RESEARCH ARTICLE

Effects of sucrose crystallization and moisture migration onthe structural changes of a coated intermediate moisture

food

Tiancheng LI1,2, Peng ZHOU (✉)1,3, Theodore P. LABUZA (✉)3

1 State Key Laboratory of Food Science and Technology, Jiangnan University, Wuxi 214122, China2 School of Food Science and Technology, Jiangnan University, Wuxi 214122, China

3 Department of Food Science and Nutrition, University of Minnesota, Saint Paul, MN 55108, USA

© Higher Education Press and Springer-Verlag 2009

Abstract The purpose of this study was to investigatewhether moisture migration and sugar crystallization playan important role in the changes of IMF matrix structure.The migration of water was monitored with changes ofwater activity in different physical domains of samplesduring storage, while the crystallization of sucrose wasdetermined with X-ray powder diffraction (XRD). Theformation of both a hard inner-layer and agglomeratedparticles in the inner matrix was observed during storage.Our results suggested that both moisture loss and sucrosecrystallization were mainly responsible for the formationof the crusty intermediate inner layer, and the agglomer-ated matrix particles were the result of sucrose crystal-lization.

Keywords sugar crystallization, moisture migration,intermediate moisture foods, texture, water activity

1 Introduction

Intermediate-moisture foods (IMFs) were described asproducts with a moderate moisture content and a reducedwater activity (aw) created to be shelf stable withoutrefrigeration [1,2]. IMF has no precise definition based onwater content or water activity, but their moisture isgenerally in the range of 10% to 40%, and their aw is from0.5 to 0.9 [3,4]. Since the texture may become too hardwhen aw is lowered to 0.5 to 0.7, the use of humectants/plasticizers such as polyols, fructose and sucrose wasintroduced. However, the sugar-containing foods aresubject to re-crystallization, which may cause changes in

food structure and texture [5,6]. Prior research on softcookies suggested that as the sugar crystallizes, the amountof plasticizer decreased [7]. Moreover, since the starchgranules of the wheat flour did not swell, they absorbedwater so the 3 molecules of water released from the sucroseas it crystallizes migrate to the starch granule, furthercausing toughening.In addition, IMF is often coated with an out-layer such

as a chocolate, with the functions of reducing moisture lossand providing flavors and color. However, the difference inthe aw of these multi-domain (outer coating and innermatrix) food products would result in moisture migrationwithin products, which can also cause the changes in foodstructure and texture unless they were both at the samewater activity. The purpose of this study was to investigatethe extent that moisture migration and sugar crystallizationplay in the physical state changes of a coated IMF modelsystem product.

2 Material and methods

2.1 IMF samples

The products in this study included the fresh IMF samplewith a flavored coating (aw in the range of 0.3 to 0.4) and asample stored at room temperature for one month with aflavored coating. The IMF sample contained sucrose andwheat flour as the major components for the inner matrix,with the aw in the range of 0.75 to 0.8, while the flavoredcoat had the aw between 0.35 and 0.4.The freshly prepared sample contained two main

physical domains (Fig. 1): (a) the flavored coating and(b) the IMF matrix. After storage at room temperature forone month, the formation of a hard intermediate inner layer

Received July 12, 2009; accepted August 25, 2009

E-mail: tplabuza@umn.edu, zhoupeng@jiangnan.edu.cn

Front. Chem. Eng. China 2009, 3(4): 346–350DOI 10.1007/s11705-009-0256-8

and some matrix hard particles were observed. Thus, thesample stored for one month contained three majorphysical domains (Fig. 2): (a) the flavored coating; (b)the intermediate inner layer between the coating and themain matrix, which is a thin and hard layer right below thecoating, and this part is called “matrix layer”; and (c) themain matrix, which is composed of both matrix material(“matrix base”) and agglomerated particles (“matrixparticle”) which appear in storage.

The coating was totally removed from samples with aknife and collected for water activity measurement. Thematrix of the fresh sample was smooth (Fig. 1(B)). For thestored sample, the intermediate inner layer of the matrixwas scraped off carefully with a knife (Fig. 2(C)) and thenground with a mortar and pestle. The main matrix of thestored sample contained large particles stuck to each otherwith a diameter varying between 2 and 6 mm (Fig. 2(D)).Because of this, the matrix of the stored samples wasfurther separated by hand into the free matrix base and thematrix agglomerated particles (Figs. 2(E) and 2(F)). Thematrix particles were then ground with a mortar and pestle.Each of the systems was then used for the XRD or wateractivity measurements.

2.2 Water activity measurements

The water activity of the various parts of samples wasdetermined using the AquaLab 3TE Water Activity Meter(Decagon Devices, Inc., Pullman, WA, USA). Theinstrument was calibrated just prior to the testing usingthe procedures specified by Decagon.

2.3 X-ray powder diffraction (XRD) measurements

XRD analysis was done on the samples with a SiemensD5005 wide-angle diffractometer (Bruker AXS Inc.,Karlsruhe, Germany). The diffractometer had a sealed2.2 kW Cu source, diffracted beam monochrometer, and ascintillation counter detector. The Braggs-Brentano paraf-ocusing geometry was used with a 1 mm aperture slit and0.6 mm detector slit. X-ray patterns were used with a stepscan from 4.0° to 30.0° 2θ with a 0.02° step and a 1 seconddwell time for each step.

3 Results and discussion

3.1 Changes in IMF structure during storage

The coated IMF sample contained two major physicaldomains: the flavored coating and the IMF main matrix(Fig. 1). After the storage at room temperature for onemonth, changes in matrix structure were observed(Fig. 2). Firstly, a hard intermediate inner layer wasformed between the coating and the main matrix,which was a thin and hard layer right below thecoating. Moreover, some agglomerated hard particleswere also observed in the main matrix, which had adiameter varying between 2 and 6 mm (Fig. 2). Thus,after the storage, the IMF matrix was further dividedinto three different parts: matrix layer, matrix particle,and matrix base (Fig. 2). The changes in IMF matrixstructure are indicative of some physical changessuch as moisture migration and crystallization of sugar[5,6].

Fig. 1 Image of freshly prepared sample: (A) the out-layercoating and (B) the main inner matrix

Fig. 2 Image of sample that was stored at room temperature forone month: (A) the coated out-layer; (B) the cross-section of mainmatrix, which can be separated into (C) and (D); (C) theintermediate inner layer between matrix and coating that formedduring storage; (D) the inner matrix, which can be furtherseparated into (E) and (F); (E) the matrix base; and (F) the matrixparticles

Tiancheng LI et al. Effects of sucrose crystallization and moisture migration 347

3.2 Moisture migration during storage

The flavored coating of IMF samples has the functions ofreducing moisture loss because of the fat content andproviding flavor and color. As seen in Table 1, for thefreshly prepared sample, the coating had the water activityaround 0.366, while the water activity of main matrix wasaround 0.734 and much higher than that of the coating part.Thus, the difference in the aw of flavored coating and mainmatrix made moisture migration possible in the direction ofthe matrix loosing moisture to the coating. After thestorage for one month, the aw of flavored coatingincreased, while the water activity of the newly formedcrust layer was significantly lower than that of matrix(Table 1). Thus, part of the changes that occurredmight be due to moisture migration from the inner layer ofmatrix to the flavored coating with low aw. This, in itself,could also cause the inner layer of matrix to harden overtime [8]. Although the agglomerated particles weredifferent in texture, i.e. very hard, their water activitywas not different statistically (P< 0.05) from that of theinner matrix, which suggested that moisture migrationwould not be the major factor causing the formation ofhard matrix particles.

3.3 Sugar crystallization during storage

Figure 3 shows the XRD library file for sucrose crystal,and Figs. 4 and 5 show the XRD results of different innerphysical domains of the fresh and stored IMF samples,respectively. In the XRD graph of pure sucrose (Fig. 3), theintensity of sucrose was expressed as a percentage (% onthe left Y axis) and with the dashed line to show each keyidentifying peak. The strongest peak (based on peak area)is assigned an intensity value of 100, and other peaks arescaled relative to that value. For the XRD pattern of typicalsucrose crystals (Fig. 3), only one peak is observed below10° 2θ, at about 7° 2θ, while there are strong peaks at about11° to 14° 2θ, another series at 18°–22°, and a strong one at~ 25° 2θ.In Figs. 4 and 5, the X-ray pattern of measured samples

is expressed as the black solid line, and the intensity wasexpressed as counts for 1 second (right Y axis) in the 1-secdwell time at each 0.02-degree increment. To identify

whether the crystallization of sucrose occurred duringstorage, the diffraction data obtained from the PDF files forsucrose (#24–1977, dotted line) were also drawn on eachgraph to show the likely position (X axis) and relativeintensity (left Y axis) of each crystal peak. Note that peaksmay shift slightly to the left or right if the crystal sizes arenot homogeneous.The XRD pattern for the interior matrix for the fresh

sample is shown in Fig. 4 and serves as a control in thepresent study. Some sucrose peaks were observed at thecharacteristic 2θ angles of ~11.7°, 18.9°, and 19.6°, butthese had low intensities as compared to pure sucrose. Thisphenomenon might be attributed to the incompletedissolving of the powdered sugar in the formula duringthe mixing or some crystallization that started during andright after the preparation of samples. These small crystalscan serve as nucleation sites for the growth of crystalsduring later storage.During storage, some particulated/agglomerated regions

Table 1 Changes in water activity of the IMF domains

sample ID parts water activity

fresh IMF sample coating 0.366�0.020

matrix 0.734�0.004

stored IMF sample coating 0.406�0.040

matrix

matrix layer 0.693�0.016

matrix base 0.733�0.011

matrix particle 0.740�0.009

Fig. 3 Standard XRD patterns for sucrose obtained from thelibrary powder diffraction files (#24–1977)

Fig. 4 The XRD pattern of the main inner matrix of fresh IMFsample

348 Front. Chem. Eng. China 2009, 3(4): 346–350

were formed in the base matrix, so the matrix wasseparated into matrix base with no particulates and matrixagglomerated particles for the XRD studies (Fig. 5). TheXRD patterns for matrix base of samples stored for onemonth (Fig. 5(A)) indicated several distinct peaks forsucrose (those at 11.7°, 13.2°, 18.9°, 19.6°, and 24.8°) withhigher intensities than the fresh sample. It suggestedcrystallization of sucrose during storage, which most likelycaused the formation of the large agglomerated particles inthe matrix since the intensity of the separated agglomeratedparticles was about 350 counts for the major sucrose peaks

(Fig. 5(B)), while for the separated matrix base, it was onlyabout 150 counts (Fig. 5(A)). Compared with the freshmatrix base, the intermediate inner layer of stored samplecontained a greater amount of sucrose crystals, and severaldistinct sucrose peaks were observed (Fig. 5(C)). Becauseof the lower moisture content of the coating, moisturemigrated from the surface of matrix into the coating, whichprobably accelerated sucrose crystallization in the matrixsurface and caused the formation of the in-layer part duringsample preparation and storage [8,9].

4 Conclusions

The difference in the aw of flavored coating and mainmatrix made moisture migration take place and resulted inthe formation of a hardened crust on the outer surface ofthe inner matrix. In addition, sucrose crystallizationoccurred in both the crusty intermediate inner layer andwithin the rest of the matrix which formed agglomeratedmatrix particles. Thus, the control of crystallization isimperative in IMF products using sucrose in a high awmatrix.

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Fig. 5 The XRD pattern of different physical domains of IMFsample that stored at room temperature for one month: (A) the mainmatrix base; (B) the matrix particles; and (C) the crust inner layer

Tiancheng LI et al. Effects of sucrose crystallization and moisture migration 349

Dr. Theodore P. Labuza is a MorseAlumni Distinguished Teaching Profes-sor of Food Science in the Depart-ment of Food Science and Nutrition atthe University of Minnesota. Hereceived a B.S. (1962) and a Ph.D.(1965) in Food Science at Massachu-setts Institute of Technology (MIT).Dr. Labuza taught Food Engineering

at MIT until July of 1971, when he went to the University ofMinnesota. He is an author of over 250 scientific refereedresearch articles, 17 textbooks, 75 book chapters, 7 patents,and 105 other semitechnical articles. He has also given over510 invited technical lectures since 1971 as well as over 330 moregeneral lectures on food science and technology. Three of hispapers have been cited over 200 times, an accomplishment only

0.5% of all of the> 36 million scientific publications since 1950have achieved.

Dr. Labuza served as the President of Institute of FoodTechnologists (IFT) during 1988–1989 and received the NicholasAppert Award, IFT's highest award for food science and technologyworldwide in 1998. He was the editor of the Journal of FoodProcessing and Preservation from 1976 to 1984 and has been on theBoard of Editors of Nutrition Research Newsletter (1982–1990),Cereal Foods World (1987–1989), Journal of Packaging Technology(1986–1991), Journal of Food Additives and Contaminants (1980–1990), Journal of Nutrition and Cancer (1975–1995), Polish Journalof Food Science, and the Journal of Food Science (1984–1986). Heis currently on the Board of Editors of Trends in Food Science andTechnology, Italian Journal of Food Science, Journal of InnovativeFood Science & Technologies, International Journal of FoodProperties, and Food Biophysics.

350 Front. Chem. Eng. China 2009, 3(4): 346–350