QUALITY OF APPLE SLICES PROCESSED BY COMBINED TECHNIQUES

D. MASTROCOLA', P. PITTIA and C.R. LERICI

Dipartimento di Scienze degli Alimenti, Universita di Udine,

Via Marangoni 97 33100 Udine, Italy

Accepted for Publication May 1 , 1995

ABSTRACT

The effectiveness of single pretreatments and combinations ofpretreatments on the prevention of enzymatic browning during freezing and thawing of apple slices was investigated. The combinations of blanching and dipping, either of which alone could not control enzyme activity, were studied. Blanching of apple slices caused a significant decrease offirmness even before the freezing-thawing phase. The use of a "light" dip after a short blanch allowed the product to be cooled and helped the antioxidant agents (ascorbic and citric acids) to enter apple tissue. The anti-browning action of sodium chloride, even at very low levels in dipping solutions, was confirmed. Partial drying avoided the loss of firmness but negatively affected the color. "Mild pretreatments ' I , including partial drying, gave satisfactory results in terms of firmness and color.

INTRODUCTION

Fruit pieces are widely used as components in many food formulations such as pastry, confectionery products, ice-cream, frozen desserts and yogurt. Maintaining color, flavor and texture is important for consumer acceptance (Maltini et al. 1993). To achieve acceptable quality, combined methods of preparation and preservation have been studied. Controlling microbial growth by combining the effects of low pH, low water activity (a,,,) and anti microbial agents has been recommended (Leistner 1985, 1990). Such a combination improves shelf stability while retaining the organoleptic characteristics, i.e. color and texture, of fresh food with a reduction in processing costs (Gould and Jones 1989).

'Please address correspondence to Dr. Dino Mastrocola. FAX 0039-432-501637.

Journal of Food Quality 19 (1996) 133-146. All Rights Reserved. OCopyright I996 by Food & Nutrition Press, Inc., Tiurnbull, Connecticut 133

134 D. MASTROCOLA. P. PITTIA and C.R. LERICI

Enzymatic and nonenzymatic browning reactions are among the main factors that limit acceptability of products preserved by such combined methods (Monsalve-Gonzales et af. 1993). Sulfiting agents have been the conventional chemicals used to inhibit browning in fresh and processed fruits and vegetables. However the harmful effects of sulfites on sensitive consumers (Taylor er al. 1986) are well-known; so several alternatives have been proposed (Ponting e? al. 1972; Sapers er al. 1989). Also, the use of partial drying, in order to obtain a reduction of 50 % of the initial weight of the product prior to freezing has been extensively studied (Howard and Campbell 1946; Lazar er af. 1961; Monzini and Maltini 1986; Bolin and Huxsoll 1993). After removing water from fruits or vegetables, less refrigeration energy is required for processing and distribu- tion. In addition, the combination of partial drying and removal produces good flavor and color retention and a good maintenance of texture after thawing (Ponting e? al. 1966; Huxsoll 1982; Bolin and Huxsoll 1993).

The objective of our research was to verify the effectiveness of single physical or chemical pretreatments and different combinations of pretreatments on the prevention of enzymatic browning during freezing and thawing of apple slices. In particular the combination of two or more physical and chemical "mild pretreatments" (blanching and dipping), separately incapable of obtaining complete control of enzymes and microbial growth in vegetal tissue, was studied, The impact of the adopted techniques on product quality after each technological stage and after thawing was also investigated.

MATERIALS AND METHODS

Raw Material

Trials were performed on medium sized apple (Malus Dornestica, cv. Golden Delicious).

Preparation of Samples

The apples were refrigerated at 2-3C, and prior to use were removed from refrigeration and equilibrated to room temperature (22-23C) for at least 3 h. After hand-washing, slicing to give uniform 0.5 cm-thick slices, samples were subjected to different single or combined pretreatments before freezing. The aim of these pretreatments was to obtain complete prevention enzymatic browning by blanching and dipping and to improve freeze-thaw texture by partial drying. Treatments performed on different samples of apple slices are reported in Table 1.

QUALITY OF FROZEN APPLE SLICES 135

TABLE 1. TREATMENTS PERFORMED ON DIFFERENT SAMPLES OF APPLE SL ICES

SINGLE PRETREATMENTS FREEZING THAWIN( SAMPLES

B 1 anch ing Dipping A i r-dry ing (90 C , 90") (1% asc.ac.

1% citr.ac..)

Cont ro l X X B X X X 0 X X X A X X X -

COMBINED PRETREATMENTS

Blanching Dipping Ai r -dry ing

0 . 5 % c i t r . a c . 1 (90 C, Sol') ( 0 . 5 % asc.ac.

B t D X X X X ID+A X X X X B+A X X X X B t D+A X X X X X - -

Blanching was performed in a steam blancher (laboratory plant) at 90C for 90 s when it was the only pretreatment for prevention of enzymatic browning. When combined with dipping, the blanching time was reduced to 60 s. After blanching, the apple slices were immediately cooled in a cold air flow (B, B+A) or directly in a dipping solution (B+D, B+D+A). See Table 1.

For the dipping treatment, apple slices were placed in a water solution containing 1 g of ascorbic acid and 1 g of citric acid in 100 ml at room temperature and agitated constantly for 3 min. When combined with blanching a solution containing 0.5 g of ascorbic acid and 0.5 g of citric acid in 100 ml was used. Dippings in solutions containing fixed amomts of ascorbic and citric acid (0.5%) and different concentrations of sodium chloride for two different time periods were also performed. A diagram illustrating the different trials is shown in Table 2. The surfaces of the apple slices were blotted upon removal from the solutions.

Some samples were hot-air dried until a weight reduction of about 50% was obtained. The air-drier used was a static suction-flow type (Sandvik Process Systems, Milan, Italy) where the air flow runs downward, perpendicular to the product. Circulating air speed was about 3 m / s with a temperature of 70C. Apple slices were placed in single layers on a perforated tray in the dehydrator.

136 D. MASTROCOLA, P. PI'ITIA and C.R. LERICI

TABLE 2. COMPOSITION OF SOLUTIONS USED FOR D I P P I N G OF APPLE

SL ICES FOR TWO DIFFERENT T IMES

D I P P I N G g o f s o l u t e s i n 100 ml o f s o l u t i o n T I M E

SAMPLES ( m i n ) S o d i u m A s c o r b i c C i t r i c C h l o r i d e A c i d A c i d

- 0.5 0.5

0.5 0.5

Raw or pretreated apple slices were frozen in a forced-air cabinet (Alaska, mod. EF 600, Bologna, Italy) operating at -4OC. After freezing, samples were packed in polyethylene laminate pouches and held at -2OC until analyzed (from 2 to 5 days later). The frozen samples were thawed at room temperature for two hours before the analyses.

Analysis

Dry matter and acidity (expressed as tartaric acid) determinations were carried out in accordance with AOACs Methods 22.013,22.080,22.084 (1980). Chlorides were determined according to the Volhard method (Koltothoff et al. 1974). The pH values were measured on homogenized fruit puree with a pH meter (Beckman model 3560).

Water activity (q) was evaluated at 25C with an electric hygrometer (Hygroskop DT Rotronic, Zurich) calibrated with saturated solutions of different salts (Labuza et al. 1976). Prior to a, determination the samples were cut into small pieces and equilibrated for 12 h. For color evaluation a tristimulus color analyzer (Minolta Chroma Meter I1 Reflectance, Minolta Camera Co. Osaka, Japan) was used. The instrument was equipped with a CR 100 measuring head connected to a microcomputer via an optical fiber cable. In these trials the illuminant C (6774 K) was adopted and the instrument was calibrated using the standard white reflector plate (L* = 93.5; a* = -1; b* = 0.8) (Barbanti et al. 1990). Results are expressed as L* (brightness variable) and a* (red-to-green degree of cotor) as chromaticity coordinates (Clydesdale 1978; Mastrocola and Lerici 1991).

QUALITY OF FROZEN APPLE SLICES 137

The firmness of apple slices after each technological stage of the process was measured with an Instron Universal Testing Machine (Instron International LTD, High Wycombe, UK) Model 4301, using a standard Kramer Shear press cell (Model CS1) with a crosshead speed of 20 cdmin on 50g of fruit. Extrusion force expressed as Newtons (N) (Maltini ef al. 1993).

Statistical Analyses

The results reported in this work are the average of multiple measurements. The values for water content, G, acidity and chlorides were the average of three repetitions; the values of pH and firmness were the average of five measure- ments, and the data for color were the average of ten repetitions. Analysis of variance was carried out for all treatments. Significance between means was determined using Student's "t-test".

Sensory Evaluation

The color and the salt content of apple slices after freezing and thawing were judged by a sensory panel of eight trained testers. Panelists were asked to evaluate the products using "acceptable" or "not acceptable" as judgment (Moskowitz 1988).

RESULTS AND DISCUSSION

In the first part of this work we evaluated the effectiveness of single or combined pretreatments in inhibiting enzymatic browning (EB) of apple slices before freezing by means of color measurements.

As is well-known, the variations of chromaticity coordinates L* and a* are well-correlated to color changes in fruit tissues (darkening) due to EB (Sapers and Douglas 1987, Mastrocola and Lerici 1991). As browning increases, L* values decrease and a* values increase. Mastrocola and Lerici (1991) pointed out that the L* parameter has a broader scale but it is more significantly influenced than a* by the water content of the product. For these reasons both L* and a* values were considered.

Tables 3 and 4 report, respectively, the values of L* and a* coordinates of different samples of frozen-thawed apple slices. These tables also indicate the samples considered not acceptable in terms of color by the panel test.

138 D. MASTROCOLA, P. PIT'TIA and C.R. LERICI

TABLE 3. COLOR ( L * ) CHANGES I N APPLE SL ICES FROZEN-THAWED RAW, AFTER

DIFFERENT PRETREATMENTS OR COMBINATIONS OF PRETREATMENTS

SAMPLES COLOR ( L * )

raw a f t e r a f t e r a f t e r a f t e r blanching d ipp ing p a r t i a l f reez ing-

drying thawing

- - o n t r o l 79.8k3.1 - 60.1 i2 .7b*

- 79.8f3.1 67.520. 4a - 87.721 .6'

79.8f3.1 - 78.5i0.5b - 75. 020 .6=

79.8f3.1 - - 63.022. Ba* 55.321 . 8a*

79.823.1 70.1+2.4b 69.0+2.6a - 67.8t1 .3'

79.823.1 - 78.5i0.7b 78.3t0.9' 77.021 .Qe

79.8k3.1 71.021 .gb - 72. 121 .3b 71.9f0 .2d

CONTROL = frozen-thawed B = blanched, frozen-thawed; D = dipped, frozen-thawed; A = p a r t i a l l y a i r dr ied, frozen-thawed. B+D = blanched, dipped, frozen-thawed; D+A = dipped, p a r t i a l l y a i r - d r i e d , frozen-thawed; B+A = blanched, p a r t i a l l y a i r - d r i e d , frozen-thawed; B+D+A = blanched, dipped, p a r t i a l l y a i r - d r i e d , frozen-thawed. a-e

s i g n i f i c a n t l y d i f f e r e n t ( P r 0 . 0 5 ) * = samples considered not acceptable (browned)

= means i n a column w i t h a common superscr ipt a r e not

Thawing caused a significant decrease of L* values when no pretreatments were performed (control sample). In fact, the breaking of apple cells during freezing causes a drip of cell liquid constituents during thawing and makes for easier contact between enzymes and substratum. Consequently, the browning in the product can appear very rapidly (Mastrocola and Pittia 1992).

The dried samples showed evident browning prior to the freezingthawing phase because of the relatively high temperature (40-45C) of the product during drying-favored enzyme activity. Dipped and blanched apple slices maintained an "acceptable" color after thawing although blanching caused a slight decrease in L* values. This phenomenon was probably correlated to water uptake in apple tissues during steam blanching (Mastrocola and Lerici 1991).

Turning to the effects of different combinations of mild pretreatments on color, it is possible to note that all treated samples presented slight color

QUALITY OF FROZEN APPLE SLICES 139

TABLE 4 , COLOR (a*) CHANGES IN APPLE SLICES FROZEN-THAWED RAW, AFTER

DIFFERENT PRETREATMENTS OR COMBINATIONS OF PRETREATMENTS

-4.521.5 -5.220.3a - - -5.1 to. 7a

-4.521.5 -4.3f0. qb - -3.620. gb

-4.521.5 - - 4.021 .gC* 7.9+0.6d4

-4.521 .5 -5.420. 5a -5.5+0. 7a - -5.320. 2a

-4.521 .5 - -4.320. gb -4.220. gb -4.3+1 . 1 at -4.521 .5 -5.220. ga - -5.4i0.4a -5.520.3a

B+D+A -4.521 .5 -5. 320.Ea -5.520. 7a -5. 420.Ea -5.421. 2a

CONTROL = frozen-thawed B = blanched, frozen-thawed; D = dipped, frozen-thawed; A = p a r t i a l l y a i r d r i e d , frozen-thawed. BtD = blanched, dipped, frozen-thawed; DtA = dipped, p a r t i a l l y a i r - d r i e d , frozen-thawed; B+A = blanched, p a r t i a l l y a i r - d r i e d , frozen-thawed; BtD+A = blanched, dipped, p a r t i a l l y a i r - d r i e d , frozen-thawed. a-d

* = samples considered not acceptable (browned)

means i n a column w i t h a common superscr ipt a r e not s i g n i f i c a n t l y d i f f e r e n t ( P L 0 . 0 5 )

differences when compared with the raw product. Moreover, the drying phase, particularly when preceded by dipping, did not give rise to browning. In general, to be effective, single anti-browning pretreatments have to be very severe. In these cases it is possible to generate negative effects on product quality, such as texture collapse by blanching, excessive acidification by dipping or considerable sugar loss by blanching and dipping (Table 5 and 6). These disadvantages can be avoided or reduced when these treatments are combined in a "mild" form. As is known, freezing, too, causes cellular breakdown in apple tissue that results in softening upon thawing.

Under our experimental conditions, blanching of apple slices reduced firmness even before freezing-thawing (Table 5) . On the other hand, partial drying prevented the loss of firmness (Table 5 ) but affected the color of apple slices (Table 3 and 4). By contrast, the combination of these two pretreatments gave satisfactory results in terms of firmness and color.

140 D. MASTROCOLA. P. PITTIA and C.R. LERICI

TABLE 5, FIRMNESS CHANGES I N APPLE S L I C E S FROZEN-THAWED RAW, AFTER DIFFERENT

PRETREATMENTS OR COMBINATIONS OF PRETREATMENTS

SAMPLES FIRMNESS ( N )

raw a f t e r a f t e r a f t e r a f t e r blanching dipping p a r t i a l f r e e z i n g

drying thawing

C o n t r o l 525f21 - - - 141f12b

525+21 - 522+20b - 1 55f22b

525?21 - 539f 16' 524f23f

52522 1 150f11a 110f13a - -

270214'

51 5i2Ob 52021 8' 470230e

2e0+22a 275226'

CONTROL = frozen-thawed B = blanched, frozen-thawed; D = dipped, frozen-thawed; A = p a r t i a l l y a i r dr ied, frozen-thawed. BtD = blanched, dipped, frozen-thawed; D+A = dipped, p a r t i a l l y a i r -dr ied , frozen-thawed; BtA = blanched, p a r t i a l l y a i r - d r i e d , frozen-thawed; Bt +A blanched, dipped, p a r t i a l l y a i r -dr ied , frozen-thawed. a-p 8eans i n a column wi th a common superscript a r e not

s ign i f icant ly d i f f e r e n t (Pr0.05)

The use of a "light" dipping (with a solution containing 50% less acid than the one used for the dipping alone) after a short blanching allowed the product to cool and helped the antioxidant agents (ascorbic and citric acids) to enter apple tissue.

The lowering of water content and a,,, in partially-dried products (Table 6) can make these products compatible with formulation at low a,,, and low freezing point (bakery products, ice cream) (Maltini et al. 1993).

In the second part of our research we investigated the possibility of obtaining a complete control of EB using only the dipping as a pretreatment before freezing-thawing of apple slices. Partial drying before the freezing of fruit pieces can be omitted only if the softening of the products after thawing and their high water activity are not considered as limiting factors. Such semiprocessed apple slices could be used, for example, as topping for fruit flans.

QUALITY OF FROZEN APPLE SLICES 141

TABLE 6, PHYSICAL/CHEMICAL ATTRIBUTES OF APPLE SLICES FROZEN-THAWED RAW, AFTER DIFFERENT PRETREATMENTS OR COMBINATIONS OF PRETREATMENTS

Samples W a t e r W a t e r T i t r a b l e T o t a l a c t i v i t y C o n t e n t a c i d s S u g a r s

(awl (%I (% o n D r y M a t t e r )

C o n t r o l 0 .Wa 84 .5020 .57e 2 .21k0 .08c 69 .9021 .45c

D 0. 9Sa 8 6 . 2 2 t O . 34f 2.9420. 15d 6 2 . 6 5 t O . 9Eb

8 0.9Sa 87.02t0.278 1.45+0.05a 6 0 . 3 5 f Q . 5 5 a

A 0.92b 65.01+0.42a 1 . 9 4 t 0 . 1 2 b 69.212O.9Sc

B+D 0 . ssa a 7 . 2 4 t o . 3 6 9 2.7020. i 4 d 6 1 . 4 7 f i . 9 8 a t

D+A 0 .93b 66 .3420 . 43b 2 .71 +Q. 1 gd 62.33tO .93b

B+A 0.93b 67 .17 t0 .34 ' 1 .50k0.08a 60.1020.63a

B+D+A 0. 93b 68.08f0. 36d 2 .7620 .1 gd 59 .43 tO . 76a

CONTROL = frozen-thawed D = dipped, frozen-thawed; B = blanched, frozen-thawed; A p a r t i a l l y a i r dr ied, frozen-thawed; BtD = blanched, dipped, frozen-thawed; D t A = dipped, p a r t i a l l y a i r -d r i ed , frozen-thawed; BtA = blanched, p a r t i a l l y a i r -d r i ed , frozen-thawed; BtDtA = blanched, dipped, p a r t i a l l y a i r -d r i ed , frozen-thawed.

= means i n a column w i t h a common superscr ip t are not s i g n i f i c a n t l y d i f f e r e n t (P20.05)

We performed dippings in citrichscorbic acid aqueous solutions containing different amounts of NaCl (Table 2). The use of small amounts of salt in fruit dipping is justified by the great inhibiting action of NaCl on polyphenoloxidase (PPO) (Lerici ef al. 1985; Mathews and Parpia 1971). Our aim was to establish the minimum level of sodium chloride able to guarantee good control of EB and at the same time the maximum level of acceptability for chlorides in apple pieces. Figure 1 shows the color changes in apple slices dipped in different solutions, respectively, for 3 min and then frozen-thawed, compared with the unpretreated product.

The color of all products did not change during dipping and freezing but was affected, as expected, only upon the thawing phase. The color changes after thawing in all dipped slices were not very noticeable and hardly influenced by the NaCl content in the dipping solutions. Besides, the color of all dipped

142 D. MASTROCOLA, P. PITTIA and C.R. LERICI

4 I * = samples considered not acceptable (browned)

'1 0

-2

-4

-6

r 75 d

E cn

B .....a ..... * Control A --*-. c -+D

FIG. 1. COLOR CHANGES IN APPLE SLICES FROZEN-THAWED OR AFTER 3 MIN DIPPINGS IN SOLUTIONS CONTAINING DIFFERENT AMOUNT OF NaCI:

A = dipping solution containing 1 % of NaCI; B = dipping solution containing 0.5% of NaCl; C = dipping solution containing 0.3% of NaCI; D = dipping solution without NaCI.

QUALITY OF FROZEN APPLE SLICES 143

T A B L E 71

PHYSICAL/CHEMICAL ATTRIBUTES OF APPLE S L I C E S FROZEN-THAWED RAW OR AFTER DIFFERENT DIPPINGS.

ac ids

% on dry m a t t e r

CONTROL 3.72+0.07a 2.18+0.08a 0.0320.02a

3.44+0. 04b 2.7920. 07b 1 .03+0. 05e

3.43+0. 03b 2.80kO.l Ob 0.6020. 03'

3.45+0. 04b 2.8220. OEb 0.41 20.04

3.46+0.02b 2.8420. l l b 0.0220.02

3.4220.02b 2.94k0.09b 1.26k0.06

3.4650. 03b 2.9OkO. OEb 0.86f0.05

3.46+0. 01 2.91 k0. Ogb 0.61 kO.07

A = dipped f o r 3 min i n a s o l u t i o n con ta in ing 1% o f NaC1; B = dipped f o r 3 min i n a s o l u t i o n con ta in ing 0.5% o f NaC1; C = dipped f o r 3 min i n a s o l u t i o n con ta in ing 0.3% o f NaC1; D = dipped f o r 3 min i n a s o l u t i o n w i t h o u t NaCl. A ' = dipped f o r 10 min i n a s o l u t i o n con ta in ing 1% o f NaCl; 6' = dipped f o r 10 min i n a s o l u t i o n con ta in ing 0.5% o f NaC1; C ' dipped f o r 10 min i n a s o l u t i o n con ta in ing 0.3% o f NaC1; D ' = dipped f o r 10 min i n a s o l u t i o n w i thou t NaCl. a-f = means i n a column w i t h a common supersc r ip t are not

s i g n i f i c a n t l y d i f f e r e n t (PZO. 05)

samples was considered acceptable by the panelists. Chemical analyses of dipped and frozen-thawed apple slices highlighted a slight increase in titrable acids and an accumulation in apple tissues of chlorides from the dipping solutions. The amount of chlorides obviously increased with the increasing of NaCl in dipping solutions and with the time of dipping (Table 7). The slices dipped in solutions containing 1 % of NaC1, for a 3 min dipping, or up to 0.5 % of NaC1, for a 10 min dipping, were not considered acceptable the panelists (Fig. 2).

144

02

0,18 0,16

8 0-14 g 0,12

g 0,l

-E - 0,oa

-0

0 2 0,06 0,04

0,02

(3

D. MASTROCOLA, P. PIT"L.4 and C.R. LERICI

* * = samples considered not acceptable (salty)

I I

1 02 0,4 086 08 NaCl in dipping solution (%)

FIG. 2. CHANGES OF NaCl CONTENT IN APPLE SLICES DIPPED FOR 3 OR 10 MIN PERIODS IN SOLUTIONS CONTAINING DIFFERENT AMOUNT OF NaCl VS NaCl

CONTENT IN DIPPING SOLUTION

CONCLUSIONS

Our trials showed that single pretreatments before the freezing-thawing of apple slices have to be fairly drastic if they are to produce an effective anti-browning action. In these cases it is possible to generate negative effects on product quality. These disadvantages can be avoided or reduced when mild forms of these techniques are combined. In fact, in our experimental conditions the blanching of apple reduced firmness even before the freezingthawing phases. Partial drying prevented loss of firmness but affected the color of the products. By contrast, combined pretreatments including partial drying gave good results in tenns of firmness and color.

The lower water content and a,,, in partially-dried products make them compatible with formulation at low water activity and low freezing point (bakery products, ice cream). The use of a "light" dipping containing reduced concen- trations of organic acids, after a short blanching, allowed the product to cool and facilitated the access of the antioxidant agents (ascorbic and citric acids) into the apple tissues. The anti-browning action of sodium chloride, even at very low concentrations in dipping solutions, was confirmed.

QUALITY OF FROZEN APPLE SLICES 145

ACKNOWLEDGMENT

This work was supported by a research grant from the Italian National Council of Research, Special Project RAISA, Sub-project n.4, Paper n.2313.

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