Abstract
The health properties of fruit are widely known. Powdered fruit may be a practical format to be offered to the consumer. Nevertheless, the process used to obtain the powder must ensure the maximum retention of the bioactive compounds and the functional value of the fruit while retaining adequate physical properties. The aim of this study was to compare freeze-drying and spray drying as the drying technologies to obtain grapefruit powder. The obtained results allow freeze-drying to be proposed as a better technology than spray drying in order to obtain a product with a higher content of vitamin C and total carotenoids. Moreover, all of the edible part of the fruit is used in this case, so a greater quantity of healthy compounds is preserved and by-product generation is avoided. Adding about 6 g water, 4 g Arabic gum and 0.6 g bamboo fibre/100 g grapefruit pulp is recommended before freeze-drying.
Introduction
Grapefruit is a very common variety of citrus fruit and an important source of phytochemicals and micronutrients. Vitamins, carotenoids and phenolic compounds, among others, could be independently or jointly responsible for the health protective effects of this fruit (Habauzit et al., 2014). The water-soluble fraction of citrus (polyphenols and vitamin C (VC)) is mainly responsible for the antioxidant and antiradical activity of fruits, while the apolar fraction (such as carotenoids, vitamin E or vitamin A) leads to the protective effects against chronic and degenerative diseases (Sdiri et al., 2012). Within the group of phenols, citrus flavonoids are associated with a reduced risk of coronary heart disease, inflammatory pathologies and tumor progression (Benavente-García and Castillo, 2008). Grapefruit is a rich source of flavonoids, especially naringin and narirutin flavanones, with physiological properties that inhibit cell proliferation, promote differentiation, function as antioxidants and are modulators of tyrosine kinases (Vanamala et al., 2006). Grapefruit is also a source of VC, commonly recognized as a major, naturally occurring nutrient and antioxidant (Dow et al., 2012). In pink grapefruit varieties, β-carotene (pro-vitamin A) and lycopene are responsible for the colour, also contributing to the antioxidant capacity (AOA) of the fruit (Xu et al., 2006). Despite the high functional value of grapefruit and the advances in the scientific knowledge of consumers regarding the binomial health-diet, the consumption of grapefruit is low, probably due to its strong, bitter taste. In this sense, it would be interesting to obtain processed grapefruit products that, while maintaining most of their functional value, could be mixed with other foods or added as an ingredient.
Powdered fruit consumption could be nutritionally equivalent to that of fresh fruit in smaller serving sizes, ranging from 30 to 43 g depending on the fruit. Epidemiological studies have found an association between fruit consumption, both fresh and dehydrated, and the prevention of diseases probably due to fruits being excellent sources of phytochemical compounds in the diet (Blasa et al., 2010). The powdered fruit market also has the advantage of being much more stable than that of fresh fruit, benefiting from the fact that the product is available all year round and is easier to store and distribute. Nevertheless, the process used to obtain the powder must ensure the maximum preservation of the bioactive or functional fruit compounds.
Spray drying (SD) is a technique that allows powder to be obtained from fruit. It is a rapid dehydration method with intensive water evaporation on the surface of the droplets which keeps them cool until the dry state is reached, thereby leading to high-quality powders (Fang and Bhandari, 2012). It is the most commonly used encapsulation method in the food industry (Rajam and Anandharamakrishnan, 2015). The properties of spray-dried powders are mainly affected by the process conditions, such as the air inlet and outlet temperatures, the air flow rate, the type of atomizer and the feed properties (Igual et al., 2014). This method has a number of advantages, some of which are the production of free flowing powders with a controlled particle size, rapid drying and easy scale up (Rajam and Anandharamakrishnan, 2015). Freeze-drying (FD) may also be used to obtain powdered foods. FD involves the removal of water by sublimation of the frozen material, usually under low applied pressure. This technique is considered as a benchmark for powders of high quality, since one of its main advantages is to preserve attributes such as taste, nutrients, color or flavor. Nevertheless, in some previous studies, losses of food vitamins, antioxidant compounds and nutritional or functional value due to FD have been reported (Shofian et al., 2011). Besides, the main disadvantage of this technique is its high cost, both in terms of time and energy. Nevertheless, the application of pre-treatment for the purposes of removing some of the water present in the food contributes to a reduction of the cost involved (Benlloch-Tinoco et al., 2013; Donsi et al., 2001; Fahloul et al., 2009).
When a dehydration process is shorter than that needed for the crystallization of solutes, an amorphous matrix is obtained, which can be found in a glassy or rubbery state, depending on the glass transition temperature (Tg). Powdered foods in the rubbery state may suffer structural collapse phenomena, quickly exhibiting stickiness and caking problems (Adhikari et al., 2003; Gabas et al., 2007). Fruits, with a high content of both organic acids and low molecular weight sugars, have a low Tg and, for dehydrated products, the rubbery state prevails under the usual storage conditions (Roos, 1995). In order to increase the Tg and promote the much easier-to-handle and stable glassy state, adding high-molecular weight additives to the product before drying is a widely used alternative (Truong et al., 2005; Yousefi et al., 2011). Both spray and FD processes lead to amorphous fruit powders. The addition of carriers is particularly necessary in the case of SD so as to avoid the adhesion of dust particles, not only to each other but also in the team, in order to increase product yield and avoid operational problems (Gabas et al., 2007; Yousefi et al., 2011). Some of the materials commonly used to increase the Tg can, at the same time, act as encapsulating agents. Arabic gum (AG), a natural plant exudate of Acacia trees, has been the encapsulating agent of choice for many years because it is an excellent emulsifier, has a bland flavor and prevents water adsorption, oxidation and the volatilization of compounds (Gabas et al., 2007; Rascón et al., 2011; Singthong et al., 2009). This solute also contributes to an increase in the Tg and reduces the hygroscopicity, thus increasing the stability of the fruit powder. Bamboo fibre (BF) is extracted from the plant Bambusoideae subfamily and is mainly composed of hemicellulose, cellulose, pectin and lignin (Liu et al., 2012). Despite the fact that this solute has not been used with this purpose in mind, its high molecular weight makes it a possible candidate to increase the Tg, with the added value of being a healthy vegetable fibre.
The aim of this study was to compare FD and SD as drying technologies in order to obtain grapefruit powder with the highest content of bioactive compounds, such as the total phenolics, total carotenoids and VC, and the highest functional value measured through the AOA. Moreover, yield information together with the water content, hygroscopicity, colour and porosity of the powder have also been considered.
Materials and methods
Raw material
This study was carried out with grapefruit (Citrus paradise var. Star Ruby) purchased in a local supermarket (Valencia, Spain). Two different batches were used in the study, one for all FD and one for all SD experiments. The grapefruits were washed and peeled with the careful removal of the albedo. AG (Scharlau, Spain) and BF (VITACEL®, Rosenberg, Germany) were added to the grapefruit pulp.
FD
Preparation of feed mixture and process conditions
Matrix of the central composite design of freeze-dried powder
Note: x1, x2 and x3 are the feed water content (gwater/100 gfeed), Arabic gum (g/100 g pulp + solutes) and bamboo fibre (g/100 g pulp + solutes), respectively, and the experimental results (with standard deviation in brackets) are TP: Total phenolic content (mg GAE/100 gGS), VC: Vitamin C content (mg/100 gGS), TC: total carotenoid content (mg β-carotene/100 gGS), A0A: Antioxidant capacity (mmol TE/100 gGS), xw: Water content (gwater/100 gGS), Hg: hygroscopicity (gwater/100 gGS), ɛ: porosity (air volume/total volume), L*: lightness and ΔE*: colour difference with respect to the freeze-dried fresh fruit. GS are the grapefruit’s own solutes.
SD
Preparation of feed mixture and process conditions
Matrix of the central composite design of spray dried powder
Note: x1, x2 and x3 are the temperature (℃), Arabic gum (g/100 g liquidized) and bamboo fibre (g/100 g liquidized), respectively, and the experimental results (with standard deviation in brackets) are TP: Total phenolic content (mg GAE/100 gGS), VC: Vitamin C content (mg/100 gGS), TC: Total carotenoid content (mg β-carotene/100 gGS), A0A: Antioxidant capacity (mmol TE/100 gGS), xw: Water content (gwater/100 gGS), YP: Product yield (g powder/100 g feed), YD: Drying yield (g water/100 g feed), Hg: Hygroscopicity (gwater/100 gGS), ɛ: Porosity (air volume/total volume) and L*: Lightness. GS are the grapefruit’s own solutes.
Analytical determinations
Ground grapefruit (GG) and liquidized grapefruit (LG)
GG and liquidized grapefruit (LG) were characterized, in triplicate, as to their water content (xw), soluble solids (xs), bioactive compounds (total phenolics, VC, total carotenoids) and AOA. The mass fraction of water was obtained by drying the samples in a vacuum oven (Vaciotem, J.P. Selecta) at 60 ℃ ± 1 ℃ under p < 100 mm Hg until constant weight (AOAC, 2000, method 934.06). The mass fraction of soluble solids was obtained at 20℃ by measuring the °Brix (Refracto 30 PX, Mettler Toledo at 20 ℃) of the previously homogenized samples. The content of bioactive compounds was determined following the methodology described in the next section.
In order to make the results of GG and LG comparable, those of the liquidized sample were referred to the corresponding ground fruit (equation (1)).
Powdered samples
The response variables considered for each obtained powder were analyzed in triplicate. The analysis of the total quantity of phenols (TP) was based on the Folin-Ciocalteu reagent. For the extraction, 35 g of the sample were homogenized (T25D Ultra-turrax, IKA, Germany) for 5 min with 40 mL of methanol, 10 mL of HCl (6 N) and NaF (2 mM) to prevent the phenolic degradation caused by polyphenol oxidase action. The homogenate was centrifuged at 12.857 × g and 4 ℃ for 10 min (Eppendorf centrifuge 5804 R, Germany). For quantification purposes, 15 mL of distilled water and 1.25 mL of Folin Ciocalteu reagent (Sigma-Aldrich, Germany) were added to 250 µL of the supernatant. The samples were mixed and allowed to stand for 8 min in darkness before 3.75 mL of 7.5 % sodium carbonate aqueous solution was added. Water was added to adjust the final volume to 25 mL. The samples were allowed to stand for 2 h at room temperature before absorbance was measured at 765 nm in a UV-visible spectrophotometer (Thermo Electron Corporation, USA). The total phenolic content was expressed as mg of gallic acid equivalents (GAE) per gram of sample, using a standard curve range of 0–800 mg of gallic acid (Sigma-Aldrich, Germany)/L.
VC was determined by high-performance liquid chromatography (HPLC) (Jasco, Italy). To quantify the total VC content, dehydroascorbic acid was the reduced to ascorbic acid by mixing 0.5 g sample with 2 mL of a 20 g/L DL-dithiothreitol solution for 2 h at room temperature and in darkness (Igual et al., 2014). Afterwards, 1 g of this mixture was extracted with 9 mL 0.1% oxalic acid for 3 min and immediately filtered through a 0.45 µm membrane filter before injection (Xu et al., 2008). The HPLC conditions were: Ultrabase-C18, 5 µm (4.6 × 250 mm) column (Análisis Vínicos, Spain); mobile phase 0.1 % oxalic acid, volume injection 20 µL, flow rate 1 mL/min, detection at 243 nm and at 25 ℃. A standard solution (Panreac, Spain) was prepared. The VC content was calculated as mg/g sample.
The total quantity of carotenoids (TC) present in the samples was extracted following the methodology recommended by Olives Barba et al. (2006). Briefly, 5 g of the sample were mixed with 100 mL of hexane/acetone/ethanol (50:25:25, v/v/v) for 30 min. Distilled water (15 mL) was added, and an upper layer aliquot of 0.6 mL was dried under a stream of liquid nitrogen. The residue was dissolved with tetrahydrofuran/acetonitrile/methanol (15:30:55 v/v/v) solution to a final volume of 1 mL. The spectrophotometric AOAC reference method (2000) was used for quantification. The sample absorbance was measured at 446 nm in a UV-visible spectrophotometer (Thermo Electron Corporation, USA). The total carotenoid content was expressed as mg of β-carotene (Fluka-Biochemika, USA).
The AOA was assessed by using the free radical scavenging activity of the samples evaluated with the stable radical 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH, Igual et al., 2014). Briefly, the samples were homogenized and centrifuged (Eppendorf centrifuge 5804 R, Germany) at 12,857 × g and 4 ℃ for 10 min. The supernatant (0.1 mL) diluted in methanol was added to 3.9 mL of DPPH diluted in methanol (0.030 g/L, Sigma-Aldrich, Germany). At 25 ℃, the same spectrophotometer mentioned before was used to measure the absorbance at 515 nm at 0.25 min intervals until the reaction reached the steady state. Appropriately diluted samples were used on the day of preparation. The percentage of DPPH was calculated following equation (2). The final results were converted to mmol trolox equivalents (TE), using a trolox (Sigma-Aldrich, Germany) calibration curve in the range 6.25–150 mM.
The water content was obtained as previously described in section “analytical determinations of GG and LG”. For the hygroscopicity (Hg), about 2 g of each powder were placed in a Petri dish at 25 ℃ in an airtight plastic container containing a Na2SO4 saturated solution (81% RH). After 24 h, each sample was weighed and hygroscopicity was expressed as the g of water gained by the sample.
The colour of the powder was measured in a previously compressed sample as described by Telis and Martínez-Navarrete (2010) by using a Minolta CM-2002 Camera Co. (Japan). From the CIE L*a*b* colour coordinates obtained with a D65 illuminant and 10° observer, the Lightness (L*) of the samples and, in the case of the freeze-dried samples, the total colour difference (ΔE*) with respect to a freeze-dried grapefruit sample without added solutes or water, were considered. The aim of this measurement was to evaluate the effect of added solutes on the colour of the obtained powder. This ΔE* study was not carried out with the spray dried samples as it is very difficult in this case to obtain a powdered fruit sample without solutes added acting as a process carrier.
The porosity (ɛ) was calculated from the true and bulk densities (equation (3)). The true density (ρ) of a mixture was calculated from its individual components. In this case, water and carbohydrates, their own and added, were considered to be the main components of the samples (equation (4)). For the purposes of bulk density (ρb) determination, approximately 2 g of the powder were transferred to a 10 mL graduated test tube and stirred for 10 s at 1600 r/min in a Vortex (Velp WX F202A0230, Italy). The bulk density was calculated by dividing the mass of the powder by the volume occupied in the tube after stirring.
For spray dried samples, the process yield was also considered, taking into account both the product yield (YP, equation (5)) and the drying yield (YD, equations (6) and (7)).
As each sample had a different composition of added solutes (Tables 1 and 2), all the compositional results were referred to the grapefruit’s own solutes (GS) (equations (8) and (9)) to make the results comparable.
Experimental design and statistical analysis
For this study, RSM was used to evaluate the effect of three independent process variables on different response variables, mainly related to the functional, nutritional and physical quality of the powder. In the case of FD, the feed inlet moisture (70–90 g water/100 g feed, x1), AG concentration (4–12 g AG/100 g feed, x2), and BF concentration (0–2 g BF/100 g feed, x3) were selected as independent variables. In order to reach the feed inlet moisture level, water was added, or microwave energy (Moulinex 5141 AFW2, Spain) was applied to dehydrate the samples. The independent variables for SD were the inlet air temperature (120–180℃, x1), AG (4–12 g AG/100 g LG, x2) and BF concentration (0–2 g BF/100 g LG, x3). The value ranges considered for the independent variables respond to previous studies (Agudelo et al., 2014; Igual et al., 2014; Kha et al., 2010; Quek et al., 2007). Twenty-three experimental runs were generated for each process based on the corresponding central composite design rotable and orthogonal (Tables 1 and 2). The experiments were randomized.
Both an analysis of variance (ANOVA) and a regression surface analysis were conducted to define the statistical significance of the model terms and to fit a regression relationship relating the experimental data to the independent variables. The generalized polynomial model proposed for predicting the response variables as a function of the independent variables was given by equation (10)
The terms which were statistically non-significant (p > 0.05) were dropped from the initial model, and the experimental data were refitted only to significant (p < 0.05) independent variable effects in order to obtain the final reduced model (Mirhosseini et al., 2009). The lack of fit of every selected final model (p > 0.05) confirmed the suitability of the fitted model and the non-significance of the Durbin-Watson proved that there was no significant autocorrelation or serial correlation. The goodness of the fit of the final reduced models to the experimental data was evaluated from the coefficient of determination adjusted (R2adj) and the standard error of estimate (EE) between the predicted and experimental values.
For the multiple response optimization, a response optimizer was used to determine the combination of input variable settings that jointly optimized the significant response variables. Through this optimization procedure, a combined level of the considered FD or SD independent variables was obtained to produce the grapefruit powder with the largest quantity of phenols and carotenoids and the highest VC content and AOA. Furthermore, in order to discover the significant differences (p < 0.05) between the FD and SD processes, an ANOVA was performed considering the losses of each compound caused by each process. All the statistical analyses were conducted using Statgraphics Plus 5.1 (Statgraphics Plus 5.1. for Windows, 2000).
Results and discussion
Characterization of the two grapefruit batches used for freeze-drying (FD) or spray drying (SD)
Note: Different superscripts within the same row indicate significant differences (p < 0.05) between both batches (A or B) and between grinded or liquidized (a or b).
Regression coefficients, adjusted determination coefficient (R2) and standard error of the estimate (EE) for the final reduced models of freeze-dried powders
βi: estimated regression coefficient for the main linear effects; β2i: estimated regression coefficient for the quadratic effects; βij: estimated regression coefficient for the interaction effects. Subscripts i = 1: water content of the sample incoming to the freeze-drier (gwater/100 gpulp + solutes); i = 2: gum Arabic (g/100 g pulp + solutes); i = 3: bamboo fibre (g/100 g pulp + solutes). TP: Total phenolic content (mg GAE/100 gGS), VC: Vitamin C content (mg/100 gGS), TC: Total carotenoid content (mg β-carotene/100 gGS), A0A: Antioxidant capacity (mmol TE/100 gGS), xw: Water content (gwater/100 gGS), Hg: Hygroscopicity (gwater/100 gGS), ɛ: Porosity (air volume/total volume); L*: Lightness and ΔE*: Colour difference with respect to the freeze-dried fresh fruit. GS are the grapefruit’s own solutes.
Regression coefficients, adjusted determination coefficient (R2) and standard error of the estimate (EE) for the final reduced models of spray-dried powders
βi: estimated regression coefficient for the main linear effects, β2i: estimated regression coefficient for the quadratic effects, βij: estimated regression coefficient for the interaction effects. Subscripts i = 1: temperature (℃); i = 2: gum Arabic (g/100 g liquidized grapefruit); i = 3: bamboo fibre (g/100 g liquidized grapefruit). TP: Total phenolic content (mg GAE/100 gGS), VC: Vitamin C content (mg/100 gGS), A0A: Antioxidant capacity (mmol TE/100 gGS), xw: Water content (gwater/100 gGS), YP: Product yield (g powder/100 g feed), YD: Drying yield (g water/100 g feed), Hg: Hygroscopicity (gwater/100 gGS), ɛ: Porosity (air volume/total volume) and L*: Lightness. GS are the grapefruit’s own solutes.
The total phenolic content of FD powders varied between 192 and 574 mgGAE/100 gGS. As shown in Tables 1 and 4, the water content of the sample coming into the freeze-drier was observed to have no effect on TP content. The positive linear and the negative quadratic effect of the AG and BF content (Table 4) lead to intermediate levels of both solutes being the most convenient with which to obtain a product with a greater amount of phenolic compounds (Figure 1(a)). The TP content of SD powders varied between 100 and 504 mg GAE/100 gGS (Table 2). In this case, only the temperature showed a positive linear and negative quadratic effect on total phenols so that intermediate temperatures favour the extraction of these compounds (Figure 1(b)). The same behaviour has been observed by Sharma et al. (2015) working with dehydrated onion.
Response surface for the total phenol (TP) content of the freeze-dried (a) and spray dried (b) powders. Values of TP are referred to grapefruit’s own solutes (GS) as a function of Arabic gum (AG) and bamboo fibre (BF) content (g/100 gpulp + solutes or liquidized) when the water content of the feed is 80 g/100 g (a) or as a function of Arabic gum (AG) and temperature (℃) when BF is 1 g/100 gliquidized (b).
The content of VC ranged from 439 to 831 mg/100 gGS in FD samples (Table 1) and from 314 to 700 mg/100 gGS in SD powders (Table 2). In the case of FD powders, a positive effect of increasing the water and AG content of the sample coming into the freeze-drier was observed, while the positive effect of BF was offset by the strong negative quadratic effect (Table 4). In this way, the powder with the greatest VC content was obtained when the sample was processed with the highest water and AG content and with an intermediate BF content. Figure 2(a) shows the change in the VC content of the FD powder, dependent on the added solutes, for an intermediate water content. In SD samples, the increase in temperature caused a clear decrease in the VC content of the powders while a protective effect of BF was observed up to an intermediate content of this solute (Table 5). The effect of VC degradation caused by the high temperature applied during SD was also observed by Langrish (2009) and Solval et al. (2012) and the protective effect of AG by Ali et al. (2010), among others.
Response surface for the vitamin C (VC) (a) and total carotenoid (TC) content (b) of the freeze-dried powders. Values are referred to grapefruit’s own solutes (GS) as a function of Arabic gum (AG) and bamboo fibre (BF) content (g/100 gpulp + solutes) or water content (g/100 g). An intermediate value of the variable that does not appear is always considered.
The total carotenoids of the FD samples ranged from 9 to 34 mg/100 gGS (Table 1). They were positively affected by all the independent variables, while some negative quadratic effects and interactions were detected (Table 4). As a result, an increase in the water content prior to FD together with an intermediate-high BF content and a low amount of AG lead to the highest carotenoid content in the obtained powder (Figure 2(b)). As shown in Table 2, the TC of the SD samples varied between 0 and 30.2 mg β-carotene/100 gGS and they were not significantly (p > 0.05) affected by any of the independent variables considered. Kha et al. (2010) also found no statistical difference in TC of spray-dried gag fruit at temperatures between 140 ℃ and 200 ℃.
The AOAs of the extracts obtained with methanol were evaluated by DPPH method. Despite this procedure allows to evaluate the hydrophilic AOA, rather than the total AOA, hydrophilic phenols and ascorbic acid are the major compounds of fruits contributing to AOA (Boeing et al., 2014; Pulido et al., 2003). FD and SD samples showed values between 12.1–194 mmol and 42–184 TE/100 gGS, respectively (Tables 1 and 2). In this case, AOA was promoted when the sample coming into the freeze-drier had the highest water content and an intermediate AG concentration, with no significant effect of BF (Table 4). Despite the corresponding response surface plot, taking into account the regression coefficients shown in Table 4, for AOA has not been included to avoid increasing in excess the number of figures in the manuscript, this behavior of AG, similar to that previously described in the case of TP (Figure 1(a)), is due to the positive linear effect and the negative quadratic effect shown. As for the SD powder, only a significant (p < 0.05) negative linear effect of AG was observed. No significant correlation between AOA and the bioactive compounds was found.
From the above results, it seems that the presence of solutes is of greater necessity as protection for the bioactive compounds, especially VC and phenols, during the long FD process than during the short SD process, where the temperature is a critical variable. BF may play a steric role, while AG interacts with water, both of which avoid contact between the different substrates involved in deteriorative reactions.
The water content of FD and SD powders varied between 2.5–7.92 g water/100 gGS and 1.68–15.53 g water/100 gGS, respectively (Tables 1 and 2). These values correspond to 1.1–4.2 g water/100 g FD powder, which are normal values for a freeze-dried product (Benlloch-Tinoco et al., 2013) and 0.8–7.8 g water/100 g SD powder (Igual et al., 2014). A clear negative effect of the water content of the sample coming into the freeze-drier or the temperature used for SD was observed (Tables 4 and 5, respectively). The greater the xw or the T, the lower the water content of the powders. The same effect of T has been observed for other fruits and vegetables, and it has been related to a higher rate of heat transfer into particles, causing faster and intense water removal (Kha et al., 2010; Quek et al., 2007). As for FD, the greater water content prior to the freezing of the samples leads to a more diluted system, which eases the water to crystallize and sublimate (Fabra et al., 2009). FD samples also exhibited an interaction with the added solutes (Figure 3(a)). If only AG is added, the greater the AG content the greater the powder water content. This could be linked to a cryoprotective role played by AG, leading to a smaller amount of ice formed during the freezing step prior to FD (Benlloch-Tinoco et al., 2013; Mosquera et al., 2012). Nevertheless, when BF is added, the powder water content decreases. A steric role of BF preventing the interaction of water with the gum could justify this result. In this way, the powders with the lowest water content were obtained when the greatest amount of both solutes was added.
Response surface for the water content (xw) of the freeze-dried powders (a) and the hygroscopicity (Hg) (b) or product yield (YP) (c) of the spray dried powders. Values are referred to grapefruit’s own solutes (GS) as a function of Arabic gum (AG) and bamboo fibre (BF) content (g/100 gpulp + solutes or liquidized) or temperature (℃). An intermediate value of the variable that does not appear is always considered.
The hygroscopicity of all the powders was increased after AG addition and, in the case of the SD powders, intermediate temperatures and BF content lead to the greatest Hg (Figure 3(b)). The porosity of the FD or SD powders increased when BF or AG, respectively, was added up to an intermediate level; moreover, a positive effect of the water content of the sample coming into the freeze drier was observed (Tables 4 and 5). A greater porosity corresponds to a more free-flowing powder. The colour of the samples was affected by the solute content because of its white colour, which increased the luminosity of the powders (Tables 4 and 5), although as observed by Kha et al. (2010), L* was not significantly influenced by SD T. Despite the drying yield of all the SD samples being high (between 87 and 93 g water evaporated/100 g feed), the product yield was very low in every case (between 1 and 7 g powder/100 g feed), due to the composition of the fruits, as described in the introduction section. A clear increase in YP was observed when AG was added and T was increased (Figure 3(c)).
The powdered grapefruit products with the best functional quality should be those with the maximum amount of the analyzed bioactive compounds and AOA. As regards the other analyzed properties, a powder with a low xw, Hg, L*, ΔE and a high ɛ and YP would be preferred. In this sense, the response variables that were significantly (p < 0.05) correlated with the independent variables were maximized or minimized for both FD and SD processes in an optimization of multiple response. In the case of FD, the optimum combination of the independent variables with which to obtain the best powder were 90 g water/100 g feed, 4 g AG/100 g grapefruit pulp + solutes and 0.56 g BF/100 g grapefruit pulp + solutes. For SD, the best grapefruit powder will be obtained by adding 4 g AG and 2 g BF to 100 g LG and using an inlet air temperature of 120℃.
For the purposes of knowing which of the two processes, FD or SD, has the greatest effect on both the content of bioactive compounds and the AOA of the obtained powders, a statistical comparison was carried out. As commented on above, due to the differences found between the batches used for each process, the comparison was made between the losses caused by each drying technique (equation (11)). Data from Tables 1 to 3 were used to this end. No significant differences (p > 0.05) were observed in the case of TP and AOA, while the losses in VC and TC were greater (p < 0.05) in the case of the SD process.
Conclusion
FD may be proposed as a better technology than SD with which to obtain grapefruit powder with the highest VC and total carotenoid content. The liquidizing step, which is necessary before SD, leads to a loss not only in the insoluble solutes of the fruit, which include fibre and carotenoids but also in VC, phenolics and AOA. Moreover, VC and carotenoids turned out to be more sensitive to the high temperatures used in SD, despite the presence of added potential microencapsulating agents. Nevertheless, the use of these agents in SD is necessary in order to increase the product yield. The freeze-dried powders with the highest functional quality were those obtained from grapefruit pulp with approximately 6 g water + 4 g AG + 0.6 g BF/100 g grapefruit pulp. The water and solutes added improve the preservation of total phenols, VC and total carotenoids and decrease the water content of the more free-flowing obtained powders.
Footnotes
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors thank the Ministerio de Economía y Competitividad for the financial support given through the Project AGL 2012-39103.