissue instead of drying on a Büchner funnel as in 3.3 and 3.4. The reason was that experiments comparing the two methods for removing excess water implied that vacuum filtration resulted in drier fruit. The rehydrated fruit was weighed in the same way as the dried fruit, described above. For equipment see appendix II c) and appendix III, figure 5.. 4. Results For the graphs for RC, RR, WHC and SC, see appendices IV, V and VI. Experiments of longer rehydration times, than shown in the graphs, were made for the fruits at 20°C and 45°C (see 3.3). No curves of these experiments are shown in this report, but the calculated RCmax and SCmax are shown in tables 2 – 5 below. To calculate percentage volume (V [%]) in tables 2 – 5 below, the volume of rehydrated fruits was divided by the original volume of the fruits, i.e. fresh fruit. Table 2. Results from experiments of banana. Banana. 20°C. 45°C. 60°C. 95°C. RCmax [kg H2O/kg dry matter] RRstart [RC/min] SCmax V [%] WHCaverage Net WHCaverage Glass Beads. 2,5 0,029 2,3 71 0,92 0,89. 2,5 0,038 2,1 65 0,89 0,84. 1,9 0,040 1,7 52 0,94 0,88. -. Table 3. Results from experiments of cooking banana. Cooking banana RCmax [kg H2O/kg dry matter] RRstart [RC/min] SCmax V [%] WHCaverage Net WHCaverage Glass Beads. 20°C. 45°C. 60°C. 95°C. 1,2 0,018 0,4 21 0,95 0,96. 1,4 0,024 0,5 26 0,95 0,95. 1,0 0,025 0,5 26 0,95 0,94. 1,8 0,041 1,1 56 0,97 0,97.  SIK. 12 (18).

(13) Table 4. Results from experiments of mango. Mango RCmax [kg H2O/kg dry matter] RRstart [RC/min] SCmax V [%] WHCaverage Net WHCaverage Glass Beads. 20°C. 45°C. 60°C. 95°C. 4,8 0,058 4,3 98 0,88 0,81. 4,4 0,058 3,8 86 0,86 0,79. 3,3 0,063 3,0 68 0,84 0,77. 2,4 0,069 2,2 50 0,86 -. 20°C. 45°C. 60°C. 95°C. 4,3 0,047 4,6 73 0,80 0,76. 3,4 0,060 4,0 64 0,77 0,78. 2,6 0,064 3,1 49 0,77 0,73. 2,7 0,067 2,3 36 0,78 0,81. Table 5. Results from experiments of pineapple. Pineapple RCmax [kg H2O/kg dry matter] RRstart [RC/min] SCmax V [%] WHCaverage Net WHCaverage Glass Beads. 5. Discussion For all experiments, duplicates were made. Standard deviation for the tests has not been calculated, but none of the tests differed very much from each other. 5.1. Drying Since main focus has not been on the procedure of drying, the only test that has been made on the dried fruits was measurement of water activity. This is an important test to ensure that growth of microorganisms is avoided. The dried cooking banana had much lower water content than the other fruits in these experiments. This is shown in appendix I, table I3, where a comparison between the water content of the dried fruit is made. 5.2. Rehydration Capacity Different pieces are used for each experiment of RC. Advantages with not using the same piece every time are less handling of the pieces and that a larger volume of samples will be used from different parts of the fruit. The fruit pieces may also be derived from different fruits, which may have achieved different degree of ripeness. This could be considered to be an advantage, but also a disadvantage, dependent on the purpose of the experiment. To receive as similar results as possible, the fruits should have achieved similar degree of ripeness, but since it is difficult to receive fruits that are exactly alike, this method will show results that could be well compared to further experiments..  SIK. 13 (18).

(14) RC differs at long rehydration times and therefore, it can be a difficulty to establish RCmax. This is due to leaching of soluble substances during rehydration and the fruits will dissolve and lose its original appearance. This explains why RCmax not always shows the largest value at the longest rehydration times for the same temperature. All fruits in the experiments of rehydration show the highest RR at 95°C, and lowest RR at 20 °C. It is shown in appendix IV that the velocity of rehydration is dependent on temperature and follows the theory in 2.1.2. Except for the banana, the fruits were more affected, considering appearance and firmness, by longer rehydration time than at high temperatures. Hence, rehydration time seems to be a more important factor than temperature. For comparisons between dried and rehydrated fruit, see appendix III, figures 6 – 9. 5.2.1. Banana Large differences in degree of ripeness could not be visually observed and results should not have been influenced from different pieces. There were some difficulties to obtain juice from the bananas, and therefore only three tests of BRIX could be performed, se appendix I, table I1. During drying, the banana became brown-coloured and during rehydration, it did not retrieve its original colour and firmness. The banana became very soaked, especially at 95°C, where it dissolved and therefore we decided not to go further with experiments at this temperature on the banana. The banana has a higher RCmax at lower temperatures and this follows the theory in 2.1.1. It is observed that the maximum RC is equal at 20°C and 45°C but RCmax is reached faster at 45°C. There is a possibility that the banana needs more time of rehydration at 20°C, and therefore RCmax for this temperature shows a lower value than expected. 5.2.2. Cooking Banana It is important to use fruit as ripe as possible, but it is difficult to measure degree of ripeness because fresh cooking bananas are very dry fruits and no fruit juice could be obtained from the fresh fruit. Therefore, the refractometer could not be used for measurements of degree of ripeness for the cooking banana. After a few hours of rehydration, the cooking banana became more yellow-coloured than the fresh fruit. It also became spongy, but it did not dissolve during rehydration. It did not become brown-coloured during drying, as the banana did (5.2.1). The cooking banana does not follow the theory that means that RC should show higher values at lower temperatures. There is no following trend, and perhaps this is to a certain extent dependent on different degree of ripeness for the fruits in the experiments..  SIK. 14 (18).

(15) 5.2.3. Mango For rehydration of mango, it is important to use mangoes that are firm-fleshed and not too ripe, otherwise the fruit will dissolve and no exact results are able to be obtained. The firm pieces did not dissolve and they retrieved their original appearance, but they became grey-coloured. The mango follows the theory of RC well and the highest RC is obtained at 20°C; it will take longer time to achieve RCmax than for the other temperatures. 5.2.4. Pineapple Large differences in degree of ripeness could not be observed and, like the banana, the results should not have been influenced by different pieces. After a few hours of rehydration, the pineapple pieces dissolved and fell apart. The pineapple lost a lot of its original colour and became very pale during rehydration. The original appearance of the pineapple is almost entirely retrieved, except for the colour, after a few hours of rehydration. At the highest temperatures, 60°C and 95°C, RCmax is analogues; otherwise it follows the theory well. Maybe RCmax for 60°C is still not reached and this temperature could be tested for longer time for the pineapple. However, according to the RC graph of pineapple at 60°C, see appendix IV, it seems that there is no more increase in RC. Perhaps some more tests should be made to investigate this fruit more. 5.3. Water Holding Capacity For the centrifugation, only one fruit piece per tube has been used to avoid the fact that two or more pieces will be influenced from each other. The centrifugation was implemented at approximately 250 g for ten minutes for the cooking banana, while the other fruits could only bear a centrifugation at 100 g for ten minutes, otherwise they became damaged. As an example, damaged banana and mango are shown in appendix III, figures 10 – 11. Two methods were used for the centrifugation; first a method with nets and afterwards, instead of the nets, glass beads were used. The beads should not be too large; otherwise an even surface will not be obtained. A comparison between the nets and the glass beads shows that for those fruits where the glass beads do not stick, the results are similar to the net method. However, there is a larger difference between the nets and the beads for mango and banana. For these fruits, the beads got stuck deep into the fruit and when the beads were removed, some pulp from the fruits was also removed which affected the results. It was laborious to remove all beads from the wet fruit pieces. We tried to separate the glass beads from the fruit by a filter paper, but the filter absorbed some of the water being released from the fruit and this influenced the results. Therefore no further experiments were made with the filter papers. The weight of released liquid was not exactly the same as the mass difference of the fruit piece before and after centrifugation respectively. In all experiments, the weight of the liquid was less than the difference between the fruit piece before and after.  SIK. 15 (18).

(16) centrifugation. Perhaps some of the released water evaporated from the tube. For all calculations of WHC, the differences in masses of the fruit pieces have been used. Because of an analogue setting and measurement of the centrifugation speed and a difficulty to measure the exact radius of the centrifuge, only an approximate value of the g-force could be calculated. In appendix VII it can be seen that the WHC alters during the experiments. Since both filter and pearls experiments follow each other, presumably the settings on the centrifugation speed varied from one experiment to another. No immediate conclusion can be drawn regarding to the different rehydration temperatures. However, it is possible to see a decrease in WHC by rehydration time. 5.3.1. Banana The WHC does not decrease by time of rehydration; it is stable during the entire experiment. Since the banana dissolved at 95°C, no further experiments of WHC were made at this temperature. At the other temperatures the banana retained a good shape. It is important to use larger glass beads (5 – 7 mm) because smaller beads will be buried into the banana slice. However, the results from the beads for the banana were not very good, because the beads deformed the banana slices. 5.3.2. Cooking Banana The cooking banana hardly released any water during centrifugation, and the WHC is very high, almost 1,0. This fruit is very durable and it retained a very good shape, even at the relatively high centrifugation rate for the cooking banana. An explanation to the high WHC can be that during rehydration, the cooking banana might have formed a “skin”, called case hardening, which restrained water from being released. This could also be an explanation for why the cooking banana did not absorb very much water during rehydration. 5.3.3. Mango The WHC decreases by time of rehydration. This happened especially when the mangoes were too ripe and started to dissolve. In this case, the fruits were deformed during centrifugation and therefore, perhaps a slower centrifugation speed should be used. This problem did not occur when firm mango was used. For WHC, it is especially important to use as similar mangoes as possible, to receive comparable results. 5.3.4. Pineapple The WHC decreases marginally by the rehydration time. The pineapple pieces retained a good shape during the centrifugation experiments. The pieces should always be centrifuged with the fibres in the same direction to achieve comparable results. 5.4. Swelling Capacity When the under-water weighing is performed, it is important that the net basket is immersed slowly into the water to avoid air-bubbles. It is preferred to have an entirely dry net basket before every weighing, so the weight of the absolutely dry net can be measured..  SIK. 16 (18).

(17) It can be difficult to make the fruit pieces equally dry every time since superfluous water from the fruits are removed with paper towels. We do not recommend removing excess water by vacuum filtration, as in the experiments of RC and WHC, since the fruit piece probably will be too dry and absorb water during the under-water weighing. A comparison between SC and RC has been made by making a graph where SC is a function of RC. This graphs show that SC is almost linear dependent on RC, see appendix VIII. 5.4.1. Banana The difference in volume was not very large. The banana neither shrunk during drying nor swelled very much during rehydration. A comparison between the SCmax and RCmax shows that these two properties follow the same temperature dependence. At maximal RC, also the maximal volume increase is achieved, see table 2. Compared to the volume of fresh banana, this fruit swells between 52 % and 71 % of the original volume, see table 2. 5.4.2. Cooking Banana The cooking banana was very unreliable during measurements of SC (especially at 95°C) with the under-water weighing method. Probably the fruit absorbed water during the measurement of mass under water and the balance did not show stable results. Since the cooking banana seems to be very hydrophilic, perhaps an organic solvent should be used instead of water. The temperature dependence of SC of the cooking banana does not follow the RC temperature dependence exactly, see table 3. This could be explained by different ripeness of fruits, although it could not be established since degree of ripeness was not able to be measured. In these experiments, some more ripe fruits could have been compared to other, less ripe fruits and hence have affected the results. However, it should be pointed out that SC is low and does not differ that much at the three lowest temperatures compared to 95°C. Perhaps the possibly formed case hardening, see 5.3.2, prevents water from being absorbed by the cooking banana at low temperatures, while it may be softened by the water at 95°C. Another explanation could be that the SC at high temperatures might be influenced by swelling of starch, since the cooking banana has a high starch content. 5.4.3. Mango The temperature dependence of SC follows RC well and swells the most when the RC has the highest value. It could clearly be visually seen that the mango swelled to almost its original size, see table 4. However, the mango swelled to only 50 % of the volume of fresh fruit at 95°C. This implies that SC is highly dependent on the temperature of rehydration. 5.4.4. Pineapple The temperature dependence of SC does not follow RC at the highest temperatures. However, the results of SC are as expected, while there is a minor error in the RC graph. At 95°C, the volume is only 36 % of the original volume of the pineapple pieces. This.  SIK. 17 (18).

(18) means that the pineapple has the lowest SC of all examined fruits in these experiments. In appendix VI, it can be seen that the SC still increases. Perhaps the pineapple should need a longer time of rehydration to see if it may achieve a higher SC at this temperature.. 6. Conclusions and Outlook The three methods that have been developed have shown reliable results and can be used for measurements of properties of dehydrated fruits. The same procedures that have been developed for each properties of RC and SC can be performed for every kind of fruit, while WHC must be adjusted for each product that will be examined (e.g. centrifugation speed, centrifugation time etc). During the experiments studies were made how well the fruit pieces retained appearance and discovered that rehydration time seems to be a more important factor than temperature. After a long time of rehydration, the fruit pieces eventually will dissolve and decompose. To receive more reliable results it is important that the fruits have achieved the same degree of ripeness; otherwise, the results differ because of different properties of the fruits. To find out how much leaching that occurred during rehydration, some additional experiments could be made by measuring water content in rehydrated fruit pieces. For further development of WHC, it could be an idea to perform tests that study how WHC depends on revolutions per minute and time of centrifugation. Perhaps the optimal results from the WHC may be obtained at a lower velocity and not at the highest speed that can be implemented without damaging the fruit. It could also be interesting to calculate standard deviation for the implemented tests to obtain the differences of the pieces..  SIK. 18 (18).

(19) Appendix I Results from experiments during sample preparation Table I1. Value over degree of ripeness. Fruit. BRIX*. Banana 19,8 20,4 22,4 Cooking banana Not able to measure Mango 15,5 17,4 17,4 18,2 Pineapple 12,0 12,4 12,6 13,0 *The more sugar content, the higher value of BRIX. Table I2. Water activity for the dried fruit. Fruit Banana Cooking banana Mango Pineapple. Water activity 0,372 0,135 0,460 0,575. Table I3. Water content for the dried fruit. Fruit Banana Cooking banana Mango Pineapple. Water content % 13 6,8 14 19.

(20) Appendix II Equipment a) RC • • • • • • • • •. Water bath Beakers with lids Büchner funnel Filter papers (Munktell filter 1F) Erlenmeyer flask with vacuum connection Rubber packing 2 Stop-watches Balance Spoons. b) WHC • • • • •. All equipment of RC Centrifuge with timer Centrifuge tubes Nets with fine mesh Glass beads, 2 mm, 5 – 7 mm. c) SC • • • • • • • •. Water bath Beakers with lids Stop-watch Net basket Balance with a hook on underside Large beaker filled with distilled water Rack to place the balance and the large beaker on Spoons.

(21) Appendix III Pictures of equipments and fruits Equipment. Figure 1. Tools for sample preparations.. Figure 2. Equipment for vacuum filtration at measurement of rehydration capacity.. Figure 3. Inside of centrifuge.. Figure 4. Centrifuge tubes with filters and pearls.. Figure 5. Equipment for measurement of swelling capacity..

(22) Appendix III Comparison between dried and rehydrated fruit. Figure 6. Banana. Figure 7. Cooking banana. Figure 8. Mango. Figure 9. Pineapple. Comparison between unsuccessful and successful centrifugation. Figure 10. Banana. Figure 11. Mango.

(23) Appendix IV Rehydration Capacity Cooking Banana 5,0. RC (kg H2O/ kg dry matter). 4,0. 3,0. 20°C 45°C 60°C 95°C. 2,0. 1,0. 0,0 0. 1. 2. 3. 4. 5. Rehydration Time (h). Velocity of Rehydration Cooking Banana 0,08. Velocity (RC/time). 0,06 20°C 45°C 60°C 95°C. 0,04. 0,02. 0,00 0. 1. 2. 3. Rehydration Time (h). 4. 5.

(24) Appendix IV Rehydration Capacity Pineapple 5,0. RC (kg H2O/ kg dry matter). 4,0. 3,0. 20°C 45°C 60°C 95°C. 2,0. 1,0. 0,0 0. 1. 2. 3. 4. 5. Rehydration Time (h). Velocity of Rehydration Pineapple 0,08. Velocity (RC/time). 0,06 20°C 45°C 60°C 95°C. 0,04. 0,02. 0,00 0. 1. 2. 3. Rehydration Time (h). 4. 5.

(25) Appendix IV Rehydration Capacity Mango 5,0. RC (kg H2O/ kg dry matter). 4,0. 3,0. 20°C 45°C 60°C 95°C. 2,0. 1,0. 0,0 0. 1. 2. 3. 4. 5. Rehydration Time (h). Velocity of Rehydration Mango 0,08. Velocity (RC/time). 0,06 20°C 45°C 60°C 95°C. 0,04. 0,02. 0,00 0. 1. 2. 3. Rehydration Time (h). 4. 5.

(26) Appendix IV Rehydration Capacity Banana 5,0. RC (kg H2O/ kg dry matter). 4,0. 3,0 20°C 45°C 60°C. 2,0. 1,0. 0,0 0. 1. 2. 3. 4. 5. Rehydration Time (h). Velocity of Rehydration Banana 0,08. Velocity (RC/time). 0,06. 20°C 45°C 60°C. 0,04. 0,02. 0,00 0. 1. 2. 3. Rehydration Time (h). 4. 5.

(27) Appendix V. Water Holding Capacity Banana 1,2 1. WHC. 0,8 20°C 45°C 60°C. 0,6 0,4 0,2 0 0. 1. 2. 3. 4. 5. Rehydration Time (h). Water Holding Capacity Cooking Banana. 1,2 1. WHC. 0,8 20°C 45°C 60°C 95°C. 0,6 0,4 0,2 0 0. 1. 2. 3. Rehydration Time (h). 4. 5.

(28) Appendix V Water Holding Capacity Mango 1,2 1. WHC. 0,8 20°C 45°C 60°C 95°C. 0,6 0,4 0,2 0 0. 1. 2. 3. 4. 5. Rehydration Time (h). Water Holding Capacity Pineapple 1,2 1. WHC. 0,8 20°C 45°C 60°C 95°C. 0,6 0,4 0,2 0 0. 1. 2. 3. Rehydration Time (h). 4. 5.

(29) Appendix VI Swelling Capacity Banana 3,5 3,0 2,5 2,0 SC. 20°C 45°C 60°C. 1,5 1,0 0,5 0,0 0. 1. 2. 3. 4. 5. Rehydration Time (h). Swelling Capacity Cooking Banana 3,5 3,0 2,5 20°C 45°C 60°C 95°C. SC. 2,0 1,5 1,0 0,5 0,0 0. 1. 2. 3. Rehydration Time (h). 4. 5.

(30) Appendix VI Swelling Capacity Mango 3,5 3,0 2,5 20°C 45°C 60°C 95°C. SC. 2,0 1,5 1,0 0,5 0,0 0. 1. 2. 3. 4. 5. Rehydration Time (h). Swelling Capacity Pineapple 3,5 3,0 2,5 20°C 45°C 60°C 95°C. SC. 2,0 1,5 1,0 0,5 0,0 0. 1. 2. 3. Rehydration Time (h). 4. 5.

(31) Appendix VII Water Holding Capacity Mango 60°C 1. 0,8. WHC. 0,6 Filter Glass pearls. 0,4. 0,2. 0 0. 0,5. 1. 1,5. 2. 2,5. 3. 3,5. 4. 4,5. Rehydration Time (h). Water Holding Capacity Pineapple 95°C 1. 0,8. 0,6 WHC. Filter Glass pearls. 0,4. 0,2. 0 0. 0,5. 1. 1,5. Rehydration Time (h). 2. 2,5.

(32) Appendix VIII. Banana. 2,5 2 1,5 SC. 20°C 60°C 45°C. 1 0,5 0 0. 0,5. 1. 1,5. 2. 2,5. RC. Cooking banana 3 2,5 2 SC. 20°C 60°C. 1,5. 45°C 95°C. 1 0,5 0 0. 0,5. 1 RC. 1,5. 2.

(33) Mango 3,5 3 2,5 20°C. 2 SC. 60°C 45°C. 1,5. 95°C. 1 0,5 0 0. 0,5. 1. 1,5. 2. 2,5. 3. 3,5. RC. Pineapple 3,5 3 2,5 20°C. 2 SC. 60°C 45°C. 1,5. 95°C. 1 0,5 0 0. 0,5. 1. 1,5. 2 RC. 2,5. 3. 3,5.

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No results found