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International Journal of FoodEngineering

Volume3, Issue2 2007 Article 6

Effects of Heat Treatment and Dehydration onPineapple (Ananas comosus L. Merr) Cell

Walls

Antoni Femenia∗ Susana Simal†

Carme Garau Taberner‡ Carmen Rossello∗∗

∗Universitat de les Illes Balears, [email protected]†Universitat de les Illes Balears, [email protected]‡Universitat de les Illes Balears, [email protected]

∗∗Universitat de les Illes Balears, [email protected]

Copyright c©2007 The Berkeley Electronic Press. All rights reserved.

Effects of Heat Treatment and Dehydration onPineapple (Ananas comosus L. Merr) Cell

Walls∗

Antoni Femenia, Susana Simal, Carme Garau Taberner, and Carmen Rossello

Abstract

The effects of thermal processing on the physico-chemical properties of cell walls from pineap-ple flesh tissues have been investigated. Commercially canned pineapple exhibited a similar cellwall composition to the fresh pineapple sample, although a marked increase in cell wall solubility,from 21 to 34%, was detected. Dehydration promoted important changes in cell wall compo-nents and related functional properties, in particular when relatively high air-drying temperatureswere applied. Thus, samples dried at 60oC and, in particular at 80oC, exhibited a larger solu-bilisation/degradation of pectic polysaccharides, probably due to eitherβ-elimination processesor enzyme-catalyzed degradation. On a fresh weight basis, about 14% and up to 39% of cellwall pectins were not recovered for the dried pineapple at 60oC and 80oC, respectively. Pectinsfrom the latter samples also exhibited a notable decrease in the degree of esterification. Thesephysico-chemical changes were probably reflected on the decrease of functional properties suchas swelling (Sw), water retention capacity (WRC) and fat adsorption capacity (FAC). Neverthe-less, fresh, canned and dehydrated pineapple at 40oC exhibited higher WRC and FAC values,about 30 g water/g AIR and 15 g oil/g AIR, respectively. A gradual decrease of Sw, WRC andFAC values was observed for the functional properties of pineapple samples dried at 60 and 80oC.Moreover, high air-drying temperatures also promoted a significant decrease in cell wall solubility.Therefore, the influence that these effects might have on the nutritional properties of cell walls ordietary fibre of thermally processed fruits such as canned and/or dehydrated pineapple needs to beconsidered.

KEYWORDS: pineapple, cell walls, dehydration, canning, functional properties

∗The authors would like to acknowledge the financial support of CICYT (AGL2003-03889)

1. INTRODUCTION

Since the middle of the 1970s, the role of dietary fiber in health and nutrition has stimulated a wide range of research activities and caught public attention. Accumulating evidence favors the view that increased intake of dietary fiber can have beneficial effects against chronic diseases, such as cardiovascular diseases, diverticulosis, diabetes and colon cancer (Delzenne and Cani, 2005; Brooks et al, 2006; Johnson et al, 2006). Cell wall components or dietary fibre, respectively, are natural constituents of fruit and vegetables. Thermal treatments of fruits such as canning and dehydration, apart from ensuring microbial stability and minimizing chemical and physical changes of material during storage can be used as a tool to produce fiber-rich foods (Bourne, 2004; Sablani, 2006)

Model studies and investigations for the improved manufacture of dietary fibre preparations, as well as dehydrated fruit and vegetable products, show that both the pre-drying treatments and the drying step itself might alter the physico-chemical properties of the cell wall compounds, modifying the nutritional properties of the processed products. (Silveira et al, 1996; Kunzek et al, 2002; Marques and Freire, 2005).

The knowledge of the structure and chemical composition of the cell wall polysaccharides might highlight the main modifications that occur during fruit and vegetable processing (Femenia et al, 2003).

So far, the investigations have shown that improved technologies for the production of cell wall and dietary fibre rich products as well as innovative foods with selectively adjusted functional properties require fundamental knowledge of the changes in structure and properties caused by processing. By understanding functional properties of cell wall components, one can increase its use in food applications and aid in developing food products with high consumer acceptance.

An improvement of the quality of cell wall and dietary fiber derived from processed fruits might contribute to obtain products with optimized physico-chemical properties.

Pineapple is one of the most important fruits in the world, and most of its production is used in processing (Larrauri et al, 1997). Pineapple is consumed as canned slices, chunks, dice, or fruit salads and in the preparation of juices, concentrates and jams. Dehydration is also an alternative method for pineapple preservation.

The dietary fibre content and cell wall composition of pineapple flesh has been reported by different authors (Englyst et al, 1988; Bartolomé and Rupérez, 1995; Smith and Harris, 1995 and 2001). However, there are few composition data in the literature based on processed pineapple samples

The objective of this investigation was to evaluate the main modifications undergone by cell wall polysaccharides from flesh pineapple fruit tissues after

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Femenia et al.: Effects of Processing on Pineapple Cell Walls

Published by The Berkeley Electronic Press, 2007

thermal treatments: canning, and dehydration carried out at different temperatures.

2. MATERIAL AND METHODS

2.1 Plant material and dehydration Fresh and commercially available canned pineapple (Ananas comosus L. Merr, “smooth cayenne” variety) were purchased from a local retailer. The canned pineapple slices were drained of syrup and gently blotted with absorbent paper before being used.

Cubes of ~1 cm of edge from fresh pineapple were dehydrated at different temperatures (40ºC, 60ºC and 80ºC). Dehydration procedure was terminated when a moisture content of 10 kg water/100 kg dm was achieved. Final moisture contents of the dehydrated products were obtained using the AOAC method 934.06 (AOAC, 1990).

Drying experiments were performed in a laboratory scale hot air drier, described previously by Simal et al (1996), operating at air mass flux of 3.1 kg m-

2 s-1, ensuring that drying was controlled by the internal resistance and that the drying rate was not dependent on mass transfer from the solid surface to the gas phase. The drier was equipped with an automatic controller (±0.1ºC). The air passed perpendicular to the bed. A monolayer loading was used. The average air room characteristics were of 26±1ºC and 33±5% humidity. Water losses were measured by weighing the basket and its content automatically. Drying curves obtained through the experiments are shown in figure 1. 2.2 Color Color was measured using a Minolta C.M. 2002 spectrocolorimeter with specular component included, C illuminant, and a n observer with an angle of 2º, using CIEL*a*b* coordinates. The changes in lightness of each color parameters were calculated as follows (Chua et al, 2001):

0LLL −=Δ whereas the total color difference (ΔE) was then determined using the following equation:

( ) ( ) ( )202

02

0 -- bbaaLLE ++−=Δ The subscript “0” in both equations refers to the fresh pineapple sample

sample.

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International Journal of Food Engineering, Vol. 3 [2007], Iss. 2, Art. 6

http://www.bepress.com/ijfe/vol3/iss2/art6

2.3 Alcohol Insoluble Residues (AIRs) AIRs were prepared from fresh and dehydrated pineapple samples. AIRs were obtained by immersing the samples (ca. 10 g) in boiling alcohol (final concentration 85% (v/v) aq.) as described by Waldron & Selvendran (1990). Prior to further analysis, the AIRs were milled using a laboratory type grain mill and were used in subsequent analysis. 2.4 Analytical methods Carbohydrate analysis. Carbohydrate analysis was performed as in Femenia et al (1998) for neutral sugars from AIRs. Sugars were released from polysaccharides by acid hydrolysis. AIRs (~5 mg) were dispersed in 72% H2SO4 for 3 h followed by dilution to 1 M and hydrolyzed at 100ºC for 2.5 h (Saeman et al, 1954). In the case of the AIRs, a 1M H2SO4 hydrolysis (100ºC for 2.5 h) was also included to estimate the cellulose content by difference. Neutral sugars were derivatised as their alditol acetates and isothermally separated by GC at 220ºC on a 3% OV225 Chromosorb WHP 100/120 mesh column. Uronic acids were colorimetrically determined, as total uronic acid (Blumenkrantz & Asboe-Hansen, 1973), using a sample hydrolyzed for 1 h at 100ºC in 1 M H2SO4. The values for carbohydrates given in this paper correspond to the means of duplicate determinations.

Degree of esterification. The degree of esterification (DE) of pectic polysaccharides, i.e. the percentage of total uronic acids which are esterified, was colorimetrically determined by the method of Lurie et al (1994). DE analyses of AIRs were performed in triplicate.

Functional properties and solubility. Hydration properties, swelling (Sw) and water retention capacity (WRC), and fat adsorption capacity (FAC) of AIRs from fresh and dehydrated pineapple samples were determined as in Femenia et al (1997). Solubility was measured in conjunction with WRC, as % loss in the original sample dry weight after recovery of insoluble material used to determine WRC. 2.5 Statistical analysis Results were analyzed by means of one-way and multifactor analysis of variance, using LSD test with a 95% confidence interval for the comparison of the test means.

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3. RESULTS AND DISCUSSION In order to assess the effects of processing on color, and physico-chemical properties of pineapple cell walls, commercially canned and dehydrated pineapple samples were compared with fresh pineapple of the same variety (“smooth cayenne”). 3.1 Dehydration Pineapple fresh samples were dehydrated using different temperatures, namely 40ºC, 60ºC and 80ºC. Drying curves obtained for each sample are shown in figure 1. Although not applied to the analysis of the drying behaviour of the pineapple samples, profiles of drying were similar to those expected from model systems developed to describe rates of water loss during drying. As it can be observed, air drying temperature had a marked influence on the drying rate.

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

0 5000 10000 15000 20000 25000 30000 35000 40000 45000

time (s)

moi

stur

e co

nten

t (kg

/kg

dm)

40ºC

60ºC

80ºC

Figure 1. Drying curves of pineapple at different air-drying temperatures

Drying at above 80ºC promoted case-hardening effects on the pineapple

samples. This phenomenon hinders the water release and slows down the drying rate, thus, dehydration performed a higher temperature does not promote any further increase in the drying rate. This was observed for the pineapple samples

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dried at 80ºC and 90ºC which exhibited almost identical drying rates. Similar effects. have been observed for several vegetable products (Femenia et al, 2003). 3.2 Color measurements Browning development is most probably not related to any substantial modification of polysaccharides from pineapple cell walls (Weerahewa and Adikaram, 2005). However, browning might influence the organoleptic properties of dehydrated samples, reducing its potential applications. Thus, this effect was measured through the CIEL*a*b* colour coordinates.

A relatively important browning development was only observed for the sample dried at 80ºC, as indicated by the significant decreases in the L* and b* values.

This was reflected on the high values observed for ΔL and ΔE parameters. All other samples underwent minor modifications of the typical yellowish color of pineapple fresh fruit. Table 1. CieL*a*b* colour coordinates for fresh, canned and dehydrated pineapple samples

dehydrated

fresh canned 40ºC 60ºC 80ºC

L* 63.1 ± 0.2 62.1 ± 0.1 61.9 ± 0.3 58.2 ± 0.3 44.3 ± 0.7

a* 0.1 ± 0.1 1.2 ± 0.1 -0.2 ± 0.0 -0.7 ± 0.1 8.2 ± 0.1

b* 39.1 ± 0.3 36.0 ± 0.5 30.0 ± 0.7 29.6 ± 0.4 21.9 ± 0.6

ΔL1 - -1.0 -1,2 -4.9 -18.8

ΔE1 - 3.4 9.2 10.7 26.7

1 Values of ΔL and ΔE are calculated taking as a reference the fresh pineapple sample

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3.3 Carbohydrate composition of Alcohol Insoluble Residues (AIRs) obtained from fresh, canned and dehydrated pineapple Since pineapple tissues were shown to be effectively free of starch by I2/KI staining, AIRs were considered as suitable preparations of cell wall polysaccharides. The yield of AIR for fresh pineapple sample was ~1.2% fresh weight, representing approximately 40% of the pineapple dry weight. On a fresh weight basis, AIR yields obtained for the canned sample and also for pineapple dehydrated at. 40ºC, were fairly similar to that corresponding to the fresh sample.

Table 2. Effects of drying on AIR recovery and polysaccharide composition of pineapple cell walls (milligrams of sugar per gram of fresh weight)

dehydrated

fresh canned 40ºC 60ºC 80ºC

AIR yield1 1.21 1.17 1.19 1.10 0.96

Rhamnose 4.5 ± 0.0 4.7 ± 0.1 4.5 ± 0.3 4.0 ± 0.3 3.1 ± 0.7

Fucose 6.9 ± 0.0 6.7 ± 0.1 6.1 ± 0.0 6.4 ± 0.1 6.0 ± 0.1

Arabinose 125.9 ± 0.3 126.3 ± 0.5 121.5 ± 0.7 118.4 ± 0.4 114.5 ± 0.6

Xylose 173.5 ± 1.7 170.1 ± 1.2 171.5 ± 1.0 163.5 ± 1.1 154.1 ± 1.1

Mannose 20.7 ± 0.2 19.7 ± 0.1 21.0 ± 0.1 18.9 ± 0.2 18.4 ± 0.1

Galactose 73.9 ± 1.0 70.8 ± 0.9 71.9 ± 1.0 63.5 ± 1.0 60.3 ± 1.1

Glucose 294.8 ± 2.5 291.3 ± 2.7 289.8 ± 2.7 288.3 ± 2.3 286.6 ± 3.1

Glc (1M)2 (27.6 ± 1.7) (28.4 ± 1.5) (27.9 ± 1.3) (25.6 ± 1.10) (23.6 ± 1.2)

Uronic acids 62.6 ± 3.7 63.6 ± 3.0 61.4 ± 3.2 52.6 ± 2.9 44.9 ± 2.7

TOTAL 762.8 ± 6.1 753.2 ± 5.2 747.7 ± 7.0 715.6 ± 6.3 697.9 ± 5.9

D.E.3 46 ± 2 38 ± 2 45 ± 2 41 ± 2 29 ± 2

1 AIR yield is given as g AIR/g fresh weight 2 Glc (1M): glucose released using 1M sulfuric acid hydrolysis 3 Degree of esterification

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In contrast, dehydration at higher temperatures (60ºC and 80ºC) promoted

a significant and gradual decrease of AIR yields (Table 2). In fact, about 10 and up to 21% of the total amount of cell wall material was not recovered on samples dried at 60ºC and 80ºC, respectively. Although this effect may be due to some loss of material, resulting from modifications to the sample during heat drying, it may also reflect the activation of several polysaccharide-degrading enzymes, which could contribute to the depolymerisation of cell wall components. Nevertheless, these results indicate the high susceptibility of pineapple fruit fibre to be solubilised and/or degraded during heat treatment.

Sugars in the AIRs were released using two hydrolytic procedures, which allowed to distinguish the sugars from noncellulosic polysaccharides and cellulose. The cell walls from fresh pineapple tissues were mainly composed by an heterogeneity of hemicellulosic polysaccharides and cellulose. Moreover, the presence of relatively important amounts of galacturonic acid, arabinose, galactose and, to a lesser extent, rhamnose, suggested the occurrence of pectic substances.

Hemicellulosic polymers were mainly composed by glucuronoarabinoxylans (GAXs) and, in small amounts, xyloglucans. The presence of GAXs could be inferred from the occurrence of important amounts of glucuronic acid (about 45% of the total uronic acids found in the fresh fruit), arabinose and xylose (Bhaduri and Sen, 1983). The existence of xyloglucans was deduced from the presence of noncellulosic glucose, xylose and small quantities of fucose, sugar characteristic of xyloglucans. The fact that most of the glucose could only be released after Saeman hydrolysis suggested that cellulose was also present in large quantities in the pineapple flesh cell walls. These results are in broad agreement with those reported by Englyst et al (1988), Smith and Harris (1995) and Bartolomé and Rupérez (1995) for pineapple flesh tissues. In addition, the occurrence of small amounts of mannosyl residues could be due to the presence of glucomannans or galactoglucomannans (Smith & Harris, 1995).

On a fresh weight basis, the carbohydrate composition of fresh and dehydrated samples at 40ºC was fairly similar. On the contrary, the decrease observed in the AIR yields of samples dried at higher temperature was reflected on the carbohydrate composition of these samples. Interestingly, in terms of solubilisation or degradation, the different type of cell wall polysaccharides exhibited different behavior. Thus, pectic substances and hemicelluloses, the main components of the cell wall matrix in the primary walls of fruits showed a major susceptibility to be degraded or solubilised, whereas cellulose, which forms the microfibrillar phase, was almost not affected by the heat treatment.

The pectic backbone underwent important modifications during dehydration as it can be inferred from the losses of uronic acids, and rhamnose

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units, and also by the decrease in the degree of esterification. Pectin side-chains were also affected as indicated by the decrease in galactose units. Degradation/solubilisation of pectic substances may be due to β-elimination reaction promoted by heating, although temperatures above 40ºC might have also enhanced the activity of pectic polysaccharide degrading enzymes (Krall and McFeeters, 1998). The presence of pectin side-chains significantly affects a variety of functional properties modifying the functionality and, probably, the nutritional properties of the fibre preparations. A pre-treatment of pineapple samples with CaCl2 before drying could have been used to maintain the levels of pectic substances (Boas et al, 1998), although it was not applied in this study.

Hemicellulosic xyloglucans were also modified during dehydration as indicated by the decrease in the amounts of xylose and noncellulosic glucose. 3.5 Functional properties of fresh and dehydrated pineapple cell walls Functional properties are related to the chemical structure of the plant polysaccharides. Dehydration may alter the physicochemical characteristics of the original products, modifying their functional properties (Guillon & Champ, 2000). Therefore, in order to evaluate possible changes in the structural arrangement of cell wall polymers from pineapple tissues, swelling (Sw), water retention capacity (WRC) and fat adsorption capacity (FAC) were measured.

0

10

20

30

40

50

60

fresh canned dried 40 dried 60 dried 80

ml w

ater

/g A

IR

Figure 2. Swelling values for fresh and processed pineapple samples

Important differences either for Sw and/or WRC were observed among the

different dehydrated samples (see figures 2 and 3, respectively). Canning and dehydration at relatively low temperature (40ºC) maintained the capacity of the

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dehydrated samples to retain water, however a significant decrease occurred when samples were dried at higher temperatures (60 and 80ºC).

0

5

10

15

20

25

30

35


g w

ater

/g A

IR

Figure 3. Water retention capacity for fresh and processed pineapple samples

0

2

4

6

8

10

12

14

16

18


g oi

l/g A

IR

Figure 4. Fat adsorption capacity for fresh and processed pineapple samples

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Hydration properties such as Sw and WRC are related to the mechanical effect of dietary fibre on gastrointestinal function (increased faecal weight, decreased transit time, etc) (Surel and Couplet, 2005).

FAC values exhibited a similar pattern to that of the hydration properties (figure 4). Although the ability of pineapple polymers to retain organic molecules, such as fatty acids, was significantly lower than the capacity to retain water. The same effect has been observed in different fruit and vegetable fibres. 3.6 Solubility The term solubility refers simply to polysaccharides that are dispersible in water, thus the term is somewhat inaccurate (Figuerola et al, 2005). However, the soluble/insoluble ratio is important for both dietary and functional properties derived from dietary fibre.

0,0

5,0

10,0

15,0

20,0

25,0

30,0

35,0

40,0


% so

lubl

e

Figure 5. AIR solubility for fresh and processed pineapple samples

Heat processing of pineapple produced important modifications in the

solubility of pineapple cell wall polysaccharides depending on the method used (see figure 5). In comparison to the amount of soluble AIR in the fresh sample, about 21%, canning promoted a significant increase in cell wall solubility, up to 34%, whereas an increase in the temperature of dehydration contributed to reduce the percentage of soluble cell wall material, in particular for the sample dried at 80ºC. It should be pointed out that drying at a mild temperature of 40ºC did not

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cause a significant modification of the solubility, being similar to that measured for fresh pineapple AIR.

During canning, β-eliminative degradation of pectic polysaccharides might have occurred, catalyzed by several cations and anions (Femenia et al, 1998), and might have contributed to the extractability of pectic polysaccharide. The decrease in the degree of esterification, probably enzyme-mediated observed for samples dried at 60 and 80ºC (see Table 2), may limit the potential for subsequent eliminative degradation and hence restrict the extractability of cell wall polysaccharides from these heat-dried samples. 4. CONCLUSIONS Processing through heating or drying treatments can affect the physico-chemical properties of cell wall polysaccharides from pineapple flesh tissues, notably pectic polysaccharides and functional properties, including solubility.

The results suggested that heating at above 40ºC may lead to enzyme mediated deesterification of pectic polysaccharides and decreases solubility probably due to rearrangement of pectic polymers within the cell wall. On the other hand, canning, heating in an aqueous environment, increases the solubility of cell wall polymers probably through the β-eliminative degradation of pectic polysaccharides.

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