The use of atomic force microscopy to measure the formation and development of chocolate bloom in pralines

Atomic force microscopy (AFM) has been used to study the surface of chocolate as well as the progress of chocolate bloom over time. Fresh chocolate was found to be relatively smooth but with deep holes. These could be pipes leading deep down into the body of the chocolate, perhaps reaching the filling. After storage for a few weeks, we observed the growth of small drops around these holes. With increasing time, these drops became larger and more structured. After further storage, a crystalline structure and bloom were revealed. These results suggest that bloom growth in pralines is a two-phase process, with drops initially forming on the surface and then bloom crystals nucleating and growing from them. Further, we deduced pipes leading down into the center of the chocolate through which the migration of filling fats can preferentially occur.     

Effect of milk fat fractions on fat bloom in dark chocolate

Anhydrous milk fat was dissolved in acetone (1∶4 wt/vol) and progressively fractionated at 5°C increments from 25 to 0°C. Six solid fractions and one 0°C liquid fraction were obtained. Melting point, melting profile, solid fat content (SFC), fatty acid and triglyceride profiles were measured for each milk fat fraction (MFF). In general, there was a trend of decreased melting point, melting profile, SFC, long-chain saturated fatty acids and large acyl carbonnumbered triglycerides with decreasing fractionation temperature. The MFFs were then added to dark chocolate at 2% (w/w) addition level. In addition, two control chocolates were made, one with 2% (w/w) full milk fat and the other with 2% (w/w) additional cocoa butter. The chocolate samples were evaluated for degree of temper, hardness and fat bloom. Fat bloom was induced with continuous temperature cycling between 26.7 and 15.7°C at 6-h intervals and monitored with a colorimeter. Chocolate hardness results showed softer chocolates with the 10°C solid fraction and low-melting fractions, and harder chocolates with high-melting fractions. Accelerated bloom tests indicated that the 10°C solid MFF and higher-melting fractions (25 to 15°C solid fractions) inhibited bloom, while the lowermelting MFFs (5 and 0°C solid fractions and 0°C liquid fraction) induced bloom compared to the control chocolates.     

Prediction of migration fat bloom on chocolate

The objective of this study was to investigate whether it is possible to predict migration fat bloom based on measurements shortly after production. At different storage times shortly after production (0, 1, 4 h), the chocolate batches, varying in tempering method, tempering degree and amount of added butter oil, were evaluated by DSC, pNMR and texture analysis. Discriminant analysis and principal component analysis were combined to investigate the potential towards prediction. The batches were classified into groups depending on the time when white spots appeared (<8 wk, 8–13 wk, >13 wk). A good separation (100% correct classifications, 100% using cross‐validation) was obtained using the afore‐mentioned analyses and storage times. It was also shown that it is possible to exclude DSC analyses or analyses at 0 h storage time without compromising the classification performances too drastically. The study further elucidated that the tempering method has no significant effect on visual fat bloom development. Furthermore, undertempered chocolates bloomed quicker than well‐tempered ones, while fat bloom was delayed on overtempered chocolates. Addition of 6% butter oil promoted fat bloom development, while no significant difference was detected between chocolate with no added butter oil and chocolate with 3% butter oil added.     

Effects of minor lipids on crystallization of milk fat-cocoa butter blends and bloom formation in chocolate

Minor lipids, such as diacylglycerols, monoacylglycerols, cholesterol, and phospholipids play a key role in crystallization of fats. In this study, the effects of minor lipid components on crystallization of blends of cocoa butter (CB) with 10% milk fat or milk-fat fractions, and on bloom formation of chocolate were investigated. Both removing the minor lipids from milk fat and doubling the level of minor lipids from milk fat resulted in longer nucleation onset time, slower crystallization rate, and rapid bloom development in chocolate. Removal of minor lipids resulted in the formation of irregular primary and secondary crystals with inclusions of liquid fat, whereas the crystals were spherical and uniform in shape in the presence of minor lipids. Minor lipids from milk fat, even at the low concentrations typically found in nature, affected the crystallization of milk fat-CB blends, impacted the chocolate microstructure, and affected bloom development in chocolate.     

Fat bloom in chocolate and compound coatings

Although bloom in chocolates and compound coatings has been studied for many decades, the specific mechanisms of fat bloom still remain largely unknown. Furthermore, it is generally considered that the mechanisms for fat bloom formation in chocolate are different than those for compound coatings. After a brief review of chocolates and compound coatings, we summarize past studies on fat bloom formation in both products. A comparison of the effects of various parameters on bloom formation, either as accelerators or inhibitors, provides insight into the similarities and differences in these phenomena. Based on this analysis, a global view of the mechanisms of bloom formation in both chocolates and compound coatings is suggested.     

Influence of thermal conditions and presence of additives on fat bloom in chocolate

This study focused on the analysis of the effects of thermal history and presence of additives on fat bloom in chocolate. Magnetic resonance data obtained on specific chocolate samples were useful in evaluating the effect of thermal history on the appearance of fat bloom and also in understanding the underlying mechanism. Fat bloom was induced by thermal history such as storage for 3 d at 32 or 28°C. Increasing storage time at 21°C after each thermal treatment also promoted fat bloom. Differential scanning calorimetry experiments confirmed the appearance of polymorphic form VI in the bloomed samples. Also, three different components were added separately to the initial composition of dark chocolate and the appearance of fat bloom was monitored. 1,3-Dibehenoyl, 2-oleoylglycerol (BOB) did prevent fat bloom. However, magnetic resonance and differential scanning calorimetry data collected on chocolate samples did not confirm that either added sucrose or milk powder prevented fat bloom. Indeed, fat bloom occurred for these samples, but its characteristics were different from those with pure chocolate.     

Relation of fat bloom in chocolate to polymorphic transition of cocoa butter

A special chocolate with spray-dried sugar (50:50 w/w sucrose/20 Dextrose Equivalent corn syrup solids) was made to study the polymorphic changes in cocoa butter crystals using X-ray diffraction. Anhydrous milk fat (AMF) and high-, middle-, and low-melting milk fat fractions were used to replace 2% (w/w) of cocoa butter. Chocolates were tempered, and the consistency of temper among chocolate samples was verified by a temper meter. Chocolates were cycled between 19 and 29°C at 6-h intervals to induce fat bloom. The special chocolates were analyzed by X-ray spectroscopy and colormeter. X-ray analysis on the special chocolates showed polymorphic transition from the βV to the βVI form of cocoa butter. After a lag phase, the percentage of the βVI form rapidly increased. However, the sample made with the high-melting milk fat fraction transformed slowly to βVI. Visual bloom appeared rapidly on the special chocolates made with AMF, middle- and low-melting fractions, whereas visual bloom was very slow to appear on the special chocolates made with high-melting milk fat fraction and on the cocoa butter control. The commercial chocolate responded consistently; the control bloomed rapidly, the AMF exhibited some bloom resistance, and the high-melting fraction inhibited bloom. Despite the βV to βVI transition, the control chocolates with amorphous sugar did not bloom. Since the only difference in the chocolates was sugar microstructure, differences in bloom formation were caused by the microstructure, not the polymorphic transition.     

Growth process of homogeneously and heterogeneously nucleated spherulites as observed by atomic force microscopy

A poly(bisphenol A octane ether) (BA-C8) was synthesized. The isothermal spherulitic growth process was studied in situ using atomic force microscopy (AFM) at room temperature. For spherulites formed by homogeneous nucleation, the growth process includes the birth of a primary nucleus, the development of a founding lamella and the growth of the founding lamella into a spherulite. An embryo below a critical size is unstable. A stable embryo grows into a founding lamella. There is only one founding lamella in each spherulite. All other lamellae originate from this founding lamella. Two eyes can be seen at the center of a spherulite. For spherulites formed through heterogeneous nucleation, many lamellae grow at the nucleus surface and propagate outward radially. The spherulites acquire spherical symmetry at the early stage of crystallization. No eyes are found for this kind of spherulites.     

Surface morphology and nodule formation mechanism of cellulose acetate membranes by atomic force microscopy

By the use of atomic force microscopy (AFM), formation mechanism of nodular structure in cellulose acetate membranes was systematically investigated. Elementary factors affecting the nodule formation were delineated on the basis of both kinetic and thermodynamic considerations. It was shown that (1) the exact nature of nodular structure is thermodynamic equilibrium glassy state; nodular structure will vanish in the rubbery state; (2) the thermodynamic factor affecting nodule formation is the membrane formation temperature; with the membrane formation temperature decreasing, more chain segments are able to form nodular structures; (3) nodule formation is dependent on the segment rearrangement; variation of the solvent environment is the major kinetic factor affecting the segment rearrangement and nodule formation. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1328–1335, 2003     

Direct observation of the main cell wall components of straw by atomic force microscopy

Cellulose, hemicellulose, and lignin are the main cell wall components of straw. After removal of the wax and the major portion of lignin, the remaining components of the cell wall surface of straw were determined by atomic force microscopy, which revealed a network structure of cellulose and hemicellulose, and some lignin localized on the surface of the network, consistent with the cell wall model suggested by other researchers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2055–2059, 2003     

Crystal growth of faujasite observed by atomic force microscopy

Atomic force microscopy (AFM) was employed to reveal the crystal growth mechanism of faujasite. The seeded growth of faujasite in dilute aluminosilicate solutions was observed. Two solutions were prepared: one was near equilibrium with the seed and the other was in growth mode for the seed. Morphological changes during the seeded growth were observed along with the growth period at the same position on the seed (ex situ observation). These observations showed the rough surface of the seed changing into a well-ordered (1 1 1) face in the solution that was near equilibrium with the seed. This surface ordering proceeded by thermodynamic stabilization of the top-surface structure via the mutual transfer of aluminosilicate species between the solution and solid phases, and/or by the dissolution of the amorphous matter on the seed. In growth mode, most of the top surfaces of the seeded crystals were terminated by double six-membered rings (D6Rs), while some were by complete or incomplete sodalite cage. These results showed that aluminosilicate species equal to or smaller than 6R contributed to the crystal growth.     

Quasi-static and hydrodynamic interaction between solid surfaces in polyisoprene studied by atomic force microscopy

Forces across polymer melts are poorly understood despite their importance for adhesion and the structure of composite materials. Using an atomic force microscope (AFM) this interaction was measured for 1,4-polyisoprene (PI, M_W = 1.9-10.2 kDa). Weak repulsive forces which decayed with characteristic decay lengths of 0.4-1 nm were observed on silicon wafers, HOPG, and mica. This indicates that, unlike poly(dimethylsiloxane) (PDMS), PI does not form an immobilized layer. The results confirm theoretical predictions that no long-range force exist across polymer melts in thermodynamic equilibrium. In addition to quasi-static experiments (with microfabricated silicon nitride tips at low approaching velocity), hydrodynamic experiments (with attached glass microspheres as probes at high approaching velocity) were carried out with PI and PDMS (M_W = 5.9, 8.0, 18.8 kDa). In some cases slip was observed. Slip was correlated with the quasi-static forces: weak quasi-static forces (observed with PI and short-chain PDMS) were correlated with slip in hydrodynamic experiments, while strong repulsive forces observed with long-chain PDMS are correlated with the absence of slip.     

A study of adhesion forces by atomic force microscopy

An atomic force microscope (AFM) was used to investigate Si₃N₄ tip interactions with various materials in four different liquid media (water, ethanol, ethylene glycol, and formamide). The adhesion forces calculated using surface energies and the values measured experimentally were compared. For all materials, the calculated adhesion force closely correlated with AFM measurements, except in water. In the case of water, the AFM experiments showed strong adhesion, whereas theoretically (van Oss-Chaudhury-Good model) repulsion is predicted. The difference observed is discussed in terms of the chemical interactions between Si₃N₄ and water.     

Surface roughness during storage of chocolate: Fractal analysis and possible mechanisms

Laser scanning microscopy and fractal analysis were used to determine roughness in the surface of chocolate samples stored under cycling temperature conditions (16 to 26°C) for 24 d. The four samples varied in the source of fat: 100% cocoa butter (CB), lauric and nonlauric fat replacers, and CB with 2% of nonlauric fat replacer. The response variable was the area-scale fractal complexity (Asfc), equivalent to a fractal dimension. Asfc increased with time to an asymptotic value (AV) in much the same way as whiteness index, both being accepted proxies of surface bloom. Images produced from topographical data revealed clearly the increase in roughness. Chocolate samples prepared with CB replacers exhibited an induction period and a slower rate of change in surface roughness than chocolate containing only CB. A linear relationship between a normalized roughness and the square root of time was followed by CB chocolate samples for the period before reaching AV. This result suggests that either diffusion or capillary flow may be the mechanism involved in fat migration to the surface.     

Cellulosic nanocomposites. II. Studies by atomic force microscopy

Dissolving‐grade wood pulp fibers were partially esterified by mixed p‐toluene sulfonic/hexanoic acid anhydride in a nonswelling suspending agent. A biphasic morphology was revealed by atomic force microscopy (AFM) for the compression‐molded, partially modified pulp fibers. The AFM phase images indicated distinct periodicity on the scale of several 10's of nanometers. Surface etching with cellulolytic enzymes of the modified pulp fibers produced height images that had virtually the same periodicity. These results indicate that the modified pulp fibers are nanocomposites comprising unmodified cellulose and cellulose hexanoate. Regenerated lyocell fibers (from N,MMNO solvent) subjected to the same esterification system as applied to pulp fibers, by contrast, exhibited AFM phase images that indicated a high level of surface (skin) versus core reactivity. Modified lyocell fibers with an average diameter of about 12 μm and having an overall DS of 0.6 had surface layers that were approximately 1 μm thick. The latter represented a transitional phase in which the chemical composition and the physical properties were intermediate between a highly substituted surface (skin) and an unsubstituted core. When a compression‐molded sheet of the modified lyocell fibers was analyzed by microthermal analysis, the thermoplastic matrix on the lyocell fiber surface was revealed to have an apparent T_g of 75°C corresponding to cellulose hexanoate, whereas no significant thermal transition was determined for the (unmodified) fiber core. These results suggest that both partially modified lyocell fibers and partially modified pulp fibers are capable of producing composites with morphologies that have grossly different scales. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2254–2261, 2000     

Adhesion force measurement of a DPI size pharmaceutical particle by colloid probe atomic force microscopy

The method to determine the adhesion characteristics of fine drug particles for dry powder inhalation (DPI) was established using a colloid probe which mounted a 1-3 μm drug particle on a commercial atomic force microscope (AFM) cantilever. A new preparation system of colloid probes for fine particles smaller than 2.5 μm in diameter was developed with the aid of a micromanipulator and a video microscope. Using this colloid probe, adhesion force distribution between a spherical polycrystalline drug particle and a plate of lactose monohydrate representing for DPI carrier materials or stainless steel for device wall materials was measured. Atmospheric humidity as well as the material and surface roughness of a target plate affected the determined adhesion force. With increasing surface roughness of a lactose plate, the adhesion force between a drug particle and the plate distributed more widely and their mean value decreased. Adhesion force increased meaningfully with atmospheric humidity. Adhesion force for stainless steel was higher than that for lactose.     

Real time observation of crystallization in polyethylene oxide with video rate atomic force microscopy

Video rate atomic force microscopy (VideoAFM), with a frame rate of 14 frames/s and a tip velocity of up to 15 cms⁻¹, is used to image polyethylene oxide films during crystal growth. The capabilities of VideoAFM when applied to semicrystalline polymer surfaces are explored. Image quality comparable to that found with conventional contact AFM is achieved but with a nearly 1000 times improvement in time resolution. By applying the technique to the real-time observation of crystal growth, different modes of rapid crystallization are followed in real time. Observation of the spherulite growth front allows measurement of growth rates at the lamellar scale, from which a factor of two difference in the rate of radial growth to the rate of tangential growth is observed, confirming that the elongated nature of spherulite lamellae is due to geometric constraints rather than an inherent fibrillar character. Measurements on screw dislocation growth, when large amounts of crystallizable material is trapped at the surface show that the terrace height does not influence the rate of crystal growth, confirming that under these conditions processes at the lamellar growth front control the rate of growth. When only a thin film of molten material is left on the surface of the already crystallized film dendritic growth is observed, implying a diffusion controlled process under these far from equilibrium conditions.     

The nanoscale phase distinguishing of PCL-PB-PCL blended in epoxy resin by tapping mode atomic force microscopy

In this work, we investigated the bulk phase distinguishing of the poly(ε-caprolactone)-polybutadiene-poly(ε-caprolactone) (PCL-PB-PCL) triblock copolymer blended in epoxy resin by tapping mode atomic force microscopy (TM-AFM). We found that at a set-point amplitude ratio (r_sp) less than or equal to 0.85, a clear phase contrast could be obtained using a probe with a force constant of 40 N/m. When r_sp was decreased to 0.1 or less, the measured size of the PB-rich domain relatively shrank; however, the height images of the PB-rich domain would take reverse (translating from the original light to dark) at r_sp = 0.85. Force-probe measurements were carried out on the phase-separated regions by TM-AFM. According to the phase shift angle vs. r_sp curve, it could be concluded that the different force exerting on the epoxy matrix or on the PB-rich domain might result in the height and phase image reversion. Furthermore, the indentation depth vs. r_sp plot showed that with large tapping force (lower r_sp), the indentation depth for the PB-rich domain was nearly identical for the epoxy resin matrix.     

Morphology of extruded high-density polyethylene pipes studied by atomic force microscopy

Atomic force microscopy (AFM) was used to study the structure of extruded polyethylene (PE) pipe. During extrusion, the outer surface of the pipe was cooled with water. Two cross sections, parallel and transverse to the extrusion direction, were examined in order to spatially follow the structural development during extrusion. The morphology revealed was spherulitic, and the spherulites had a mostly banded appearance when viewed under the AFM. We were not able to distinguish an oriented skin layer at the surface of the pipe, either by AFM or polarizing microscopy. The changes in the pipe's structure resulting from the cooling conditions were found to be rather gradual, and no clearly defined zones were observed. A slight orientation towards the extrusion direction was detected only in the area of the pipe crystallized under the lowest degree of undercooling. Measured spherulitic size, band period, and lamellae thickness showed a gradual increase in their values from the cooled to the noncooled surface of the pipe. Transmission electron microscopy (TEM) was used to verify the band period and lamellae thickness measurements done by AFM. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 515–523, 1997     

Photodegradation of lauric acid at an anatase single crystal surface studied by atomic force microscopy

The photodegradation of lauric acid at an anatase single crystal surface was visualized using atomic force microscopy (AFM). Photooxidation was performed for lauric acid thin films with thickness about 80–90 nm to simulate more realistic processing conditions rather than using submonolayer films. It was noticed that lauric acid deposited by spin coating technique formed domain structure at the TiO₂ surface. The phenomenon of domain surface decrease without change in the film thickness was observed. This suggests that only molecules at the crystal–air–lauric acid contact line and extended therefrom were degraded.