lable at ScienceDirect

LWT - Food Science and Technology 59 (2014) 1186e1190

Contents lists avai

LWT - Food Science and Technology

journal homepage: www.elsevier .com/locate/ lwt

Research note

Effect of temperature fluctuations on ice-crystal growth in frozenpotatoes during storage

Javid Ullah a, Pawan S. Takhar a, *, 1, Shyam S. Sablani b

a Food Science and Human Nutrition, University of Illinois at Urbana Champaign, USAb Biological Systems Engineering, Washington State University, Pullman, WA, USA

a r t i c l e i n f o

Article history:Received 16 January 2014Received in revised form23 May 2014Accepted 6 June 2014Available online 17 June 2014

Keywords:FreezingMicro computed tomographyFreeze/thaw cyclesImage analysisStorage

* Corresponding author.E-mail address: ptakhar@illinois.edu (P.S. Takhar).

1 Author has previously published as Pawan P. Sing

http://dx.doi.org/10.1016/j.lwt.2014.06.0180023-6438/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

Temperature fluctuation during storage and distribution of frozen foods has been a major concern forfrozen food manufacturing industry. The fluctuation of temperature results in thawing and recrystalli-zation of ice crystals, which is the main cause of frozen food quality degradation during storage. Theeffect of temperature fluctuations on ice recrystallization in frozen potatoes was investigated. Usingmicro computed tomography, growth in pore area due to increase in ice crystal sizes was determined.Four treatments (constant at �80 �C; and fluctuations between �17 to �16 �C, e17 to �11 �C and �17to �7 �C) were used to observe their effect on pore size distribution. The growth of ice crystals with theincrease in amplitude of temperature fluctuations resulted in damaging the microstructure of frozenpotatoes. The damage pore solid walls resulted in reduction of the number of pores and increase in thesize of pores.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Potato is a starchy tuberous crop, known as Solanum tuberosum,belonging to the family Solanaceae. For centuries potatoes havebeen consumed and have become an integral part of food globally.After rice, wheat and maize, potato is the next important food crop.More than 30% of the potato crop is used for making French friesworldwide (Saguy & Pinthus, 1995). For more than one and a halfcentury potato chips have been a popular snack in the US and itsretail sale is about $6 billion/year, which is 33% of the total sale inthe market (Garayo & Moreira, 2002). For longer storage, potatoesrequire specialized care in stores. If potatoes are held for severaldays at 4.5 �C, or below, reducing sugars accumulate, resulting indark color of processed product (Talburt & Smith, 1959). Temper-ature above 24 �C for long periods of time may increase certaintypes of storage rot diseases and, in fairly airtight areas, may resultin blackheart, a discolored break down of the tissues in or near thecenter of the tuber. Fluctuation in transit temperature may causepotatoes to be unacceptable at their destination due to accumula-tion of reducing sugars to amounts, which result in chips of darkcolor. Storage hasmultiple effects on potatoes sugar content, starch,

h.

enzymes, respiration and organic acids. It is difficult to maintainraw potato quality during storage. On the other hand French friesand dehydrated potatoes are somewhat less susceptible tobrowning (Talburt & Smith, 1959).

Freezing and thawing may result in textural changes of frozenfoods during storage and transportation due to the growth in size ofice crystals. The rate of growth in crystal size depends upon fluc-tuation of temperature during storage and transportation. Small icecrystals are formed during quick freezing, which are unstableduring freeze/thaw operations. The small ice crystals become largerin size resulting in more stable crystals during storage (Ablett,Clarke, Izzard, & Martin, 2002; Adapa, Schmidt, Jeon, Herald, &Flores, 2000; Chevalier, Le Bail, & Ghoul, 2000; Do, Sagara,Tabata, Kudoh, & Higuchi, 2004; Hagiwara, Hartel, & Matsukawa,2006). The advantage of quick freezing may attenuate by fluctua-tion of storage temperature resulting in state and phase transition.Foods are considered to be more stable in their glassy state, inwhich their molecular motion is reduced. Reduced molecular mo-tion helps to maintain food quality. The transition of food fromglassy to rubbery state and vice versa is unavoidable under freeze/thaw conditions. The increased molecular mobility in rubbery stateresults in the enlargement of crystals and subsequent qualitydegradation (Syamaladevi, Kalehiwot, Balasigam, & Sablani, 2012).Glassy and rubbery states are important in frozen foods as the re-actions are reduced to a large extent in the glassy state (Hagiwara,Mao, Suzuki, & Takai, 2005; Syamaladevi, Sablani, Tang, Powers, &

J. Ullah et al. / LWT - Food Science and Technology 59 (2014) 1186e1190 1187

Swanson, 2010). The effect of freezing and thawing during storagehas also been studied in meat, sea foods, ice cream and sugar so-lutions, which causes undesirable changes in texture of food (Adapaet al. 2000; Hagiwara et al. 2006; Mousavi, Miri, Cox,& Fryer, 2007;Syamaladevi et al., 2012).

The X-ray micro computed tomography system allows visuali-zation and measurement of complete three dimensional objectstructures with minimal sample preparation. The techniques havebeen used for many materials such as rocks, bones, ceramics,metals (Salvo et al., 2003), and food proteins (Mousavi, Miri, Cox, &Fryer, 2005; Mousavi et al., 2007). The approach of CT scanning hasalso been used for fresh fruits (Lin et al., 2008), for measuringdistribution of fat in beef muscles (Frisullo, Marino, Laverse,Albenzio, & Del Nobile, 2010) and other food materials (Lida,Matsuoka, Shimizu, Wakisaka, & Katsumata, 2013). This commu-nication discusses the effect of fluctuating storage temperature onmicrostructural changes in potatoes due to ice recrystallization.

Fig. 1. CT scan micrographs of freeze dried potatoes showing the p

While freeze/thaw studies have been performed in the past, thismanuscript's emphasis is on quantifying the porous microstructureformation due to recrystallization by performing image analysis.The quantitative analysis discussed in the study will be useful forengineers designing the freezing operation and in future studiesaimed at modeling the freeze/thaw process. The results will alsohelp food industries in maintaining food quality during storage andtransportation by understanding the effect of temperature fluctu-ations during storage and shipping on crystal formation andgrowth.

2. Materials and methods

Fresh potatoes (Russet var.) were purchased from a local grocerystore. The potatoes were cut to French fry cuboid shapes(11.9 mm � 11.9 mm � 63.5 mm). The cut potatoes were washedwith fresh tap water to remove starch from the surface to avoid

ores sizes as a function of fluctuation in freezing temperature.

Fig. 2. Pores size distribution in samples stored at (a) �80 �C; (b) �17 to �16 �C; (c) �17 to �11 �C; and (d) �17 to �7 �C.

J. Ullah et al. / LWT - Food Science and Technology 59 (2014) 1186e11901188

browning. Next, they were blanched at 83e84 �C for 2 min in asteam jacketed kettle. After blanching, the cuts were put in Ziplocstorage bags and stored in a freezer at �80 �C (T1) to be consideredas control. Another batch of potatoes similarly prepared was storedin a second freezer with fluctuating temperature. The temperatureof the second freezer was fluctuated between �17 and �16 �C (T2)for 10 days; between �17 and �11 �C for the next ten days (T3) andfinally between �17 and �7 �C (T4) for the last ten days. Thetemperature of the freezer during storage was controlled usingprogrammable temperature controller (Model BCS-462, EmbeddedControl Concepts, Huntington Beach, CA). Thermocouples attachedto the temperature controller were put in the plastic bags con-taining potatoes and inside the freezer. The controller was alsoconnected to a computer for continuous data recording and remotemonitoring of the freezer operation. This allowed studying the ef-fect of increase in temperature fluctuations on ice crystal size andits effect on microstructure of the cut potatoes. The end point offreezing (Tm0) of potato was determined as �13 �C using the pro-cedure detailed in Syamaladevi et al. (2010). The low temperaturetransition temperature of potato was determined using differentialscanning calorimeter (DSC, Q2000, TA Instruments, New Castle,DE). The calorimeter was calibrated by checking standard temper-atures and enthalpies of fusion for indium and sapphire. Initially,the potato samples were cooled to �50 �C without annealing toidentify the apparent Tm0. A linear base line to the melting endo-thermwas drawn to identify apparent Tm0. The baseline intersectedwith the endotherm and the intersection at the left side was taken

as the apparent Tm0 of the potato. Annealing was performed at atemperature (apparent Tm0�1 �C) for 0, 30 and 60 min and anannealing time of 30 min was chosen for further analysis. Afterannealing, potato samples were scanned from (apparent Tm0�1 �C)to �50 �C at the rate of 5

�C/min and then heated to 25 �C at 5 �C/

min to identify Tm0. Therefore, the selected fluctuating tempera-tures during storage were kept above and below Tm0 to study theireffect on crystal grown on both sides of the temperature rangearound Tm0. The temperature of the freezer during storage wascontrolled by programmable temperature controller (Model BCS-462, Embedded Control Concepts, Huntington Beach, CA). The cutpotatoes were taken out of the freezer after each 10-day intervaland freeze dried (Model FreeZone 6, Labconco freeze dryer, KansasCity, MO). The temperature of �50 �C and vacuum of 2.1 Pa wasused during freeze-drying.

The freeze-dried samples were scanned using a high resolutionXradia micro-computed tomography (Model Micro XCT-400 Xra-dia, Pleasanton, CA) to see the distribution of pores sizes. A voxelsize of 33.68 mm was used for image acquisition. The images wereanalyzed using Image Pro Plus software (Media Cybernetics, Inc.Rockville, MD). The pores were separated from solid portions. Allpore pixels were selected within a specific range from pores tosolid and classified as one selection type. The solid pixels wereselected within another specific range from pores to solid andclassified as second selection type. There were contiguous groupsof pixels in each selection for both the pores and solid features,which allowed creating an enclosed outline around each

Fig. 3. Average pore area versus freezer temperature during storage. Five images wereanalyzed at each temperature level with total number of pores being: 1890, 664, 334and 302 for temperature levels shown from left to right along x-axis. Error barsindicate one standard error.

J. Ullah et al. / LWT - Food Science and Technology 59 (2014) 1186e1190 1189

individual feature for providing the data for the software toquantify. The pore area data generated by Image Pro Plus wereimported into Jmp Pro (Ver 11, SAS Inc., Cary, NC, U.S.A.) andMatlab (R2012b, Mathworks Inc., Natick, MA, U.S.A.), where poresize distribution was analyzed.

3. Results and discussion

The crystal growth was estimated by comparing the pore areasof samples subjected to freeze/thaw cycles with control samplefrozen rapidly in �80 �C freezer. Freeze dried potatoes, show anetwork of fibers and holes representing ice crystals (Fig. 1). Imagesshow that ice crystals are small and not uniform in size soon afterfreezing. Ice crystals become larger and bigger as the amplitude ofstorage temperature fluctuations and time is increased. Freezethaw cycles are expected to be the reason for increase in size of icecrystals due to recrystallization in the potatoes (Fig. 1). In com-mercial frozen foods, thawing and refreezing is the unavoidablereason of recrystallization and crystal growth. As the fluctuation intemperature increased, the growth of ice crystals occurred andsubsequently the number of ice crystals reduced (Figs. 1 and 2).Analysis of 5 micro CT images at each temperature level shows thatthe total number of pores (N ¼ 1890) in control stored at �80 �C ishigher as compared to the number of pores in samples storedat �17 to �16 �C (N ¼ 664) (Fig. 2a, b). In control sample, a thickfibrous network is visible which is representing the solid portion ofthe potatoes (Fig. 1). As the fluctuation is increased to temperaturesbetween �17 and �11 �C, a reduction in the number of pores(N ¼ 334) due to growth in pore sizes of ice crystals is observed(Fig. 2c). A further reduction in number of pores (N ¼ 302) isobserved for samples stored between �17 and �7 �C (Fig. 2d). Thereduction in number of pores was deduced to be the result of ice

crystal growth because the smaller pores merged to form biggerpores as a result of solid wall disruption (Fig. 1). These resultsclearly indicate that as the fluctuation in temperature is increasedthe size of ice crystals becomes larger. Enlargement of ice crystals,damages the internal fibrous structure of potatoes, and ultimatelydegrades the quality and reduces the storage life. A similar crystalgrowthwas reported by Syamaladevi et al. (2012) for frozen salmonand Ablett et al. (2002) for frozen sugar solutions. Syamaladeviet al. (2012) also reported that when the ice crystal size increases,the number of ice crystals decreased in salmon. Our results are alsosupported by the hypothesis of Do et al. (2004) and Mousavi et al.(2005). The authors reported the enlargement of ice crystals size instored beef and other frozen solid foods during storage and re-ported the destruction of internal structure affecting the qualityand storage stability.

Fig. 2aed also show pore size distribution for control and timevarying storage temperature samples. Figure shows that for control(�80 �C) the pore size distribution is over smaller pore area rangespanning from 0 to 6 mm2. As the temperature fluctuation isincreased, the distribution spreads toward larger pore areas(Fig. 2bed), which is expected to have been caused by increase inice-crystal size. This damage in microstructure of potatoes is ex-pected to result in subsequent reduction in the storage life of po-tatoes. This may be the result of recrystallization without a state/phase transitions. Do et al. (2004) also investigated the ice crystalgrowth in frozen beef with increase in temperature fluctuationsresulting structural damage.

For control samples stored in �80 �C freezer, smaller ice crystalsare formed due rapid lowering and negligible fluctuation of tem-perature (Fig. 3). The smaller crystals caused less damage to thestructure of potatoes. As shown in Fig. 3, the average pore size is0.254mm2 in samples stored at�80 �C.When the temperaturewasincreased to fluctuating values between �17 and �16 �C, theaverage pore size increased to 0.586 mm2. With further increase intemperature fluctuation from �17 to �11 �C, the average pore sizeincreased to 1.105 mm2. The average pore size further increased to1.147 mm2 for samples stored at fluctuating temperaturebetween �17 and �7 �C. This showed that as the fluctuation intemperature is increased, the size of ice crystal is increased. This isexpected to be due to increased molecular mobility and recrystal-lization (Syamaladevi et al., 2012). Syamaladevi et al. (2012) re-ported that during storage and transportation with increase intemperature, the thawing and recrystallization are the reason forincrease in crystal size, which results in damage to the internalstructure. The damage to the internal structure is expected to causequality degradation and reduced storage life of food materials.

It was concluded from the present study that temperaturefluctuations during storage and transportation is affecting themicrostructure of potatoes. During shipment from the plant to thetrucks and then to the vendors the increase in temperature willresult in increasing the size of ice crystals, which rupture the cellstructure and subsequently degrades the quality of food.

Acknowledgments

This work was financially supported by the Higher EducationCommission of Pakistan (HEC). Thanks to the University of Illinoisat Urbana-Champaign and The University of Agriculture, Peshawar,Pakistan for providing the opportunity to conduct this study and forproviding study leave, respectively to Dr. Ullah. The authors wouldlike to thank Dr. Roopesh Syamaladevi for performing DSC exper-iments. Micro CT experiments and image analysis was performed atthe Beckman Institute at University of Illinois.

J. Ullah et al. / LWT - Food Science and Technology 59 (2014) 1186e11901190

References

Ablett, S., Clarke, C. J., Izzard, M. J., & Martin, D. R. (2002). Relationship between icerecrystallization rates and the glass transition in frozen sugar solutions. Journalof the Science of Food and Agriculture, 82, 1855e1859.

Adapa, S., Schmidt, K. A., Jeon, I. J., Herald, T. J., & Flores, R. A. (2000). Mechanisms ofice crystallization and recrystallization in ice cream: a review. Food ReviewsInternational, 16(3), 259e271.

Chevalier, D., Le Bail, A., & Ghoul, M. (2000). Freezing and ice crystals formed in acylindrical food model: part I. Freezing at atmospheric pressure. Journal of FoodEngineering, 46(4), 277e285.

Do, G., Sagara, Y., Tabata, M., Kudoh, K., & Higuchi, T. (2004). Three-dimensionalmeasurement of ice crystals in frozen beef with micro-slicer image processingsystem. International Journal of Refrigeration, 27, 184e190.

Frisullo, P., Marino, R., Laverse, J., Albenzio, M., & Del Nobile, M. A. (2010). Assess-ment of intramuscular fat level and distribution in beef muscles using X-raymicrocomputed tomography. Meat Science, 85, 250e255.

Garayo, J., & Moreira, R. (2002). Vacuum frying of potato chips. Journal of FoodEngineering, 55, 188e191.

Hagiwara, T., Mao, J., Suzuki, T., & Takai, R. (2005). Ice recrystallization in sucrosesolutions stored in a temperature range of -21Cto -50C. Food Science andTechnology Research, 11(4), 407e411.

Hagiwara, T., Hartel, R. W., & Matsukawa, S. (2006). Relationship between recrys-tallization rate of ice crystals in sugar solutions and water mobility in freeze-concentrated matrix. Food Biophysics, 1(2), 74e82.

Lida, Y., Matsuoka, M., Shimizu, I., Wakisaka, T., & Katsumata, A. (2013). Micro-CTobservation of test food materials for videofluoroscopic swallowing studies.Oral Radiology, 29(1), 56e63.

Lin, T. T., Liao, Y. C., Huang, T. W., Ouyang, C. S., Jiang, J. A., Yang, M. M., et al. (2008).X-ray computed tomography analysis of internal injuries of selected fruits.American Society of Agricultural and Biological Engineers Annual InternationalMeeting, 7, 4056e4071.

Mousavi, R., Miri, T., Cox, P. W., & Fryer, P. J. (2005). A novel technique for ice crystalvisualization in frozen solids using X-ray micro-computed tomography. Journalof Food Science, 70(7), 437e442.

Mousavi, R., Miri, T., Cox, P. W., & Fryer, P. J. (2007). Imaging food freezing using X-raymicrotomography. International Journalof FoodScienceandTechnology, 42, 714e727.

Saguy, I. S., & Pinthus, E. J. (1995). Oil uptake during deep fat frying: factors andmechanisms. Food Technology, 49(4), 142e145, 152.

Salvo, L., Cloetens, P., Maire, E., Zabler, S., Blandin, J. J., Buffiere, J. Y., et al. (2003). X-ray micro-tomography; an attractive characterization technique in materialsscience. Nuclear Instruments and Methods in Physics Research, 200, 273e286.

Syamaladevi, R. M., Kalehiwot, N. M., Balasigam, M., & Sablani, S. S. (2012). Un-derstanding the influence of state/phase transition on ice recrystallization inAtlantic Salmon (Salmo salar). Food Biophysics, 7, 57e71.

Syamaladevi, R. M., Sablani, S. S., Tang, J., Powers, J., & Swanson, B. G. (2010). Watersorption and glass transition temperatures in red raspberry (Rubusidaeus).Thermochimica Acta, 503e504, 90e96.

Talburt, W. F., & Smith, O. (1959). Potato processing. Westport Connecticut, U.S.A: AVIPublishing Co.