Food Chemistry 128 (2011) 1087–1093

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Food Chemistry

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Effect of microwave pretreatment on the kinetics of ascorbic acid degradationand peroxidase inactivation in different parts of green asparagus(Asparagus officinalis L.) during water blanching

Hong Zheng, Hongfei Lu ⇑College of Chemistry and Life Science, Zhejiang Normal University, Jinhua 321004, China

a r t i c l e i n f o

Article history:Received 6 November 2010Received in revised form 17 February 2011Accepted 30 March 2011Available online 12 April 2011

Keywords:Asparagus officinalis L.Ascorbic acidPODBlanchingMicrowave

0308-8146/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.foodchem.2011.03.130

⇑ Corresponding author. Tel.: +86 0579 8228 2284.E-mail address: luhongfei63@yahoo.com.cn (H. Lu

a b s t r a c t

In this study, the effects of two blanching conditions on ascorbic acid (AA) and peroxidase (POD) in dif-ferent segments of asparagus (bud, upper, middle, and butt) were investigated. The blanching treatmentswere: blanching in water at 70, 80 and 90 �C (WB); microwave heating (900 W, 30 s) followed by waterblanching (MW + WB). AA degradation and POD inactivation in all segments of asparagus for both treat-ments are well described by first-order models. The degradation rate of AA and POD is graduallyincreased from butt to bud segment of asparagus. In addition, MW pre-treatment could increase the Ea

of AA degradation and decrease the Ea of POD inactivation during water blanching of asparagus. There-fore, it is recommended that the different segments of asparagus should be subjected to different blanch-ing times, and MW pre-treatment could be applied for alleviating AA degradation and accelerating PODinactivation during blanching, cooking and pasteurisation in water.

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1. Introduction

Asparagus (Asparagus officinalis L.) is a green vegetable withhigh antioxidant activity compared with other commonly con-sumed vegetables (Vinson, Hao, Su, & Zubik, 1998). However, thisvegetable is highly perishable due to its high respiratory activitythat continues after harvesting. Therefore, it is often processed toreduce post-harvest losses and increase the economic return forfarmers. Blanching is one of the many processes that take placeduring the preparation of untreated vegetables for preservationprocesses such as canning and freezing. Hot-water blanching isby far the most popular and commercially adopted process as itis the simplest and most economical technique. Its primary pur-pose is to inactivate enzymes and destroy vegetative microbialcells, allowing stabilisation and product quality retention duringstorage (Canet, 1989). Despite its preservative benefits, it may alsohave some negative effects on product quality, such as excessiveloss of texture, undesirable changes in colour, nutritional lossesand degradation of antioxidant properties (Arroqui, Rumsey, Lopez,& Virseda, 2001; Oboh, 2005; Selman, 1994).

Peroxidase (POD) catalyses the oxidation reactions using eitherperoxides or oxygen as the hydrogen acceptor (Hemeda & Klein,1991). It is recognised as being one of the most heat-stableenzymes and is widely used as an index of blanching (Bahçeci,Serpen, Gökmen, & Acar, 2005). If this enzyme is inactivated, other

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).

enzymes responsible for tissue degradation will also be inactivated(Koksal & Gülçin, 2008; Soysal & Söylemez, 2005). In addition,ascorbic acid (AA) is an important component of our nutritionand reportedly reduces the risk of arteriosclerosis, cardiovasculardiseases and some forms of cancer (Harris, 1996) because of itsantioxidant capacity (Gülçin, Bursal, S�ehitoglu, Bilsel, & Gören,2010). However, AA is one of the most labile vitamins. It is easilyoxidised by air, heat, oxidising enzymes, and multivalent cations(Arroqui, Rumsey, Lopez, & Virseda, 2002; Gökmen & Serpen,2007). It is generally observed that if AA is well preserved, the othernutrients are also well retained (Lin, Durance, & Scaman, 1998).

Therefore, this research used POD and AA as the indicator ofenzymes and nutrients to evaluate asparagus quality during waterblanching after microwave pre-treatment, respectively. The objec-tives of the present work were (i) to evaluate the kinetics of AA deg-radation and POD inactivation in different parts of green asparagusduring two blanching treatments, namely, water blanching andmicrowave heating followed by water blanching, and (ii) to studythe effect of microwave pre-treatment in alleviating AA loss andaccelerating POD inactivation during water blanching of asparagus.

2. Materials and methods

2.1. Sample preparation and blanching process

Fresh asparagus (A. officinalis L. var. UC800) was harvested froma local farm in Jinhua (Zhejiang, PR China) and transported under

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refrigeration at 8 �C for 30 min to the laboratory. Spears of uniformdiameter (0.5 ± 0.1 cm) at the base and length (20 cm) were used inthis experiment. The spears, after being sorted for size and length,were washed with tap water and drained. All spears were dividedinto two equal groups subjected to microwave (30 s) andnon-microwave heating. In the microwave treatment, every timeapproximately 200 g of asparagus was heated in the microwaveoven (Galanz WD900ASL23-5S, 23L, 900 W). After cooling to roomtemperature, the spears were separately placed in two distilledwater baths set at 70, 80 and 90 �C. The spears were separatelyremoved at different blanching times and immediately cooled inrunning water at 5 �C. Prior to measurement of ascorbic acid andPOD activity, the spears were cut into four segments (bud, upper,middle, and butt segments) at room temperature as described inour previous study (Lu et al., 2010).

2.2. Chemicals

Ascorbic acid (purity P99%), 2,6-dichlorophenol indophenol(purity P99%), guaiacol (purity P98%), H2O2 solution (purityP30%) and bovine serum albumin (purity P99%) were purchasedfrom Shanghai Sangon Biological Engineering Technology & Ser-vices Co., Ltd. (Shanghai, China).

2.3. Determination of ascorbic acid (AA)

The AA content in the spear segments was determined basedupon the quantitative discolouration of 2,6-dichlorophenol indo-phenol titrimetric method as described in AOAC methodologyNo. 967.21 (AOAC 2000, chap. 45). The AA content was measuredin triplicate and expressed as mg/100 g fresh weight (FW).

2.4. Preparation of enzyme extracts

Crude extract preparation was carried out according to themethod described by Gonçalves, Pinheiro, Abreu, Brandão, and Silva(2007). Raw and blanched asparagus samples (5 g each) wereweighed into 45 ml of 1 M sodium chloride solution. The sampleswere ground gently in a mortar surrounded by chipped ice. Thehomogenate was centrifuged in polypropylene tubes at 3500 rpm,using a cold centrifuge (Hettich, Tottlingen, Germany), at 4 �C for10 min. The supernatants were then filtered using 1.2 lmmembrane filters (Whatman) and kept on ice until the analysis.

2.5. Determination of peroxidase (POD) activity and protein content

PPO activity was assessed in triplicate based on a modifiedmethod of Gökmen, Bahçeci, Serpen, and Acar (2005). The sub-strate solution was a mixture of 0.1 ml of guaiacol and 0.1 ml ofH2O2, made up to 100 ml with 0.1 M acetate buffer (pH 5.50).The substrate solution (4.9 ml) was transferred into a cuvette andthe reaction was started by adding 0.1 ml of crude enzyme extract.One unit of POD activity was defined as an increase in absorbanceof 0.001 at 420 nm per minute per mg of protein under the assayconditions at 37 �C (S�is�ecioglu et al., 2010).

Protein content of the enzyme extracts was determined accord-ing to Bradford (1976) using bovine serum albumin as a standard(Gülçin, Küfrevioglu, & Oktay, 2005)

2.6. Kinetic models of POD activity inactivation and AA degradation

For the inactivation of POD activity, Morales-Blancas, Chandia,and Cisneros-Zevallos (2002) reported first-order inactivationkinetics showed a good fit during thermal treatments of greenasparagus. In addition, the first-order kinetic model has been alsoapplied by numerous researchers (Burdurlu, Koca, & Karadeniz,

2006; Frias & Oliveira, 2001; Giannakourou & Taoukis, 2003; Ud-din, Hawlader, Ding, & Mujumdar, 2002; Zheng & Lu, 2011) forevaluating AA degradation during food processing and storage.Therefore, the kinetic data of POD activity inactivation and AAdegradation were analysed with first-order kinetics using the fol-lowing equations:

Ct=C0 ¼ expð�k � tÞ ð1Þ

where Ct and C0 are the POD activity or AA content at time t andzero, respectively; k is the first-order rate constant; and t is theblanching time (s).

The Arrhenius equation is the most widely accepted method ofaccounting for the temperature dependence of the rate constant infood systems. The temperature and the rate constant k are relatedaccording to the Arrhenius equation:

K ¼ A expð�Ea=RTÞ ð2Þ

where Ea is the activation energy of the reaction; R is the gasconstant; T is the absolute temperature; and A is the pre-exponen-tial constant.

The decimal reduction time (D-value) is defined as the treat-ment time needed for 90% degradation of its original value. TheD-value was calculated as follows:

D ¼ 2:303k

ð3Þ

where k is the first-order rate constant.

2.7. Statistical analysis

Data were expressed as the mean ± standard deviation of threemeasurements. Differences between means were analysed withDuncan’s test using the SAS software package (version 8.1). A sig-nificant difference was considered at the level of p < 0.05.

3. Results and discussion

3.1. Kinetic models to describe changes in AA and POD activity ofdifferent parts of asparagus

First-order kinetic models were used to treat the resulting dataof AA degradation in different parts of asparagus during WB andMW + WB as represented in Fig. 1. These models seemed to be suit-able to describe AA degradation in this study because of the highdetermination coefficients (R2 = 0.8163–0.9426). In fact, manyresearchers reported that AA degradation of food materials are welldescribed by first-order models (Frias & Oliveira, 2001; Giannakou-rou & Taoukis, 2003; Zheng & Lu, 2011). Fig. 2 shows that thedegradation of POD activity in all segments of asparagus alsofollowed first-order reaction kinetics (R2 = 0.8968–0.9942). Forother vegetables such as carrots, potatoes, tomato and watercress,a first-order model was also used to describe the enzyme inactiva-tion (Anthon & Barrett, 2002; Anthon, Sekine, Watanabe, & Barrett,2002; Cruz, Vieira, & Silva, 2006).

Fig. 3 illustrates contour plots of rate constants (k) of AAdegradation and POD inactivation as a function of blanching tem-perature and different parts of asparagus for both treatments. Ascan be seen from Fig. 3A and B, the k of AA loss increases with in-creased blanching temperature, but it decreases gradually frombud to butt segment of asparagus. This result suggests that budsegment is more liable to lose AA than other segments, which inaccordance with our previous findings in green asparagus duringthermal treatments (Zheng et al., 2011). The loss in AA contentduring blanching could be attributed to the fact that AA is very sol-uble in water and not stable at high temperature (Nagy & Smooth,1977). Therefore, one possible explanation for these results is that

Fig. 1. Residual ascorbic acid changes in bud (A), upper (B), middle (C), and butt (D) segments of asparagus during water blanching (WB) and microwave heating (900 W, 30 s)followed by water blanching (MW + WB) at 70 �C (j), 80 �C (d), and 90 �C (N).

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the bud segment of asparagus provides a poorer texture than othersegments, which increases the penetration of AA and decreasesheat resistance. Torreggiani and Bertolo (2001) revealed that fruittexture improvement could improve pigment, colour, vitamin,and aroma retention during air dehydration and frozen storage.In addition, Lau, Tang, and Swanson (2000) observed that the reac-tion rate constants of the thermal degradation of asparagus texture

gradually increased from the base to the tip of the asparagusspears. According to Fig. 3C and D, the results are similar to thoseobtained on the degradation of AA, that is, the k of POD inactivationincreased with blanching temperature and decreased from bud tobutt segment of asparagus. Therefore, it is recommended that thebud segment of asparagus should be subjected to a relatively shortperiod of time during blanching, cooking and pasteurisation for

Fig. 2. Residual POD activity changes in bud (A), upper (B), middle (C), and butt (D) segments of asparagus during water blanching (WB) and microwave heating (900 W, 30 s)followed by water blanching (MW + WB) at 70 �C (j), 80 �C (d), and 90 �C (N).

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preserving AA content and inactivating POD activity. Moreover, thedifferent segments of asparagus should be blanched with differenttimes for preserving its quality.

Activation energies (Ea) were calculated by using Arrheniusplots of POD inactivation and AA degradation in asparagus givenin Fig. 4 and found higher in the middle and butt segments thanthat of the bud and upper segments (Table 1). A similar result

was obtained in our earlier study (Zheng et al., 2011) for AA lossduring thermal treatment of asparagus. High Ea in the middleand butt segments indicated that POD inactivation and AA degra-dation was more temperature dependent than the other segments.A possible reason for this is to be found in the texture of differentsegments of asparagus, that is, a better texture of the middle andbutt segments improve resistance to heat. Hayakawa and Timbers

Fig. 3. Contour plots showing the effect of blanching temperature and different parts of asparagus on rate constant of ascorbic acid degradation (A and B) and PODinactivation (C and D) during water blanching (WB) and microwave heating (900 W, 30 s) followed by water blanching (MW + WB).

Fig. 4. Arrhenius plots of ascorbic acid degradation (A and B) and POD inactivation (C and D) in different segments of asparagus (bud, upper, middle and butt) during waterblanching (WB) and microwave heating (900 W, 30 s) followed by water blanching (MW + WB).

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Table 1Activation energies (Ea) of ascorbic acid (AA) degradation and peroxidase (POD)inactivation in bud, upper, middle, and butt segments of asparagus during waterblanching (WB) and microwave heating (900 W, 30 s) followed by water blanching(MW + WB).

Ea (kJ mol�1)

Bud Upper Middle Butt

AA WB 18.44ba 19.14b 21.07a 20.81aMW + WB 23.12a 22.70a 22.68a 21.29a

POD WB 15.52a 20.66a 23.86a 22.83aMW + WB 15.23a 20.59a 18.51b 20.61b

a Different lower case letters indicate significant differences (P < 0.05) betweenWB and MW + WB.

Fig. 5. The D-value of POD inactivation in bud, upper, middle, and butt segments ofasparagus for both treatments (WB: water blanching; MW + WB: microwaveheating followed by water blanching) at 70 �C (A), 80 �C (B), and 90 �C (C). Differentlower case letters indicate significant differences (P < 0.05) between WB andMW + WB.

Fig. 6. The percentage retention of ascorbic acid (AA) in bud, upper, middle, andbutt segments of asparagus during MW + WB (microwave heating followed bywater blanching) and WB (water blanching) at 70 �C (A), 80 �C (B), and 90 �C (C) forinactivating 90% of POD. Different lower case letters indicate significant differences(P < 0.05) between WB and MW + WB.

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(1977) and Lau et al. (2000) reported that the hardness of aspara-gus gradually decreased from the base to the tip of asparagus.According to Table 1, the Ea of AA degradation during waterblanching was increased for all segments of asparagus after micro-wave pre-treatment, although a non-significant difference wasobserved for the middle and butt segments. On the contrary, the

Ea of POD inactivation was decreased during MW + WB, but therewere no significant differences for the bud and upper segments.The reason for these results will need further study before theycan be satisfactorily accounted for.

3.2. The effect of blanching method on the degradation of AA and PODactivity in different parts of asparagus

POD is the most thermally resistant enzyme in vegetables, andfor this reason, complete inactivation of POD requires considerablylong time, resulting in heavy loss of nutrients. Therefore, Singh andChen (1980) reported that 90% inactivation of POD as sufficient toprevent any deterioration in food and recommended the same asthe optimum level for blanching. Fig. 5 shows the D-value of PODinactivation in different segments of asparagus for bothtreatments. These results suggest that significant decreases(P < 0.05) in the D-value of POD in all segments of asparagus areobserved during MW + WB at all temperatures used in this study.This phenomenon results from a decrease in the Ea of POD inacti-vation during MW + WB, although this is the subject of further

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study. In addition, the percentage retentions of AA in all segmentsof asparagus during MW + WB are statistically significantly higherthan those during WB for inactivating 90% of POD, as shown inFig. 6. This result should consider that, on the one hand, the k ofPOD inactivation was increased during MW + WB, and on the otherhand, the Ea of AA degradation was also increased after microwavepre-treatment. Therefore, microwave pre-treatment could alleviateAA loss and accelerate POD inactivation during water blanching ofasparagus. Soysal and Söylemez (2005) also reported thatmicrowave heating was more effective for inactivating POD andpreserving ascorbic acid in carrot than thermal treatment. Viñaet al. (2007) suggested that microwaves pre-blanching showedno deleterious effect on ascorbic acid content of Brussels sprouts.

4. Conclusions

AA degradation and POD inactivation in blanched asparagusfollowed first-order kinetics. The bud segment of asparagus shouldbe blanched to a relatively short period of time for preserving AAcontent and inactivating POD activity. In addition, microwavesmay be an effective pre-treatment process for use prior to waterblanching to reduce the degradation of AA and to accelerate theinactivation of POD and thus maintain produce quality. Futurework will mainly cover the effectiveness of microwave pre-treat-ment to other fruits and vegetables during water blanching andtheir reasons.

Acknowledgements

This research is supported by Postgraduate Innovation ResearchProjects of Zhejiang Province (YK2008038). The authors would liketo thank Mr. Heqiang Lou, Ms. Hedan Li and Ms. Lingling Jiang inthe College of Chemistry and Life Science at Zhejiang Normal Uni-versity (Jinhua, China) for chemical analysis.

References

Anthon, G. E., & Barrett, D. M. (2002). Kinetic parameters for the thermalinactivation of quality-related enzymes in carrots and potatoes. Journal ofAgricultural and Food Chemistry, 50(14), 4119–4125.

Anthon, G. E., Sekine, Y., Watanabe, N., & Barrett, D. M. (2002). Thermal inactivationof pectin methylesterase, polygalacturonase, and peroxidase in tomato juice.Journal of Agricultural and Food Chemistry, 50(21), 6153–6159.

AOAC (2000). Vitamins and Other Nutrients. In Official Methods of Analysis (17th ed.,pp. 16–20). Washington, DC: AOAC International.

Arroqui, C., Rumsey, T. R., Lopez, A., & Virseda, P. (2001). Effect of different solublesolids in the water on the ascorbic acid losses during water blanching of potatotissue. Journal of Food Engineering, 47, 123–126.

Arroqui, C., Rumsey, T. R., Lopez, A., & Virseda, P. (2002). Losses by diffusion ofascorbic acid during recycled water blanching of potato tissue. Journal of FoodEngineering, 52, 25–30.

Bahçeci, S. K., Serpen, A., Gökmen, V., & Acar, J. (2005). Study of lipoxygenase andperoxidase as indicator enzymes in green beans: change of enzyme activity,ascorbic acid and chlorophylls during frozen storage. Journal of Food Engineering,66, 187–192.

Bradford, M. M. (1976). A rapid and sensitive method for the quantitation ofmicrogram quantities of protein utilizing the principle of protein-dye binding.Analytical Biochemistry, 72(1-2), 248–254.

Burdurlu, H. S., Koca, N., & Karadeniz, F. (2006). Degradation of vitamin C in citrusjuice concentrates during storage. Journal of Food Engineering, 74, 211–216.

Canet, W. (1989). Quality and stability of frozen vegetables. In S. Thorne (Ed.).Developments in food preservation (Vol. 5). New York: Elsevier SciencePublishing Inc.

Cruz, R. M. S., Vieira, M. C., & Silva, C. L. M. (2006). Effect of heat andthermosonication treatments on peroxidase inactivation kinetics inwatercress (Nasturtium officinale). Journal of Food Engineering, 72(1), 8–15.

Frias, J. M., & Oliveira, J. C. (2001). Kinetic models of ascorbic acid thermaldegradation during hot air drying of maltodextrin solutions. Journal of FoodEngineering, 47, 255–262.

Giannakourou, M. C., & Taoukis, P. S. (2003). Kinetic modelling of vitamin C loss infrozen green vegetables under variable storage conditions. Food Chemistry, 83,33–41.

Gökmen, V., Bahçeci, K. S., Serpen, A., & Acar, J. (2005). Study of lipoxygenase andperoxidase as blanching indicator enzymes in peas: change of enzyme activity,ascorbic acid and chlorophylls during frozen storage. LWT – Food Science andTechnology, 38, 903–908.

Gökmen, V., & Serpen, A. (2007). Reversible degradation kinetics of ascorbic acidunder reducing and oxidizing conditions. Food Chemistry, 104, 721–725.

Gonçalves, E. M., Pinheiro, J., Abreu, M., Brandão, T. R. S., & Silva, C. L. M. (2007).Modelling the kinetics of peroxidase inactivation, colour and texture changes ofpumpkin (Cucurbita maxima L.) during blanching. Journal of Food Engineering, 81,693–701.

Gülçin, _I., Bursal, E., S�ehitoglu, M. H., Bilsel, M., & Gören, A. C. (2010). Polyphenolcontents and antioxidant activity of lyophilized aqueous extract of propolisfrom Erzurum, Turkey. Food and Chemical Toxicology, 48(8-9), 2227–2238.

Gülçin, _I., Küfrevioglu, Ö. _I., & Oktay, M. (2005). Purification and characterization ofpolyphenol oxidase from nettle (Urtica dioica L.) and inhibition effects of somechemicals on enzyme activity. Journal of Enzyme Inhibition and MedicinalChemistry, 20(3), 297–302.

Harris, J.R. (1996). Subcellular biochemistry, ascorbic acid: Biochemistry andbiomedical cell biology (Vol. 25). New York: Plenum.

Hayakawa, K.-I., & Timbers, G. E. (1977). Influence of heat treatment on the qualityof vegetables: changes in visual green color. Journal of Food Science, 42,778–781.

Hemeda, H. M., & Klein, B. P. (1991). Inactivation and regeneration of peroxidaseactivity in vegetable extracts treated with antioxidants. Journal of Food Science,56, 68–71.

Koksal, E., & Gülçin, _I. (2008). Purification and characterization of peroxidase fromcauliflower (Brassica oleracea L. var. botrytis) buds.. Protein and Peptide Letter,15(4), 320–326.

Lau, M. H., Tang, J., & Swanson, B. G. (2000). Kinetics of textural and color changes ingreen asparagus during thermal treatments. Journal of Food Engineering, 45,231–236.

Lin, T. M., Durance, T. D., & Scaman, C. H. (1998). Characterization of vacuummicrowave, air and freeze dried carrot slices. Food Research International, 31(2),111–117.

Lu, H. F., Zheng, H., Lou, H. Q., Jiang, L. L., Chen, Y., & Fang, S. S. (2010). Using neuralnetworks to estimate the losses of ascorbic acid, total phenols, flavonoid, andantioxidant activity in asparagus during thermal treatments. Journal ofAgricultural and Food Chemistry, 58, 2995–3001.

Morales-Blancas, E. F., Chandia, V. E., & Cisneros-Zevallos, L. (2002). Thermalinactivation kinetics of peroxidase and lipoxygenase from broccoli, greenasparagus and carrots. Journal of Food Science, 67(1), 146–154.

Nagy, S., & Smooth, J. M. (1977). Temperature and storage effects on percentretention and percent US recommended dietary allowances of vitamin C incanned single-strength orange juice.. Journal of Agricultural and Food Chemistry,25(1), 135–138.

Oboh, G. (2005). Effect of blanching on the antioxidant properties of some tropicalgreen leafy vegetables. LWT – Food Science and Technology, 38(5),513–517.

Selman, J. D. (1994). Vitamin retention during blanching of vegetables. FoodChemistry, 49(2), 137–147.

Singh, R. P., & Chen, G. (1980). Lethality-Fourier method to predict blanching. In P.Linko, Y. Malakki, J. Olkku, & J. Larinkari (Eds.). Food process engineering (Vol. 1).London: Applied Science Publishers.

S�is�ecioglu, M., Gülçin, _I., Çankaya, M., Atasever, A., S�ehitoglu, M. H., Kaya, H. B., et al.(2010). Purification and characterization of peroxidase from Turkish blackradish (Raphanus sativus L.). Journal of Medicinal Plants Research, 4(12),1187–1196.

Soysal, Ç., & Söylemez, Z. (2005). Kinetics and inactivation of carrot peroxidase byheat treatment. Journal of Food Engineering, 68, 349–356.

Torreggiani, D., & Bertolo, G. (2001). Osmotic pre-treatments in fruit processing:chemical, physical and structural effects. Journal of Food Engineering, 49,247–253.

Uddin, M. S., Hawlader, M. N. A., Ding, L., & Mujumdar, A. S. (2002). Degradation ofascorbic acid in dried guava during storage. Journal of Food Engineering, 51,21–26.

Viña, S. Z., Olivera, D. F., Marani, C. M., Ferreyra, R. M., Mugridge, A., Chaves, A. R.,et al. (2007). Quality of Brussels sprouts (Brassica oleracea L. gemmifera DC) asaffected by blanching method. Journal of Food Engineering, 80, 218–225.

Vinson, J. A., Hao, Y., Su, X. H., & Zubik, L. (1998). Phenol antioxidant quantity andquality in foods: vegetables. Journal of Agricultural and Food Chemistry, 46(9),3630–3634.

Zheng, H., Fang, S. S., Lou, H. Q., Chen, Y., Jiang, L. L., & Lu, H. F. (2011). Neuralnetwork prediction of ascorbic acid degradation in green asparagus duringthermal treatments. Expert Systems with Applications, 38, 5591–5602.

Zheng, H., & Lu, H. F. (2011). Use of kinetic, Weibull and PLSR models to predict theretention of ascorbic acid, total phenols and antioxidant activity during storageof pineapple juice. LWT – Food Science and Technology, 44, 1273–1281.