Influence of High-Pressure Blanching on Polyphenoloxidase Activity of Peach Fruits and its Drying Behavior

Abstract

Experiments were conducted to evaluate the feasibility of using high-pressure processing as an alternate for hot-water blanching of peach slices. Peach slices were pressure processed at various combinations of pressures (50–700 MPa) at 25°C with and without citric acid for two holding times (5 and 10 min). Pressure treatment (>300 MPa), in combination with citric acid (1–1.2%), was an effective method to inactivatepeach polyphenoloxidase enzymes. Pressure-treated peach slices, with and without citric acid, were subsequently dehydrated in a cabinet dryer at 70°C. Pressure treatment enhanced the drying rate and reduced the drying time. Logarithmic model satisfactorily described the drying behavior of peach slices. Pressure-processed samples had higher moisture diffusivity than control samples. The results reveals that high-pressure processing of peach fruits suspended in citric acid medium can be used as potential alternative for hot water blanching.

Keywords:

• Peach
• High-pressure processing
• Blanching
• Citric acid
• Polyphenoloxidase
• Drying
• Moisture diffusivity

INTRODUCTION

Peach (Prunus persica L.) is one of the important temperate stone fruits grown in the world. Drying is one of the common methods of preserving peaches apart from canning and freezing. Usually peaches are dried in halves, slices, or cubes and are used in bakery product filling, fruit sauces, cake mixes, fruit leather, etc.[1] The quality of the dehydrated peaches depends on the enzymatic activity after blanching, drying temperature, and the raw material. Browning in both fresh and processed peaches are observed due to the presence of polyphenoloxidase (PPO) enzymes.[2] Polyphenoloxidases are the copper containing enzymes, can influence the color of peaches during or after processing. An addition of ascorbic acid or sulphur dioxide and blanching in hot water are used to inhibit the enzymes.[3–7] Hot-water blanching of fruits is a common method of pretreatment before dehydration or freezing. Due to heating at high temperature, the texture and color of the product may change. Application of high-pressure processing (HPP) has been studied to inhibit the PPO enzymes.[8–9] During HPP, the fruit is subjected to elevated pressure (up to 900 MPa) with or without addition of heat, so that inactivation of microorganisms and spoilage catalyzing enzymes are achieved with minimum damage to the physical and quality attributes.[10] Knorr[11] found that high pressure of 100–400 MPa led to a decrease in PPO activity in potatoes. Elevated pressures inactivated PPO activity in grapes.[12] Al-Khuseibi et al.[13] demonstrated that the quality of high-pressure blanched potato, after drying, was better than that of water blanched samples.

Control of residual PPO activity is important for preserving the quality characters of food during storage. High-pressure treatment after reducing pH of the samples was suggested for storage of avocado fruits and puree.[14–15] Pressurizing the samples in low pH immersion medium with citric acid was found to increase the inactivation of enzymes.[8] Immersion of peach fruits in 1.7% citric acid solution before dehydration maintains appearance and improves acceptability of dried fruits.[16]

HPP was reported to enhance drying rate during osmotic dehydration by rendering the cells more permeable thereby increasing the rate of mass transfer.[17] The application of pressure pretreatment before dehydration improved the mass transfer rates in potato[13] and paprika[18] due to increased cell permeabilisation. The drying kinetics of food is a complex phenomenon and requires simple representations to predict the drying behaviour. Thin-layer mathematical models describe the drying phenomena in a unified way, regardless of the controlling mechanism.[19] The drying kinetics of all fruit and vegetables cannot be described by the same equation due to the difference in moisture content and varied transport phenomena mechanism during drying. There are many studies on drying kinetics of fruits and vegetables.[20–23] However, little work on the effect of high-pressure treatments on drying kinetics of peach are reported.

The objective of this study were: (a) to investigate the combined effect of HPP and citric acid at 25^oC on PPO inactivation; (b) to study the drying behavior of high pressure blanched peach samples; (c) to evaluate a suitable thin-layer drying model for describing the drying process; and (d) to calculate the effective moisture diffusivity.

MATERIALS AND METHODS

Raw Material

Yellow-flesh peach fruits of cultivar Junegold were used for the experiments. Ripe fruits [Total Soluble Solids 12°B, moisture content 92.17% (wet basis), pH 3.56] were purchased from local market (Columbus, OH, USA) and stored at 4°C. Same batch of fruits were used for all the experiments. After peeling the fruits were cut into circular pieces of diameter 17.5 mm and height 5 mm by using a cork borer (Fisher Scientific, Pittsburgh, PA, USA) and a sharp knife.

Citric Acid Pretreatment

During HPP, citric acid (Fisher Scientific, New Jersey, USA) solutions of 1.0, 1.2, and 1.4% were used as a carrier fluid to lower the pH. The pH, measured by a pH meter (model-215, Denver instrument, Denver, Colorado, USA), was 2.13, 2.21, and 2.30 for 1.0, 1.2, and 1.4% citric acid solution, respectively. The peach slices were suspended in citric acid solution (sample solution mass ratio of 1:10) and vacuum packed in high barrier pouches. The samples were high pressure processed immediately after packaging.

To check the effect of dipping fruits in citric acid suspension[16] (without pressure treatment) on PPO activity, peach slices were dipped in citric acid solutions of 1.4, 1.2, and 1.0% (22 ± 1ºC) for 10 min. For comparison, fresh samples as control, and water-blanched samples were also used. Hot-water blanching was carried out in boiling water in water bath for 3 min with sample water ratio as 1:20.

High-Pressure Processing

High pressure pretreatment of peach slices was carried out using a Quintus high pressure food processor (QFP-6, Flow Autoclave Systems, Columbus, OH, USA). The pressure transmitting fluid was a 1:1 mixture of distilled water and food grade propylene glycol (Houghto-Safe 620-TY, Houghton International Inc., Valley Forge, PA). The samples were high pressure processed at 50, 100, 300, 500, and 700 MPa for 5 and 10 minutes pressure holding time at 25 ± 1ºC. The pressure holding time did not include pressure-come-up and depressurization time. The rate of pressurization was approximately 258 MPa/min and depressurization occurred in less than 4 s. To achieve a final process temperature of 25ºC during the pressure holding time, the initial temperature of carrier fluid and samples before pressure treatment was lowered separately (4 to 23.5°C for 700 to 50 MPa treatment) taking account the rise in temperature due to heat of compression.[24–25] Pressure-pH combination pretreatments that inactivated PPO enzyme were selected for further investigation on drying behavior of peach slices.

Polyphenoloxidase (PPO) Activity

To find the PPO activity after pretreatments, peroxidase (Guaiacol) test was conducted as per the method described by Al-Khuseibi et al.[13] for the presence of PPO enzymes. A peach slice was put into a test tube containing 6 mL of water and 1 mL of 0.5% guaiacol solution (Acros Organics, New Jersey, USA) was added to the tube without mixing. Then, 1 mL of 0.08% hydrogen peroxide (Fisher Scientific, New Jersey, USA) and then mixed by inverting the tube. Change in color was monitored to find the presence of PPO at room temperature until a time period of 15 min, after the peroxidase test. Change in color (marked as +ive) indicated inadequate treatment.

Drying Experiments

To investigate the combined effect of pressure and citric acid pretreatments on drying characteristics of peach slices, peach samples were processed using two different conditions (300 MPa with 1.2% citric acid and 500 MPa with 1.0% citric acid). The above mentioned pretreatments were selected for their optimum pressure and citric acid concentration. A pressure holding time of 5 min was used in both the cases. Untreated fresh slices (with or without dipping in citric acid solution) were kept as control. High pressure pretreatments were carried out as mentioned in section High Pressure Processing. After processing, the pouch was cut open, samples were dried with a tissue paper to remove any residual water adhered to the surface and then subjected to further dehydration. Drying experiments were performed in a cabinet drier (model TS 160 A, Cabela's Food Dehydrator, Sidney, NE, USA) at 70°C.[26] After reaching the set drying air temperature conditions, sliced peaches (diameter 17.5 mm and height 5 mm) were uniformly spread in aluminum dish in single layer for drying. Moisture loss was recorded at 30 min interval for first 3 h of drying and thereafter at 1 h interval, until there was no large variation in the moisture loss. Untreated samples were kept as control. The experiments were carried out in triplicate and mean values were reported.

Mathematical modeling

Moisture ratio of samples during drying was expressed by the following equation:

(1)

where MR is the dimensionless moisture ratio and M, M_o and M_e are the moisture content (w.b.) at any given time t, the initial and equilibrium moisture contents, respectively. Equilibrium moisture content was calculated from any two successive observations of moisture content M_n and M_{n + 1} (dry basis) and their corresponding drying times t_n and t_{n + 1} and fitting in the following equation:[27]

(2)

where, z = exp(kΔt); and Δt = t_{n + 1} – t_n, k, constant. The drying curves obtained were fitted with six theoretical thin layer drying expressions (Table 1). For estimating the model constants and correlation coefficient, r² non-linear regression was performed using a statistical analysis program (ver 11, Statistical Package for Social Scientists, Chicago, IL, USA). In addition to correlation coefficient, the goodness of fit was determined by various statistical parameters such as reduced chi-square (χ²), mean bias error (MBE) and root mean square error (RMSE). For quality fit, r² value should be higher and χ², MBE and RMSE values should be lower.[20]

Table 1 Thin-layer drying models

Equation	Name	References
MR = exp (−kt^n)	Page	Zhang and Litchfield[28]
MR = exp (−(kt)^n)	Modified Page	Overhults et al.[29]
MR = a exp (−kt)	Henderson and Pabis	Henderson and Pabis[30]
MR = a exp (−kt) + c	Logarithmic	Yaldiz et al.[31]
MR = a exp (−k0t) + b exp (−k1t)	Two-term	Sharaf-Eldeen et al.[32]
k, k0,k1 a, b, c, and n are model constants.

Moisture diffusivity

Fick's unsteady state diffusion equation can be written as:[33]

(3)

where M is moisture content; t is time; D_eff is diffusion coefficient; and x is the diffusion path. For an infinite slab being subjected to drying from both the major faces following assumptions were made: (1) uniform initial moisture distribution; (2) no shrinkage during drying, and with the following initial and boundary conditions

(4)

(5)

The solution of Eq. (6) can be written for moisture ratio (MR):[33]

(6)

where D_eff is effective moisture diffusivity; and L is half-thickness of the slab. For longer drying times (assuming n = 1), the Eq. (9) can be written in logarithmic form as:

(7)

The experimental values of ln MR was plotted against t/L² and from the slope of the curve, moisture diffusivity was calculated using Microsoft Excel 2003^TM.

RESULTS AND DISCUSSION

High-Pressure Blanching

As expected, hot water blanching inactivated PPO enzymes in peach fruits (Table 2). Pressure treatment in combination with acidic pH was also effective in inactivation of enzymes. The observations were similar to that of earlier studies, where increased inactivation of PPO by HPP with addition of citric acid has been reported for potato.[8] The results suggested that reducing the pH was helpful in the inactivation of PPO enzymes in peach fruit. Combination of high pressure and citric acid has minimized PPO activity by reducing ○-quinones back to phenolic compounds before they form brown pigments.[16] Modest citric acid concentration (1.2 and 1.0%) in combination with pressure treatment (>300 MPa) at 25º C was effective in inactivation of PPO enzymes (Table 2).

Table 2 Effect of pretreatments on polyphenoloxidase (PPO) activity

Pretreatments	Without carrier fluid	Dipped in citric acid (1.0%)		Dipped in citric acid (1.2%)		Dipped in citric acid (1.4%)
Non-pressurized samples
Control (10 min)	+
Hot water blanched ( 3 min)	−
Citric acid pretreatment (10 min)		+		−^1		−
Pressurized samples
Level of pressure	10 min	5 min	10 min	5 min	10 min	5 min	10 min
50 MPa	+	+	+	+	−^1	−^1	−^1
100 MPa	+	+	+	+	−^1	−^1	−^1
300 MPa	+	−^1	−^1	−	−	−	−
500 MPa	+	−	−	−	−	−	−
700 MPa	+	−	−	−	−	−	−
+, presence of polyphenol oxidase; −, absence of polyphenol oxidase; −^1, slow development.

Drying Behavior

The drying rate of pressure treated samples was higher than that of control samples (Fig. 1). High pressure processed samples had shorter drying time (300 min) than the control samples (420 min). Application of high pressure resulted in enhanced permeability and induced moisture movement from inner core to outside and this movement might have resulted in reduced drying time.[34] A similar result has been reported by Al-Khuseibi et al.[13] for potato cubes.

Figure 1 Comparison of drying rate of peach fruits subjected to various combined pressure-pH pretreatments.

Use of citric acid as carrier fluid has also influenced the drying rate. During drying, the rate of removal of moisture from samples processed with citric acid was higher than samples processed without citric acid. Increased moisture removal in pressure treated samples suspended in citric acid may be due to the formation of fine cracks in upper layer by the acidic medium.[17] Drying of peach slices occurred in falling rate period and no constant rate period was observed (Fig. 1), which indicated that internal mass transfer has occurred by diffusion. The results are in agreement with the results reported by Kingsly et al.[23] for drying of peach slices that diffusion is the major driving force in removing the moisture from peach fruits.

Drying Models

Moisture ratio data (Fig. 2) of peach slices dried at thin-layers were fitted into thin-layer drying models listed in Table 1. For all the equations, the r² values were greater than 0.9, indicating a good fit. Among the thin-layer drying models evaluated, the logarithmic model was found to represent the drying behaviour of peach slices with high r² values and low χ², MBE and RMSE values (Table 3) in all the treatments. Logarithmic model has been used for describing the drying behavior of rosehip[20] and pretreated peach slices.[23]

Figure 2 Moisture ratio of peach fruits subjected to various pressure-pH pretreatments.

Table 3 Values of statistical parameters of the logarithmic model

Pretreatment	Correlation coefficient, r^2	χ^2	RMSE	MBE
Control	0.99	0.0004	0.0169	5.2 × 10^−6
300 MPa	0.99	0.0017	0.0358	−9.7 × 10^−6
300 MPa with 1.2% citric acid	0.99	0.0011	0.0295	−6.2 × 10^−6
500 MPa	0.99	0.0007	0.0221	−1.8 × 10^−6
500 MPa with 1.0% citric acid	0.99	0.0011	0.0295	−9.9 × 10^−6

Moisture Diffusivity

The moisture diffusivity of untreated samples was lower than the high pressure processed samples (Table 4) and the values ranged between 6.56 × 10⁻¹⁰ – 7.65 × 10⁻⁹ m²s⁻¹. The values are within the general range of 10⁻⁹ – 10⁻¹⁰ m²/s for drying of food materials.[35] HPP peach slices had higher moisture diffusivity due to higher internal mass transfer during drying. The increased cell permeabilisation due to HPP might have increased the moisture diffusivity than the reported values[23] during drying of peach slices. Use of acidic carrier medium has slightly influenced the moisture diffusivity and the values were higher than the diffusivity of samples processed without citric acid medium. Carrier fluid might have dissociated the cell structure, which has resulted in higher water diffusion.

Table 4 Moisture diffusivity of peach slices

Pretreatment	Deff, m^2/s	r^2
Control	6.56 × 10^−10	0.90
300 MPa	7.35 × 10^−9	0.91
300 MPa with 1.2% citric acid	7.41 × 10^−9	0.91
500 MPa	7.61 × 10^−9	0.94
500 Mpa with 1.0% citric acid	7.65 × 10^−9	0.92

CONCLUSIONS

Pressure treatment (>300 MPa at 25^oC) of peach slices suspended in citric acid (1–1.2%) was found to be effective in inactivation of PPO enzymes. High pressure processed samples dried faster than the control samples. Higher permeability of cells after HPP increased the drying rate. Rate of removal of moisture from pressure treated samples suspended in citric acid was higher than control and pressure treated samples. Logarithmic model adequately described dehydration behaviour of peach slices HPP of peach slices increased the moisture diffusivity (6.56 – 7.65 × 10⁻⁹ m²s⁻¹). HPP of peach slices in acidic medium was found to be an potential alternative for hot water blanching as pretreatment of peach fruits.

ACKNOWLEDGMENTS

The author A.R.P. Kingsly was a Norman E. Borlaug International Agricultural Sciences and Technology Fellow and visiting Scholar at High-pressure food processing laboratory, Department of Food Science and Technology, The Ohio State University, USA. He acknowledges the financial support of Indo-US Knowledge Initiative in Agriculture. Author Dr. N.K. Rastogi is from Central Food Technological Research Institute (CFTRI), Mysore, India. He was a Visiting Assistant Professor at Department of Food Science and Technology, Ohio State University; and the visit was sponsored by a overseas research associateship from Department of Biotechnology (DBT), Government of India, New Delhi. Thanks are due to Dr. R.T. Patil, Director, CIPHET, Ludhiana for his constant encouragement.