Fat Crystal Growth and Microstructural Evolution in Industrial Milk Chocolate Sopark Sonwai† and De´rick Rousseau*,‡ Department of Food Technology, Faculty of Engineering and Industrial Technology, Silpakorn UniVersity, Nakornprathom, Thailand 73000, and School of Nutrition, Ryerson UniVersity, Toronto, Ontario M5B 2K3, Canada
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 9 3165–3174
ReceiVed June 3, 2007; ReVised Manuscript ReceiVed May 1, 2008
ABSTRACT: Surface microstructure, solid fat content (SFC), whiteness index, and fat phase polymorphism in four milk chocolates were studied over 1 year. The fat phase of each chocolate differed in cocoa butter, milk fat, and cocoa butter equivalent (CBE) content. The microtopography of all samples roughened with time, with randomly distributed crystalline and amorphous features gradually appearing. Conelike structures manifested themselves on all chocolates, although at different rates, depending on composition. These may have formed by “welling” and deposition of liquid-state fat pushed from within the matrix onto the surface during contraction. Phase imaging using atomic force microscopy showed that the cones hardened with age. Although only slight changes in surface whiteness index and SFC as a function of time were observed, crystal outcroppings protruded from the cones on all chocolates, suggesting that these features acted as loci for bloom crystals. After 1 year of storage, only the formulation consisting of cocoa butter, no CBE, and a lower amount of milk fat showed signs of visual bloom. This formulation demonstrated the most intense cone growth and was the first to undergo the form VfVI polymorphic transition. Introduction Fat bloom continues to be a problem in the confectionery industry as it compromises both the visual and the textural quality of many chocolates.1 This physical defect, which appears during storage as a dull, grayish-white film on the chocolate surface, has been ascribed to the uncontrolled formation of large, light-diffusing triglyceride (TG) fat crystals >5 µm in length.2–4 The three root causes of fat bloom are related to composition, processing, and improper storage.2 Compositionally, when two incompatible fats [e.g., cocoa butter (CB) and palm kernel oil] are combined, the resulting lower solid fat content (SFC) (below that of either individual fat) enhances liquid fat movement throughout the chocolate matrix.5 Alternatively, in filled confections, unsaturated TGs from soft center fillings, which are normally rich in oil (e.g., hazelnut oil), may migrate toward the chocolate surface. Both scenarios promote indiscriminate crystal growth. From a processing standpoint, fat bloom may occur if tempering and/or cooling are poorly controlled, which encourages improper crystallization (i.e., into form IV) or insufficient form V formation and the subsequent form VfVI transition. Post-processing bloom is often related to undesirably high storage temperatures (>∼30 °C) and undue temperature fluctuations.4,6 In this case, if the chocolate’s fat phase is partly or fully liquefied, subsequent cooling leads to unstable polymorph (re)crystallization.2 Temperature fluctuations during storage, even to a small degree, decrease the induction time for fat bloom and increase its rate of formation.2,6,7 Microstructurally, milk chocolate is a particulate material with sugar crystals, milk solids, and cocoa solids interspersed within a continuous fat phase consisting of crystalline and liquid fat. Tempering and cooling strongly influence the fat phase’s crystallization rate as well as its crystal morphology and aggregation behavior. These steps ultimately result in the contraction of the chocolate and the generation of mesoscopic * To whom correspondence should be addressed. Tel.: +1-416-979-5000 ext. 6940. Fax: +1-416-979-5204. E-mail:
[email protected]. † Silpakorn University. ‡ Ryerson University.
pores and hairline cracks, particularly if too fast a cooling rate is used. Kleinert8 mentioned that even cooling reduced temperature gradients within chocolate (i.e., between the cooling and the air-exposed surfaces), minimizing surface imperfections and subsequent fat bloom. For decades now, concerted efforts by the confectionery industry to partially replace CB in chocolate formulations have led to the admixture of CB with milk fat (MF) and/or cocoa butter equivalents (CBEs). MF is often blended with CB as it has an antibloom effect.9 Compatibility between the two fats allows the addition of up to 30% MF (on a total fat basis), without seriously affecting CB’s polymorphic stability.10 However, MF lowers final SFC and decreases the crystal size, crystallization rate, and rate of polymorphic transformation of the CB.11,12 Different theories have been proposed regarding MF’s antibloom effect. It is thought that a reduced fat crystallization rate may slow chocolate contraction and de facto the formation of surface cracks.8 Alternatively, MF may disrupt the necessary lamellar packing of the form VI lattice, thereby delaying this solid-state transition.13 To this day, the exact manner by which it delays bloom remains unresolved, and further research is much needed. CBEs are vegetable fats with chemical and physical characteristics similar to CB, so that they may be used to replace it in any proportion.9,14 Shortage of supply, poor quality of individual CB harvests, economic advantages, and some technological benefits have prompted their development.15 A key advantage of CBEs is their ability to inhibit fat bloom.16 According to EU Chocolate Directive 2000/36/EC, six vegetable fats (illipe´, palm oil, sal, shea, kokum gurgi, and mango kernel) may be used, at up to 5% (wt) in chocolate.16,17 Most CBEs are based on carefully blended mixtures of palm oil, palm oil fractions, shea stearin, and illipe´ oil that yield a TG profile almost identical to CB.15 If used, the appearance and bloom-free shelf life of CBE-containing products should rival, if not better, that of CBbased products.14,15 In this work, how the fat phase composition of milk chocolate influences microstructure and a number of physicochemical properties over 1 year of isothermal storage was explored. The
10.1021/cg070503h CCC: $40.75 2008 American Chemical Society Published on Web 07/23/2008
3166 Crystal Growth & Design, Vol. 8, No. 9, 2008
Sonwai and Rousseau
Figure 2. Topography of chocolate #1 at different storage times with 2D scans on the left and 3D projections on the right. 25 µm × 25 µm; z-scale ) 2 µm per division. Figure 1. Topography of the four freshly tempered chocolates (at t ) 0 weeks). The 5 µm × 5 µm scans are close-ups of the area identified by the square boxes in the 25 µm × 25 µm scans.
formation and evolution of conspicuous surface conical features led us to develop a theory that may explain liquid fat movement in solid chocolate and its role in fat bloom formation. Materials and Methods Materials. Four milk chocolates with different fat phase formulations were manufactured by Cadbury Adams Canada (Toronto, Canada). Although all chocolates consisted primarily of CB, there were notable differences in composition. Besides CB, chocolate #1 also contained the standard amount of MF for milk chocolate. Chocolate #2 contained the same amount of MF plus CBE, which amounted to ∼5% (wt) of the finished product. Chocolates #3 and #4 contained 50% less MF than chocolates #1 and #2. The rest of the fat phase of chocolate #3 was CB. Finally, chocolate #4 was made up of both CB and CBE [∼5% (wt) of the finished product].
Preparation and Storage of Chocolate Samples. All formulations were properly tempered (at the Cadbury Adams Canada factory in Toronto, Canada) before being poured into plastic molds and solidified into thin chocolate bars (0.4 cm × 3 cm × 12 cm). Once removed from the molds, each bar was packaged in a plastic bag and sealed. Roughly 190 bars were made for each formulation. The samples were then put inside a Styrofoam box and transported to Ryerson University (a 15 min journey). Once on site, they were transferred to a temperaturecontrolled cabinet, set at 25 ( 0.1 °C. Samples were stored for 1 year at this temperature, during which time their properties were monitored weekly for the first 8 weeks and monthly thereafter. Sample characterization at week 0 was performed after the samples had been kept in the cabinet for 1 h. Ten bars per chocolate formulation were required for each set of analyses: two for X-ray diffraction, three for color measurements, three for SFC measurements, and two for microtopography characterization via atomic force microscopy. All samples were sacrificed following analysis. Mesoscale Topography. Atomic force microscopy has been used to investigate the surface of glossy and bloomed chocolate.4,6,18,19 In this work, a Bioscope atomic force microscope (AFM) with Nanoscope
Fat Crystal Growth and Microstructural Evolution
Crystal Growth & Design, Vol. 8, No. 9, 2008 3167
Figure 3. Topography of chocolate #2 at different storage times with 2D scans on the left and 3D projections on the right. 25 µm × 25 µm; z-scale ) 2 µm per division.
Figure 4. Topography of chocolate #3 at different storage times with 2D scans on the left and 3D projections on the right. 25 µm × 25 µm; z-scale ) 2 µm per division.
IIIa controller (Digital Instruments, Santa Barbara, CA) operated in “tapping” mode was used to image 25 µm × 25 µm, 15 µm × 15 µm, and 5 µm × 5 µm areas of the molded side of the chocolate bars. The AFM tips had a cantilever spring constant of 40 N/m and were oscillated at ∼350 kHz with an end-point radius of 10 nm and a body angle of 30°. Height, amplitude, and phase data were recorded. The former two provided information on the microtopography of the chocolates. Phase imaging was used to ascertain relative differences in hardness between structural features seen on the chocolate. This type of imaging can provide a qualitative indication of nanometer-scale local rheological behavior of the surface being examined. All images shown represent the typical evolution seen on the chocolate surface at a given storage time. Polymorphic Transitions during Storage. A Rigaku Geigerflex (Danvers, MA) XRD unit was used in this study to identify the polymorphic form(s) of the samples as a function of storage time. The generated X-rays had a wavelength at 1.79 Å. Scans from 1.5 to 35° 2θ were performed, allowing the detection of spacings between 2.98 and 68.3 Å, which encompasses both small- and wide-angle scattering regions. To extract the chocolate fat phase, the method of Cebula and
Ziegleder3 was used, whereby each chocolate bar was chopped with a knife and sifted to obtain particle sizes 5 readings, and the WI value for each sample was then averaged from three bars. SFC. A Bruker Minispec mq20 pulsed nuclear magnetic resonance (p-NMR) unit (Bruker, Milton, ON, Canada) was used to measure the
3168 Crystal Growth & Design, Vol. 8, No. 9, 2008
Figure 5. Topography of chocolate #4 at different storage times with 2D scans on the left and 3D projections on the right. 25 µm × 25 µm; z-scale ) 2 µm per division. changes in SFC of the chocolates with time, using a previously described procedure.3 The direct method was used and the p-NMR f-factor was set at 1.5. The contribution to SFC of the nonfat material was calculated from the apparent SFC values of the melted chocolate (at 50 °C). Three bars from each formulation were coarsely chopped with a knife, and each was poured into an NMR tube (i.d. ) 1 cm). The glass tubes were stored in a waterbath set at 25 ( 0.1 °C for 30 min prior to each measurement, which took less than 5 s.
Results and Discussion Evolution in Microtopography during Storage. Irrespective of formulation, the surface of all freshly tempered and cooled chocolates (Figure 1) was fine-textured and smoother than the surface of freshly tempered CB.19 The typical root-mean-square roughness of chocolate was ∼0.22 vs ∼0.7 µm in CB. The lower roughness in milk chocolate was likely due either to its spacefilling nature, where the particulates (cocoa solids, milk powder,
Sonwai and Rousseau
and sugar particles) promoted a more regular surface with fewer imperfections, or to differences in tempering that may have yielded diverse fat crystal sizes in CB. The close-up scans of all formulations (Figure 1, right) showed distinct layerlike structures forming on top of one another, similar to CB.19 Contrary to CB, however, all freshly tempered chocolate surfaces exhibited smooth, amorphous conelike features with base diameters of 0.1-0.4 µm (as indicated by the white arrows). Samples aged up to 1 year (Figures 2–5) revealed striking changes in microtopography. Images on the first row of each of these figures show the surface of the samples after 2-10 weeks of storage. After 6 weeks, a number of small cones with base diameters of >2.5 µm were present on the surface of chocolate #1 (Figure 2a). By 4 months, the cones had increased in both number and base diameter, and several of the cones had ovalized (Figure 2b). Between 8 and 12 months (Figure 2c,d), the cone concentration continued to increase. Morphologically, the surface of many cones roughened and numerous small crystals began to protrude. Besides this obvious crystal growth, the remainder of the surface exhibited little change over 1 year. By contrast, over the first 8 months, the surface of chocolate #2 was virtually devoid of any conical feature with a base diameter >2.5 µm (Figure 3a-c). As with all chocolates, these features were randomly distributed on the surface. After 1 year, there were no large cones visible on its surface (Figure 3d). Conversely, on chocolate #3, substantial changes in microstructure were readily apparent after only 2 weeks of storage (Figure 4a). At 4 months, many of the cones were >5 µm in diameter with near-perfect circular bases and sharp, pointed tips (Figure 4b). After 8 months, some cones exceeded 12 µm in diameter and 1.5 µm in height (Figure 4c). After 1 year, numerous crystals jutted out vertically (>1 µm) from the cones and horizontally (∼5-10 µm) along the chocolate surface (Figure 4d). The evolution in the morphology of chocolate #4 (Figure 5a) was similar to that of chocolate #1. Conical features >2.5 µm were visible only after 4 months of storage (Figure 5b). After 8 and 12 months, many of the cones were textured, likely due to crystallization (Figure 5c,d). The only other mention of such cones on the surface of chocolate was by Smith and Dahlman18 who reported the formation of amorphous “drops” on the surface of pralines following 4 weeks of storage. With surface crystals and fat bloom appearing at longer storage times, they postulated that bloom growth in pralines was a two-phase process, with drops initially forming on the surface followed by the growth of bloom crystals from them. This hypothesis was based on the presence of a large number of microscopic pores or holes on the surface of chocolate that conceivably acted as routes for migration of liquid TGs onto the surface.6,18 Contrary to those findings, the conical features in this work were immediately spotted on the surface of the freshly tempered chocolate, irrespective of formulation. The fact that Smith and Dahlman18 used filled chocolates whereas we used solid chocolate may explain our divergent results. Cone nascence appeared directly related to the presence of surface imperfections. In Figure 6a-c, the white arrows point to small, newly formed cones (base diameter 105 different TGs with melting points between -40 and 40 °C.25 Such variety in composition is responsible for MF’s unique physical properties and its β′-stability. The lack of variety in CB and CBE TGs drives crystallization toward the β-forms (forms V and VI). CBE’s similar, although still different, composition will slow down the form VfVI transition.19 Chocolate #2 (full MF and CBE) was slowest to show the form VfVI transition, cone growth, and an increase in WI. Conversely, chocolate #3 (1/2 MF and no CBE) showed the earliest presence of extensive cone formation and the most rapid form VfVI transition. Thus, CBE delayed both the cone formation and the form VfVI phase transition but more so with the full amount of MF present. These results clearly indicate that chocolates demonstrating the most rapid form VfVI transition were also most sensitive to cone formation (i.e., chocolates #1 and #3). This was likely the result of the more rapid form VfVI transition leading to continued contraction within the chocolate, which promoted more welling. Overall, an increase in WI (which may be associated with crystal growth) always followed both cone formation and the form VfVI transition. Solutions to Controlling Cone Formation and Properties. With pores a necessary precursor to cone formation (and ultimately, uncontrolled crystal growth), optimizing process conditions or developing a chocolate composition that limits
3172 Crystal Growth & Design, Vol. 8, No. 9, 2008
Sonwai and Rousseau
Figure 11. Structural evolution in putative cones as a function of storage time: after a few weeks (a-c, from chocolate #4), after ∼4 months (d-f, chocolate #4), after ∼8 months (g-i, chocolate #3), and after ∼1 year (j-l, chocolate #3). The left-hand column shows 2D amplitude representations of the cones. The middle column is the cross-section across the middle of the cone, as marked by the dotted lines. The images in the right-hand column are phase images of the scans presented in the left-hand column. Darker areas correspond to rheologically harder regions.
such imperfections would be an effective means of extending the bloom-free shelf life of milk chocolate. Two complementary contraction-related phenomena in chocolate encourage the formation of cones. During the initial “large-scale” contraction that follows tempering, there is sudden welling promoted as liquid fat is pushed toward the surface of the chocolate. During storage, gradual contraction will continue due to the form VfVI transition and the additional solidification of the fat. The initial
welling “burst” may be limited by moderating cooling-induced contraction within reason so as to permit acceptable demolding while reducing the movement of liquid fat toward the surface. This may be achieved by slowing the crystallization rate of the fat phase and reducing the overall SFC of the chocolate, while still guaranteeing its desirable organoleptic properties. Second, if it is assumed that cone formation will occur, controlling composition or optimizing the manufacturing process such that
Fat Crystal Growth and Microstructural Evolution
Crystal Growth & Design, Vol. 8, No. 9, 2008 3173
Figure 12. X-ray diffraction patterns of the chocolates at different storage times: chocolate #1 (a), chocolate #2 (b), chocolate #3 (c), and chocolate #4 (d); t is in weeks.
Figure 13. Changes in surface WI of the four chocolates with storage time.
Figure 14. Changes in SFC of the four chocolates with storage time.
the cones remain liquidlike for a longer time will retard their solidification. In conclusion, the fat composition that yielded the longest shelf life of these four chocolates consisted of the legal limit of CBE (5%) along with the full amount of MF. This combination was least prone to the form VfVI transition and also demonstrated the slowest cone formation. MF alone also hindered these phenomena, but more so in the presence of CBE. The usage of fats such as MF and CBE that can slow contraction, slightly lower SFC, and retard form VI formation should help to minimize cone formation and ultimately the formation of bloom crystals.
Table 1. Storage Times at Which Key Changes Were Observed storage time (weeks) key observation
#1
#2
#3
#4
onset of form VfVI transition 2 onset of extensive cone formation (D > 2.5 µm) 6 onset of increase in WI 20 SFC low
30 – – low
2 2 10 high
20 16 25 high
Excellent Award from the province of Ontario, Canada, and from Ryerson University is also acknowledged.
References Acknowledgment. Nigel Sanders of Cadbury Adams Canada (Toronto, Ontario, Canada) is thanked for the chocolate samples. Shane Hodge is acknowledged for his technical assistance. Funding from the Natural Sciences and Engineering Research Council of Canada (NSERC), from a Premier’s Research
(1) Bricknell, J.; Hartel, R. W. J. Am. Oil Chem. Soc. 1998, 75, 1609– 1615. (2) Lonchampt, P.; Hartel, R. W. Eur. J. Lipid Sci. Technol. 2004, 106, 241–274. (3) Cebula, D. J.; Ziegleder, G. J. Am. Oil Chem. Soc. 1993, 9, 340–343.
3174 Crystal Growth & Design, Vol. 8, No. 9, 2008 (4) Hodge, S. M.; Rousseau, D. J. Am. Oil Chem. Soc. 2002, 79, 1115– 1121. (5) Talbot, G. In Industrial Chocolate Manufacture and Use; Beckett, S. T., Ed.; Blackie Academic & Professional: Oxford, 1999; pp 307322. (6) Rousseau, D. LWT-Food Sci. Technol. 2006, 39, 852–860. (7) Hettich, A. Manuf. Confect. 1966, 46, 29–36. (8) Kleinert, J. In 15th PMCA Conference; Pennsylvania Manufacturing Confectioners’ Association: Hershey, 1961; pp 1-14. (9) Haylock, S. J.; Dodds, T. M. In Industrial Chocolate Manufacturing and Use; Beckett, S. T., Ed.; Blackie Academic & Professional: Oxford, 1999; pp 57-77. (10) Metin, S.; Hartel, R. W. J. Therm. Anal. 1996, 47, 1527–1544. (11) Timms, R. W. Confectionery Fat HandbooksProperties, Production and Applications; The Oily Press: London, 2003. (12) Lohman, M. H.; Hartel, R. W. J. Am. Oil Chem. Soc. 1994, 71, 267– 276. (13) Cook, L. R. Chocolate Production and Use; Harcourt Brace: New York, 1964. (14) Smith, K. W. In Structured and Modified Lipids; Gunstone, F. D., Ed.; Marcel Dekker Inc.: New York and Basel, 2001, pp 401-422.
Sonwai and Rousseau (15) Lipp, M.; Simoneau, C.; Ulberth, F.; Anklam, E.; Crews, C.; Brereton,
(16) (17) (18) (19) (20) (21) (22)
P.; de Greyt, W.; Schwack, W.; Wiedmaier, C. J. Food Compos. Anal. 2001, 14, 399–408. Wilson, E. Manuf. Confect. 1999, 11, 47–51. Stewart, I.; Kristott, J. Lipid Technol. 2004, 16, 11–14. Smith, P. S.; Dahlman, A. J. Am. Oil Chem. Soc. 2005, 82, 165– 168. Sonwai, S.; Rousseau, D. Eur. J. Lipid Sci. Technol. 2006, 108, 735– 745. Briones, V.; Aguilera, J. M. Food Res. Int. 2005, 38, 87–94. Jewell, G. ReV. Int. Choc. 1972, 27, 161–162. Hobbs, J. M.; Register, R. A. Macromolecules 2006, 39, 703–710.
(23) Morton, D. N.; Roberts, C. J.; Hey, M. J.; Mitchell, J. R.; Hipkiss, J.; Vercauteren, J. J. Food Sci. 2003, 68, 1411–1415. (24) Wille, R. L.; Lutton, E. S. J. Am. Oil Chem. Soc. 1966, 43, 491– 496. (25) Jensen, R. G.; Ferris, A. M.; Lammi-Keefe, C. J. J. Dairy Sci. 1991, 74, 3228–3243.
CG070503H
