effect of different drying methods on physical-chemical

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Effect of d i f ferent dry ing methods on physical-chemical characteristics and drying time of mixed açaí, strawberry and acerola pulp

10.37885/200700678

Carolina Tatagiba da RochaUFES

Katia Silva MacielUFES

Mila Marques GambaUFES

Milton de Jesus FilhoUFES

Sergio Henriques SaraivaUFES

Pollyanna Ibrahim SilvaUFES

Luciano José Quintão TeixeiraUFES

Palavras-chave: Mixed Fruit Pulp; Powdered Products; Drying Methods.

RESUMO

The pulp drying to production of fruit powders allows its preservation for a long time, allowing its use in the manufacture of instant beverages among other applications of industrial interest. This study aimed to compare three drying methods: freeze-drying of pulp mix, freeze-drying of pulp mix in foam; and foam mat drying of pulp mix in a cabinet dryer at 60 oC. Drying parameters in the freeze-dryer and in the cabinet dryer were studied. Drying time, moisture, water activity, solubility and vitamin C of powders were determined. Powder coloration was assessed using the CIELAB scale in Konica Minolta colorimeter (CM-5). The morphology of powders was evaluated by scanning electron microscopy. Based on the results, the drying time of freeze-drying of pulp mix in foam was smaller than the drying time of freeze-drying of pulp mix. The drying method influenced the moisture content, water activity and solubility of dry products. The foam mat drying at 60º C promoted greater loss of vitamin C and reddish pigments, as well as increased browning caused by chemical reactions. A more porous structure was observed in the freeze-drying processes. Such results were expected once freeze-drying causes less loss of vitamin C. The foaming process made the freeze-drying faster by increasing the surface area and porosity of the pulp mix. It is concluded that the freeze-drying of pulp mix in foam was the best of the evaluated methods to drying of pulp mix of açaí, strawberry and acerola.

Tecnologia de Alimentos: Tópicos Físicos, Químicos e Biológicos - Volume 2 207

INTRODUCTION

Dehydration to produce powdered fruit allows its longer preservation and waste reduc-tion (Macedo et al., 2020). In addition is a key raw material for obtaining other products in the food industries (Carvalho et al., 2017). The use of mixed fruit pulps to produce powdered products offers the advantages of enabling supplementation with nutrients from different fruits, balancing high acidity and developing new flavours (Ribeiro et al. 2018).

The choice of drying method is important because it may affect the nutritional quality, appearance and flavour characteristics. In foam mat drying, a foaming agent is added to liquid and soft foods to transform them into stable foam following an aeration process (Tavares et al., 2019). The foam that is formed is dried using hot air in a tray dryer at atmospheric pressure. The presence of air bubbles in the foam matrix renders the foam porous, thereby increasing the surface area for evaporation (Lobo et al., 2017). Consequently, this drying process is faster than traditional dehydration using hot air, which promotes reduced loss of nutrients and smells. The reduced processing time is advantageous because drying is one of the most time and energy consuming processes in the food industry (Doymaz and Ismail, 2011).

The use of hot-air drying for food dehydration may cause loss of nutrients and undesira-ble sensory changes (Omolola et al., 2017). In contrast, freeze-drying occurs at temperatures lower than room temperature and without liquid water, thereby preventing the loss of volatile nutrients and compounds through chemical reactions (Rawson et al., 2011) and minimising damage to the structure, texture and appearance of the product.

In freeze-drying, the product is frozen, and the amount water in the product is sub-sequently reduced by sublimation followed by desorption, thus yielding a porous structure (Ishwarya and Nandharamakrishnan, 2015). The main limitation of freeze-drying is the high cost of the process, which is associated with slow drying rates, vacuum maintenance and the energy required for sublimation (Barresi et al., 2009).

The preparation of foam for freeze-drying may be an alternative for decreasing the long processing times. However, few studies regarding foam freeze-drying have been presented in the literature (Muthukumaran, Ratti and Vijaya, 2008; Raharitsifa and Ratti, 2010a; Raharitsifa and Ratti, 2010b). Additional studies are therefore necessary to elucidate the mechanisms involved in the method because many parameters are affected by the structural and porosi-ty changes in the material (Raharitsifa and Ratti, 2010b) due to the incorporation of air into foods that is necessary to form the foam.

Raharitsifa and Ratti (2010a) studied the thickness of the layer to be freeze-dried and successfully reduced the processing time of foamed apple juice compared with that of non-foamed juice when the sample volumes of juice or foam to be dried were equal. However, no studies that assessed whether foam preparation can contribute to increase the drying

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rate for a given sample weight were found in a literature review. Furthermore, the effects that adding the foaming agent to mixed fruit pulp has on the physicochemical and nutritional characteristics of the dried product have not been studied.

The present study aimed to compare the colour properties, physicochemical characte-ristics and vitamin C content of freeze-dried and foam mat dried mixed fruit pulp to identify the best process for preparing powdered mixed fruit pulp. Additionally, the effect of the foam preparation before freeze-drying on the drying time and on the characteristics of the dry product was evaluated.

MATERIALS AND METHODS

Preparation of mixed fruit pulp

Unpasteurised, frozen strawberry (Fragaria vesca), acerola (Malpighia emarginata D.C.) and açaí berry (Euterpe oleracea Mart.) pulps purchased from a local market vendor in Alegre, Espírito Santo (ES), Brazil, were used to prepare the mixed fruit pulp. The pulps were thawed and mixed in equal ratios (1/3 each), which were established in previous studies.

Foam preparation

To prepare the foam, 300g of mixed fruit pulp were placed in a bowl and albumin, which was used as a foaming agent, was added at 5% concentration (w/w). The pulp and albumin mixture was stirred using a domestic mixer at full speed for 20 min. Next, samples of the foam that was formed were dried in either a freeze-dryer or a foam mat dryer.

Freeze-drying

Freeze-drying was conducted using a FreeZone 2.5 litre benchtop freeze-dryer (LAB-CONCO). Prior to freeze-drying, the pulp mixture (20 g) and foam (20 g) were placed in fre-eze-dryer glass flasks and frozen in a freezer (BRAND) at -40 °C for 24 hours. After freezing, the flasks that contained the frozen samples were removed from the freezer and immediately placed in the freeze-dryer. The condenser temperature was -55ºC, and the chamber pressure was 0.014 mBar. The flasks were weighed every 2 hours until they reached constant weight. The dried pulp and foam were removed from the flasks, ground and packed into aluminium


containers to prevent light exposure, thus completing the process.

Foam mat drying at 60ºC

The foam (300 g) was spread on a 10mm-high stainless steel tray and placed in a con-vective dryer (Polidryer) at 60ºC. To construct the drying curves, the tray was weighed over time, outside the drying chamber, until it reached a constant weight. After the drying process, the dehydrated material was ground and packed into aluminium containers to prevent light exposure. This procedure was also performed on the mixed fruit pulp with hot air and without addition of the foaming agent to prepare a control sample.

Equilibrium moisture content and assessment of the time necessary for drying

Equation 1 was used to describe the free moisture variation over time:

where RU is the free moisture ratio; Xt is the moisture value on a dry basis of the food at time t; X∞ is the moisture value on a dry basis after the equilibrium is reached, i.e., the value that the food moisture approaches as the time t approaches +∞, and X0 is the food moisture value on a dry basis at time zero. The experimental data were fitted with mathematical models.

According to the proposed model, the free moisture ratio ranged from 1 (time zero) to zero (as time tended to infinity). Because the zero value for the free moisture ratio is actu-ally a horizontal asymptote, the end time of the procedure cannot be defined as the time at which the free moisture ratio is zero because this time would be infinite. Thus, the end of the process can be defined as the time at which the free moisture ratio reached a value that is sufficiently close to zero. To compare the times required for the drying process using the different treatments, the end time was defined as the time at which the free moisture ratio reached a value of 0.01. Thus, the drying time could be calculated from the model obtained for each curve.

Analysis performed with the powdered mixed fruit pulp

To determine the moisture, 1 g of homogenised sampled was spread on a metal capsule that was previously dried in an oven at 105ºC. The samples were dried in an oven at 105ºC until a constant weight was reached (AOAC, 1995).

The water activity was determined using an Aqualab 3TE thermo-hygrometer (Decagon

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Devices Inc, Pullman, Washington, USA) at 25ºC. The solubility assessment method was adapted from Eastman and Moore (1984). A

total of 1 g of powdered product was added to 100 mL of distilled water and homogenised using an AGI 103 mechanical stirrer (Nova Ética) at a speed of 1.550 xg for 5 minutes. The solution was placed in tubes and centrifuged for 15 minutes at 10.956 xg. A 25 mL supernatant aliquot was placed on previously weighed Petri dishes and dried at 105 °C under atmospheric pressure until a constant weight was reached. The solubility was expressed using Equation 2.

where S is the solubility, Pf is the evaporation residue weight (g) and Pa is the sample

weight (g).The colour of the samples was measured according to Caner and Aday (2009). The

colours of the treated (powder) and untreated (mixed fruit pulp and foam) samples were me-asured using a CM-5 colourimeter (Konica Minolta). The device was calibrated using black and white references. The colour was expressed using the CIELAB scale (with parameters L*, a* and b*). The L* parameter represents the brightness, which ranges from black/dark (0) to white/bright (100); a* represents the variation from red to green (the value becomes more positive as the colour approaches red); and b* represents the variation from yellow to blue (the value becomes more positive as the colour approaches yellow). The colour coor-dinates C* (chroma), h* (hue) and total colour difference (∆E) were calculated according to Equations 3, 4 and 5.

In the above equations, L0, b0 and a0 are the untreated sample values (for mixed fruit pulp and foam). The powders prepared from the foam were compared with the foam prior to drying, and the powders prepared from the mixed fruit pulp were compared with the mixed fruit pulp prior to drying.

Scanning electron microscopy was performed to assess the particle morphology. The dried sample was mounted on the stub using double-sided tape and then taken to a Balzers Union FDU 010 sputter coater. The metallised sample was observed using a JSM-6010 LA scanning electron microscope (JEOL) at 350x magnification.

Vitamin C assessment

The powders prepared after dehydrating the mixed fruit pulp and foam were reconsti-

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tuted at a 1:20 ratio (1g of powder: 20g of distilled water) to perform the following analyses: The vitamin C assessment method was adapted from the official method of the As-

sociation of Official Analytical Chemists (1995), which consists of reducing a coloured subs-tance, 2.6-dichlorophenolindophenol, in titration with a solution that contains ascorbic acid. A 10 mL aliquot of extraction solution (metaphosphoric-acetic acid) was added to 10 mL of the reconstituted sample and centrifuged at 5000 rpm for 15 minutes. 2 mL aliquot of the supernatant was added to a 25 mL volumetric flask, and the remainder of the volume was filled with distilled water. Then, 2 mL of the diluted solution were pipetted and again added to a 25 mL volumetric flask; the remainder of the volume was filled with distilled water. Finally, 2 mL of the new diluted volume were added to 5 mL of extraction solution and titrated using a solution of 2.6-dichlorophenolindophenol diluted in distilled water (1:10). The volume of 2.6-dichlorophenolindophenol spent was used to calculate the vitamin C content.

Statistical analyses

The experiment was conducted using a completely randomised design with 3 replica-tes. The model parameters of the drying curves were fitted using non-linear regression. The physicochemical properties were compared using analysis of variance (ANOVA) and, whe-never necessary, Tukey’s multiple comparisons test. A 5% significance level was adopted in all statistical procedures.

RESULTS AND DISCUSSION

Effect of foaming agent addition on freeze-drying parameters

The freeze-drying curves are similar to the conventional drying curves (Figure 1). This result was expected because both freeze-drying and conventional drying involve simultaneous transfer of heat and mass, and the transport mechanisms are similar in both cases. Figure 1 shows the freeze-drying curve obtained in the present study for the exponential model.

Figure 1. Freeze-drying curve of pulp (T2) and foam (T1).


The initial moisture content was 87.66% for the foam and 92.92% for the pulp. After freeze-drying, the equilibrium moisture content was 3.25% and 11.11%, respectively. The drying time may be calculated using the exponential model

where ts is the drying time, according to the criterion adopted.Table 1 shows the fitted models, coefficient of determination values and drying times

estimated using the fitted models according to the criterion adopted.

Table 1. Parameters fitted to the exponential model regarding the freeze-drying curves, coefficients of determination values and drying times estimated according to the criterion adopted (Eq. 6)

Raw material Fitted model r2 ts (h)

Pulp RU=e-0.184560 t 0.9955 24.95

Foam RU=e-0.253949 t 0.9961 18.13

The models were well fitted to the data, with a coefficient of determination of 0.99 (Ta-ble 1). The time to dry 20 g of foam (18.13 h) was shorter than that needed to dry the same weight of mixed fruit pulp (24.95 h). Thus, the preparation of foam for drying in a freeze-dryer enabled a reduction in the processing time of more than 6 hr. The moisture content of the freeze-dried foam (3.25%) was less than the content of the freeze-dried pulp (11.11%). A different result regarding the time could be found if the process were interrupted when the products achieved similar moisture content. In this manner, dried foam could reach the same moisture reached by dried pulp in an even shorter amount of time. Raharitsifa et al. (2010a) freeze-dried foam and apple pulp and observed an approximately 5-fold reduction in proces-sing time when drying the foam prepared with albumin. This reduction is much greater than the one found in the present study. However, the authors kept the thickness (4 mm) of the material layer to be dried constant, where as the material weight was kept constant in the present study. Because of the lower density of the foam, its weight was much less than the weight of the pulp for a fixed thickness, which explains the greater reduction in drying time.

The choice to prepare foam before the freeze-drying process will also depend on the desired characteristics of the dried product because the addition of a foaming agent changes the properties of the material.

Effect of foaming agent addition on the 60ºC drying parameters

The 60ºC drying curves of the foam and mixed fruit pulp without the foaming agent are

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shown in Figure 2.

Figure 2. Drying curves of foam and mixed fruit pulp at 60ºC.

The initial moisture contents of the foam and pulp were 87.30% and 92.56%, respecti-vely, and the final contents were 7.76% and 9.88%. The drying time may be calculated using the Page model:

where tl is the drying time, according to the criterion adopted.Table 2 shows the fitted models; coefficient of determination values and drying times

estimated using the fitted models according to the criterion adopted.

Table 2. Parameters fitted to the model regarding the 60ºC drying curves, coefficients of determination values and drying times estimated according to the criterion adopted (Eq. 2)

Material Fitted Model r2 ts (min)

Pulp RU=e(-0.002405t)1.307 0.9941 324.24

Foam RU=e(-0.002585t)1.374 0.9958 232.08

Table 2 indicates that the models were well fitted to the data, with a r² value equal to 0.99. The preparation of foam for subsequent drying decreased the processing time from approximately 324.24 minutes to 232.08 minutes. This reduction resulted from the higher rates of water removal in foam compared with pulp because water is present in foam in the form of thin films, which allows easy evaporation (Kandasamy et al., 2014). These same authors found a similar pattern when drying papaya foam and pulp. Thus, preparation of foam from mixed fruit pulp can decrease the conventional drying time.

Effect of treatments on powder solubility, moisture and water activity

There were significant differences in the moisture, water activity and solubility between treatments at the 5% significance level. Table 3 presents the Tukey’s test results for these

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variables.

Table 3. Tukey’s test results for the moisture variable

Treatment % Moisture * Water activity Solubility

FDF 3.25c 0.15c 0.51ab

FDP 11.11a 0.22b 0.48b

FMD 7.76b 0.20b 0.54a

HAD 9.88a 0.27a 0.51ab

* Means followed by the same letter in columns are not significantly different at the 5% significance level. Treatments: FFD – powder derived from foam freeze-drying (freeze-dried foam); FDP - powder derived from mixed fruit pulp freeze-drying (freeze-dried pulp); FMD– powder derived from foam mat drying at 60ºC (foam mat drying); and HAD – powder derived from drying the mixed fruit pulp at 60ºC (hot-air drying).

The solubility measured in all treatments was much less than that found by Dantas (2010) for foam mat dried pineapple (98%) and mango (91%) powders and by Ceballos, Gi-raldo and Orrego (2012) for freeze-dried soursop pulp (81.51% to 85.75%). This difference may have resulted from the high lipid levels that are present in açaí berry powder, which are approximately 40.75% (Menezes et al., 2008).

Freeze-dried foam (FDF) exhibited the lowest mean moisture content, followed by foam dehydrated using foam mat drying (FMD). The mixed fruit pulps freeze-dried and hot-air-dried at 60ºC exhibited the highest means and were not significantly different from each other. Thus, addition of the foaming agent arguably changed the equilibrium moisture content value to smaller values. This change may have resulted from the increased content of total solids due to the addition of the foaming agent. This effect may be advantageous because it could further increase the stability and storage capacity of powdered products. The moisture con-tent of freeze-dried pulp (FDP) corroborated the results from other studies of freeze-dried products (Oliveira et al., 2011).

The moisture content measured when using foam mat drying (FMD) was similar to the values found by Krasaekoopt and Bhatia (2012) for yoghurt powder and Falade and Okocha (2012) for banana powder. Menezes et al. (2009) measured moisture content of 11.37% for green acerola powder that was oven-dried at 70ºC. That result is similar to the one found in the present study for pulp dried at 60ºC (HAD).

All treatments yielded moisture contents that were less than the maximum water activity values recommended for dehydrated foods (0.35 and 0.5). However, water activity that is as low as the one found in FDF is considered detrimental because of increased lipid oxidation reactions (Damodaran, Parkin and Fennema, 2010).

Effect of treatments on colour

Table 4 presents the Tukey’s test results for the colour parameters (L*, a*, b*, C and


h) and calculated ∆E values. The ANOVA results indicated significant differences among the treatments at the 5% significance level using the F-test (p<0.05) for all colour parameters (L*, a*, b*, C and h). Thus, the drying method had a significant effect on the pulp colour.

Table 4. Tukey’s test results for colour parameters (L*, a*, b*, C and h) and calculated ∆E values

TreatmentColour parameter

L* a* b* C h ∆E

P 12.85f 20.48a 4.39c 21.10ab 13.71d -

F 45.90b 18.17ab 4.44c 18.64bc 12.76d -

FDF 55.15a 14.71c 9.24b 17.25c 32.15b 11.02

FDP 26.89d 17.72b 9.28b 20.15ab 28.11bc 14.53

FMD 37.90c 13.57c 17.03a 21.77a 51.44a 11.97

HAD 21.58e 10.88d 4.97c 11.96d 24.52c 15.19

*Means ± standard deviation followed by the same letter in columns are not different at the 5% significance level according to Tukey’s test. Treatments: P - fresh pulp; F - foam; FDF – powder derived from foam freeze-drying (freeze-dried foam); FDP - powder derived from mixed fruit pulp freeze-drying (freeze-dried pulp); FMD – powder derived from foam mat drying at 60ºC (foam mat drying); and HAD – powder derived from drying the mixed fruit pulp at 60ºC (hot-air drying).

A colour difference is clearly visible to the naked eye when the total colour difference (∆E) is greater than 1.5 (Ramos and Gomide, 2007). Thus, the colours of all of the dried pro-ducts were clearly different from the untreated materials. The untreated materials exhibited an h angle in the red colour region, whereas the dried products exhibited a colour that was between red and yellow.

Regarding brightness (L*), the foam colour was brighter than that of the pulp, and the products treated with FDF and FMD had higher brightness values than those treated with FDP and HAD. The colours of the materials treated with FDF were brighter than that of the foam, whereas those treated with FMD were darker in colour, thereby indicating that hot air contributes to darkening powders possibly because of the Maillard reaction of the sugars present in the pulp. Orak et al. (2014) also found higher brightness values for freeze-dried fruits than hot-air-dried fruits.

The red colour of the materials decreased regardless of the treatment applied, and the freeze-dried materials lost less red colour than those treated with hot air, possibly because of the loss of red pigments.

The assessment of the parameter b* indicates that the untreated materials were not dif-ferent from each other except in terms of the yellow colour. The yellow colour increased after all treatments except HAD. Heat contributes to yellow colour intensification (Argyropoulos, Heindl and Müller, 2011). The yellower colour of FMD compared with HAD may be explai-ned by the greater contact surface with the hot air provided by the foam air bubbles. There was an increase in the yellowness of the freeze-dried products compared with the untreated materials. Orak et al. (2014) explain that this observation results from an accumulation of


carotenoids during dehydration in a freeze-drier rather than from Maillard reactions.

Effect of treatments on the vitamin C levels

The results of the ANOVA performed for vitamin C indicated a significant difference among treatments (p < 0.05). Table 5 presents the Tukey’s test results for the vitamin C content.

Table 5. Tukey’s test results for the variables vitamin C

Treatment Vitamin C

1 291.83 ± 12.58b

2 296.09 ± 32.04b

3 263.76 ± 2.01b

4 262.01 ± 16.74b

5 184.02 ± 32.96a

6 142.94 ± 12.65a

*Means ± standard deviation followed by the same letter in columns are not different at the 5% significance level according to Tukey’s test. Treatments: 1 – fresh mixed juice; 2 – fresh mixed juice supplemented with 5% albumin; 3– reconstituted fruit juice from freeze-dried mixed fruit pulp; 4 – reconstituted fruit juice from freeze-dried foam; 5 – reconstituted fruit juice from mixed fruit pulp dried at 60 ºC; 6 – reconstituted fruit juice from powder derived from foam mat drying at 60 ºC.

There was no significant difference in the vitamin C content of fresh juice (treatments 1 and 2) compared with freeze-dried juice (treatments 3 and 4). Thus, the freeze-drying process caused no significant losses of vitamin C. The use of low temperatures and vacuum may account for this result (Liaotrakoonet al., 2012).

Fruit juice that was reconstituted from powder yielded by the foam mat drying process (treatment 6) had significantly lower ascorbic acid levels than fruit juice that was reconstituted from freeze-dried foam (treatment 4). This difference may result from the oxidation reactions that are accelerated by the heating and aeration that occur in the foam mat drying process. Reconstituted fruit juice from the mixed fruit pulp dried at 60ºC (treatment 5) exhibited the same pattern.

Similar results to those found in the present study were reported by Joshi et al. (2011), who used different methods to dry apple slices. The authors concluded that apples that were vacuum-dried at low-temperature exhibited no losses of ascorbic acid compared with fresh apples. However, the product dried at 70ºC without using a vacuum exhibited a reduction in vitamin C levels of approximately 69%.

In contrast with the present study, Orak et al. (2014) observed a reduction in the vitamin C content of freeze-dried arbutus berries compared with the fresh fruit. That reduction may be explained by the small losses that occur during freeze-drying because of exposure to li-ght and moisture (Liaotrakoon et al., 2012). However, the reduction in the vitamin C content


during freeze-drying that was observed by Orak et al. (2014) was less than that caused by hot air drying.

Some authors observed vitamin C losses that were greater than those found in the present study. Maharaj and Sankat (1996) reported a 70.3% loss for yams that were foam mat-dried at 60ºC. Kadam et al. (2012) reported an ascorbic acid loss of 52.28% when drying pineapple foam prepared with albumin.

The addition of the foaming agent caused no change in the vitamin C content of fresh and reconstituted juices. A similar pattern was observed by Kadan, Wilson and Kaur (2010) in a study of mango drying.

Effect of treatment on morphology

Figure 3 shows scanning electron micrographs of the products dried using different drying methods.

Figure 3. Scanning electron micrographs of powdered mixed fruit pulp: (a) FFD: derived from foam freeze-drying (freeze-dried foam); (b) FDP: powder derived from mixed fruit pulp freeze-drying (freeze-dried pulp); (c) FMD: powder derived from foam mat drying at 60ºC (foam mat drying); (d) HAD: powder derived from drying the mixed fruit pulp at 60ºC

(hot-air drying).

Analysis of the micrographs indicated that the sample derived from the FDF treatment (Figure 1a) had many cavities in its structure and an almost smooth surface. This morpho-logy indicates that freeze-drying was able to maintain the pores, which were caused by the presence of air bubbles and existed in the foam matrix prior to drying (Pinto, 2012). Thus, the freeze-drying process did not cause matrix shrinkage. A similar morphology was observed in the sample that was submitted to the FDP treatment (Figure 1b), but the cavities were smaller because of the absence of air bubbles in the fresh pulp.

The FMD and HAD treatments resulted in very similar micrographs (Figures 1c and 1d),


which exhibit a dense structure and rough surface. This morphology may have been caused by the collapse and shrinkage of the foam matrix and pulp, phenomena that may occur during hot-air drying (Aguilera, Cuadros and Valle, 1998). These results corroborate the findings of Argyropoulos, Heindl and Müller (2011), thereby indicating that the type of drying affects the morphology of the dried material.

CONCLUSIONS

The preparation of foam from mixed fruit pulp for freeze-drying reduced the drying time when samples with the same weight were compared. The drying method and addition of albumin affected the moisture and water activity of the dried product. The sample solubility was less than that found in other studies for all treatments. The vitamin C content of the fre-eze-dried pulp was not different from that of the fresh pulp. The colours of the dried samples were visibly different from those of the fresh samples for all treatments. The methods that used hot air resulted in greater darkening and degradation of red pigments. Freeze-drying preserved the foam and pulp matrix structure, thereby resulting in more porous and brittle products. Under the study conditions, freeze-drying was the best method to dehydrate mixed fruit pulp with the least loss of quality.

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