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PAPER 
 
 
 
 
 

APPLE SLICES ENRICHED WITH ALOE VERA 
BY VACUUM IMPREGNATION 

 
 
 
 
 

A. DEROSSI, I. RICCI, A.G. FIORE and C. SEVERINI* 
Department of Science of Agriculture, Food and Environment, University of Foggia, Via Napoli 25, 71122 

Foggia, Italy 
*E-mail address: carla.severini@unifg.it 

 
 
 
 
 

ABSTRACT 
 
Recently, the interest in Aloe vera has been increased accordingly with its content in 
polymannas, which show many healthy effects. The vacuum impregnation (VI) was used 
to enrich apple slices with Aloe vera gel. The effects of vacuum level, vacuum and 
relaxation times on the main chemical and physical attributes were described. Results 
showed as, applying the best operating conditions, VI allowed to introduce Aloe vera gel 
into the pores of apple tissue reaching a content of polymannan between 1 and 8 mg/100 g 
of fresh apples. 
 
 
 
 
 

Keywords: fresh cut apples, polymannans, enriched fruit, aloe vera gel 
 



	

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1. INTRODUCTION 
 
Aloe vera (Aloe barbadensis Mill.) is a perennial xerophyte belongs to the Liliaceae family, 
which consists in about 360 species. The internal fraction of the leaves is a large thin-
walled parenchyma cells in which the water is held as a gel (NEWTON, 2004). The Aloe 
vera’s gel consists of water (99.5%) and solids (0.5%) such as polysaccharides, vitamins, 
minerals, enzymes, phenolic compounds and organic acids (ESHUN and HE, 2004; 
BOUDREAU and BELAND, 2006). However, a mass fraction of 60% of the total solids is 
constituted by polysaccharides (MCANALLEY, 1993). Recently, the interest on Aloe vera 
gel has increased on the basis of its healthy properties. Back in the past, peoples used the 
Aloe vera for its curative and therapeutic properties. Recently the intake of the gel has been 
proved to have positive effects in the treatment of gastrointestinal, kidney and 
cardiovascular deseases. Also, the gel has used to reduce the cholesterol and triglyceride 
levels in human blood (LIM et al., 2003; GEREMIAS et al., 2006). Furthermore, anti-
inflammatory and antibiotic properties, as well as positive effects against several diseases, 
have been reported (REYNOLDS and DWECK, 1999; ESHUN and HE, 2004). Several 
scientific papers (HAMMAN, 2008; RODRIGUEZ et al., 2010) proved the most of the 
health benefits may be attributed to polysaccharides (DAGNE et al., 2000; NI et al., 2004; 
HABEEB et al., 2007) such as cellulose, hemicellulose, glucomannans, mannose derivatives 
and acetylated mannans (ROBERT and TRAVIS, 1995; FEMENIA et al., 1999; LEE et al., 
2001). Given these considerations, food industries have been attracted from the use of Aloe 
vera as an ingredient to improve the functional properties of food products. Moreover, the 
FDA approved the use of Aloe vera gel extracted as a “dietary supplement” 
(RAMACHANDRA and SRINIVASA RAO, 2008). Thus, a wide number of enriched foods 
with Aloe vera such as yogurt, juice, pasta are available on the market. However, food 
processing could induce irreversible modifications of the polysaccharides, reducing or 
inactivating the health effects (FEMENIA et al., 2003; ESHUN and HE, 2004; CHANG et al., 
2006; VEGA-GÁLVEZ et al., 2011).  
The vacuum impregnation (VI) is a very interesting technique that allows to introduce, 
dissolved or suspended substances in the void fraction (i.e. the pores) of food in a 
controlled manner (GRAS et al., 2002). VI occurs at room temperature, avoiding the 
thermal degradation of nutritional and functional compounds. In the last years, several 
authors studied the application of VI to obtain foods enriched with structural compounds 
(MARTINEZ-MONZO et al., 1998), probiotic microorganisms (PUENTE et al., 2009), fruit 
juices (BETORET et al., 2012; CASTAGNINI et al., 2015), phenols (SCHULZE et al., 2014), 
folic acid (MORENO et al., 2016), sugars (NERI et al., 2016), etc. SANZANA et al. (2011) 
evaluated the effects of VI with Aloe vera on the respiration rate of some vegetables 
(endive, cauliflower, broccoli and carrots). In some previous papers we applied the 
vacuum impregnation in order to accelerate the acidification of some vegetables (DEROSSI 
et al., 2013a,b) as well as to improve the quality of thawed truffles by introducing anti-
freezing proteins (DEROSSI et al., 2015a).  
Given these considerations, the main aim of this paper was to study the application of 
vacuum imprengation to improve the functional properties of apple slices by using an 
extract of Aloe vera gel. Specifically, the effects of the main process variables of VI on the 
level of enrichment as well as on the main physical properties of apples slices were 
studied using the response surface methodology. 
 
 
 
 
 



	

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2. MATERIALS AND METHODS 
 
2.1. Fresh apple and Aloe vera plant 
 
Fresh apples (cv. Golden Delicious) were purchased to the local market in September 2015 
and stored for a maximum of 3 days at 4°C. Before treatments, the apples were 
equilibrated at room temperature. Aloe vera plant (Aloe Barbadensis Mill.) of three years old 
was supplied by Ricciotti Gardens (Foggia, Italy). The gel was prepared by cutting the 
fresh leaves and separating the outer green rind from the inner parenchyma. 
 
2.2. Vacuum impregnation treatments 
 
After washing and peeling, the apples were manually cut in slices with a thickness of 0.5 
cm. The Aloe vera extract was prepared by dissolving 25 g of the Aloe vera gel in 125 mL of 
distilled water at room temperature under agitation at 300 g-1 for 90 min. A 
product/solution mass ratio of 1:5 (w/w) was used for each experiment.  
 
2.3. Experimental design 
 
A factorial design was used to study the effects of three variables (pressure, p, vacuum 
time, t1, and relaxation time, t2) on the main quality attributes of apples (BOX and 
BEHNKEN, 1960). The pressure was modified between 50 and 450 mbar while vacuum 
times and relaxation times were studied in the ranges of 1-5 min and 5-15 min, 
respectively. These values were chosen on the basis of preliminary experiments performed 
in order to define wide ranges able to significantly increase the weight of apple slices that 
is a rough index of the filling of pores. More specifically, a 3(k-p) fractional factorial design 
was implemented with k = 3 and p = 1 obtaining a total of 9 experimental conditions. Each 
VI test  was repeated in triplicate by using the experimental conditions reported in Table 1.   
 
 
Table 1. Experimental conditions of vacuum impregnation experiments. 
 

Experiments Pressure Vacuum time Relaxation time 
 (mbar) (t2, min) (t1, min) 

1 450 5 15 
2 450 1 10 
3 450 3 5 
4 250 1 15 
5 250 5 5 
6 250 3 10 
7 50 1 5 
8 50 3 15 
9 50 5 10 

 
 
2.4. Chemical and physical analysis 
 
Moisture (xw) and solid (xs) content of samples were gravimetrically measured by drying 5 
g of vegetable tissue at 65°C until a constant weight (CABEZAS-SERRANO et al., 2009).  
 



	

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2.5. Changes in porosity 
 
Porosity values of fresh and VI samples were obtained comparing apparent (ρa) and real 
solid-liquid (ρr) density values. All porosity values were expressed as kg/m3. Pycnometer 
method was used to determine the apparent density (ρa) using an isotonic sucrose solution 
as a reference. Real (ρr) density and porosity fraction (ε) were estimated as reported by 
GRAS et al. (2002). The weight increase (DE) was expressed as relative differences on the 
basis of fresh weight.  
 
2.6. Determination of total polymannans content 
 
The total polymannans content was determinated by using the colorimetric assay 
proposed from EBERENDU et al. (2005) with some minor changes. A volume of 400 mL of 
aqueous extract, 500 mL of NaOH (0.1 M) and 1 mL of Congo Red (Sigma Aldrich) dye 
(diluted by a factor of 500) were mixed and agitated for 2 h, then the changes in colour 
were analysed at 540 nm with a spectrophotometer (Perkin Elmer Lambda 25 UV/VIS). 
The calibration curve was performed by using a solution of β-glucan (Sigma Aldrich Inc.) 
in the range between 3.2 and 100 mg/L as reported by PELLIZZONI et al. (2012). 
 
2.7. Statistical analysis  
 
The effect of each independent variable on the quality indexes of apple samples was 
evaluated by ANOVA test with a significant level of 0.05. Also, the results were described 
by Pareto’s charts. Moreover, 3D plots describing the changes of each dependent variable 
as a function of experimental conditions were obtained by fitting the experimental data 
with a polynomial model as reported by DEROSSI et al., (2015b). All statistical analyses 
were performed using Statistica ver. 10.0 (Statsoft Tulsa, USA). 
 
 
3. RESULTS AND DISCUSSION 
 
Table 2 shows the main physico-chemical attributes of fresh apples. Average values of 
moisture and solids content of Golden delicious were of 0.88±0.01 g H2O /g f.w. and 
0.12±0.01 g H2O /g f.w., respectively. These values were in good agreement with other 
authors (MUJICA-PAZ et al., 2003; PAES et al., 2007; WU et al., 2007).  
 
 
Table 2. Physico-chemical properties of fresh apples. 
 

Parameter Value ± SE 
Moisture content (Xw) (g H2O/g f.w.) 0.88±0.01 
Solids content (Xs) (g/g f.w.) 0.12±0.01 
Apparent density (ρa) (kg/m

3)  822±0.04 
Real density (ρr) (kg/m

3) 1047±0.04 
Porosity fraction (ε) (%)  21.5±0.04 

 
 
Moreover, a porosity fraction of 21.4±3.69 % indicates slight variability in porosity as a 
consequence of their biological variance, which includes the ripening degree, the 
environmental conditions, varieties, etc. In general, literature has reported a significant 
variability in porosity of apples with values between ~18 % and ~27 % (SALVATORI et al., 



	

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1998; MARTINEZ-MONZÒ et al., 2000; MUJICA-PAZ et al., 2003; PAES et al., 2007) which 
are in agreement with our data. As expected, polymannans were not revealed in fresh 
apples. With the aim to quantify the content in polymannans in aqueous extracts of Aloe 
vera the distribution function of about 40 measurements performed on extracts prepared 
from different part of Aloe vera plant, is reported in Fig. 1. A normal distribution was 
proved by the Shapiro-Wilk’s test that exhibited a value of 0.98. The mean value was of 
0.355 ± 0.078 mg/g enables to state that the most of the observation fell in the range of 0.3 
and 0.4 mg/g. Also, minimum and maximum values of 0.188 mg/g and 0.546 mg/g were 
also observed.  
 
 

 
 
Figure 1. The normal probability function of polymannans content of the Aloe vera gel extracts. 
 
 
The results of statistical analysis showed that the pressure value and the relaxation time 
linearly affected the changes in porosity of apples showing standardized effects of 6.97 
and -3.53, respectively (Fig. 2a). This means that as the pressure decreased as the porosity 
decreased too, while the relaxation time showed an inverse relationship with the void 
fraction of apple tissue. Since that the driving force of VI is the difference between internal 
(i.e. inside the pores) and external pressures, the higher was the vacuum, the greater was 
the impregnation. Also, the longer was the relaxation times, the higher was the reduction 
in porosity, because there was more time for impregnation and relaxation phenomenon 
(GRAS et al., 2002; MUJICA-PAZ et al., 2003; NERI et al., 2016). Figure 2b shows the 3D plot 
describing the effects of relaxation time and pressure on the porosity fraction of apples. At 
first, taking into account the treatment performed at the lower vacuum of 450 mbar and 
the minimum relaxation time of 5 min, a significant reduction in porosity from fresh apple 
(ε = 21.4±3.69%) to 14% was observed. 
Moreover, according to Pareto chart of Fig. 2a the pressure had the highest effect in the 
reduction of porosity fraction of the samples. According to this results porosity fraction 
reduced from 0.14 to ∼0.08% when the pressures of 450 and 50 mbar were applied, 
respectively, with a minimum t2 of 5 min (Fig. 2b). 
 



	

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Figure 2. a) Estimated effects of the independent variables on the porosity fraction of apple slices submitted 
to VI treatments with Aloe vera gel extract. L and Q refer to the linear and no linear (quadratic) effect, 
respectively. b) 3D Plot describing the effects of relaxation time and pressure on the fraction porosity of 
apple slices submitted to VI treatments with Aloe vera gel extract. 
 
 
On the other hand, a negligible reduction in porosity was observed by applying relaxation 
time from 5 to 15 min for some pressure applied. For instance, appling a pressure of 50 
mbar, values of 0.08 and 0.03 were measured progressively increasing t2. Moreover, the 
results stated the very high reduction in porosity fraction of apples tissue independently 
from the length of vacuum time; in fact, by reducing t1 a change in porosity only of 0.4% 
was achieved (data not shown). However, experimental data show a high variability, 
which cannot be underestimated. This could be the result of the variance in microstructure 
properties such as the porosity of fresh apples, the presence of closed pores, the size and 
dimension of capillaries, their tortuosity, etc. However, the decrease of the porosity 
fraction after VI treatment cannot assure that the impregnation by external solution 



	

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occurred. As well known, during relaxation time the impregnation and the compression of 
capillaries are involved, both allowing to reduce the porosity fraction of fruit. The 
equilibrium between the impregnation and compression level is controlled by several 
variables such as the viscosity of the solution, the rigidity of vegetable tissue, etc., most of 
which cannot be manually controlled. The Pareto chart (Fig. 3a) shows the effect of the 
independent variables on the weight increase of apples. The pressure linearly affected the 
weight increase of apple samples exhibiting a standardized effect of -3.58, while the other 
variables did not show any effect. The variation in weight as a function of vacuum time 
and pressure are shown in Fig. 3b.  
 
 

 
 

 
 
Figure 3. a) Estimated effects of independent variables on the weight increase of apples slice submitted to VI 
treatment with Aloe vera gel extracts. L and Q refer to the linear and no linear (quadratic) effect, respectively. 
b) 3D plot describing the effect of vacuum time and pressure on the weight increase of apple slices. 
 
 



	

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According to pareto chart, only the pressure exhibited a significant effect. Even 
considering the lower t1 = 1 min the weight of apple slices increased from ∼0.18 to ∼0.26 
g/g by reducing the pressure from 450 to 50 mbar. That means that a significant amount of 
Aloe vera extract was introduced in the void phase of apple. On the other hand, any 
differences were not observed increasing the vacuum time until 5 min. Also in this case, 
the experimental data showed a not negligible variability that could be the result of 
differences in microstructure (porosity, connectivity, pore size distribution, etc.). Fig. 4a 
reports the standardized effects of the independent variables on polymannans content of 
apple slices. The pressure and the relaxation time were the most important variables 
affecting the enrichment of apple slices. The standardized effects of -3.15 and 2.55 showed 
as the pressure exibhited the greater effect on enrichment of apple samples. Fig. 4b shows 
the effect of the vacuum level on the polymannans content of apples.  
 

 

 
 
Figure 4. a) Standardized effect of the independent variables on the polymannan content of apple slice 
submitted to vacuum impregnation with Aloe vera gel extract. L and Q refer to the linear and no linear 
(quadratic) effect, respectively b) Changes in polymannas content of apple slices submitted to vacuum 
impregnation as a function of pressure. 
 



	

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More specifically, the figure was obtained by grouping all experimental data for the 
pressure values used during experiments while the other two variables, t1 and t2, were free 
to change. The positive effect of the vacuum impregnation is clearly observed. By reducing 
the pressure from 450 to 50 mbar, the average content in polymannans increased from 2 to 
5 mg/100 g f.w. This means that VI treatments may be considered a useful method to 
enrich apples with the bioactive compounds of Aloe vera. Of course, the high variability of 
the data (i.e. error bars) was caused by the different relaxation times as well as by the 
effects of the microstructure variability of fresh apple. 
Fig. 5 shows the 3D plot describing the effects of pressure and relaxation time on the 
polymannans content of apple slices. The enrichment in polymannans was obtained for 
any experimental condition with values ranged between 1 and 8 mg/100 g f.w.. For any 
relaxation time, by increasing the vacuum level, it was possible to enrich apple slices with 
polymannans. An average increase of 7 mg/100 g f.w. was obtained decreasing the 
pressure, progressively, with a fixed relation time of 5 or 15 min. For a t2 of 10 min a peak 
of polymannans content was reached according to the non-linear effect reported in figure 
4a. This means that a negative effect was observed for relaxation time longer than 10 min, 
probably because a prolonged compression damaged apple tissue favoring the release of 
Aloe vera extracts from the capillaries. On these bases, further experiments, focused on the 
changes in void and solid matrix phases as well as in terms of microstructure, would be 
necessary to improve the understanding of the behavior of apple tissues under VI 
treatment and to optimize the enrichment in polymannans.  
 
 

 
 
Figure 5. 3D Plot describing the effect of relaxation time and pressure on the changes in polymannas content 
of apples slice submitted to vacuum impregnation treatments.  
 
 
4. CONCLUSIONS 
 
By vacuum impregnation is possible to obtain fresh-cut apple slices with improved 
healthy properties by filling their pores with an Aloe vera gel extract. Pressure and 
relaxation time significantly affected the porosity fraction and the polymannans content of 
apples, while any effect was not observed modifying the vacuum time. From an initial 



	

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porosity of 21.4 % of fresh apples, values of 16% were obtained for the weakest treatments, 
while a value of 2% was reached for the strongest experimental conditions. The weight 
increase ranged between 0.16 and 0.25 g/g proved the significant impregnation of the 
pores by the Aloe vera extract. The best operative conditions were a pressure of 50 mbar 
and a relaxation time of 10 min by which a polymannans content of 8 mg/100 g was 
measured in apple slices. On the other hand a further increase in t2 produced a decrease in 
polymannans content probably caused by the damage on the vegetable tissues, which 
reduced the capacity of the capillaries to retain the external solution.  
 
 
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Paper Received June 20, 2017  Accepted November 29, 2017