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Department of Food Science, University of Wisconsin, 1605 Linden Drive, Madison 53706
Corresponding author: R. W. Hartel; e-mail: hartel{at}calshp.cals.wisc.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: milk powder • chocolate • milk fat • rheological property
Abbreviation key: AMF = anhydrous milk fat, HFW = powder made by drying cream and skim milk powder together, LSN = low-heat, spray-dried skim (nonfat) milk powder, LSW = low-heat, spray-dried whole milk powder, RDW = roller-dried whole milk powder, SMP = skim milk powder, WMP = whole milk powder
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Numerous factors, not just free fat level, affect the properties of chocolates made with milk powder addition. Table 1
summarizes the properties of milk powders that can potentially influence chocolate characteristics. Characteristics such as particle size and density, internal structure, color, and flavor all potentially can influence the processing conditions needed to make chocolate, the physical properties of the chocolates produced, and/or the sensory characteristics of the final product.
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There are several factors that impact the degree of free fat in a milk powder, with the processing conditions being key to developing a powder with high free fat. Roller-dried WMP has a characteristically high free fat level (60 to 90%), apparently due to the shearing and scraping action as the film dries on a thin surface and then is removed by the knives. Because of this, roller-dried WMP is ideal for use in milk chocolate (Dewettinck et al., 1996). Spray-dried WMP has significantly lower free fat levels (only 2 to 3%), and it does not perform as well in milk chocolate (Campbell and Pavlasek, 1987). Chocolate manufacturers that use spray-dried WMP in their formulation typically use slightly higher concentrations of cocoa butter to keep viscosity down in the desired range. Thus, there is a strong economic incentive for chocolate manufacturers to use the optimal dairy ingredients for chocolate manufacture.
Processing parameters during spray drying can impact the level of free fat to some extent, although this has not been explored at great depth. Twomey and Keogh (1998) suggest that free fat in spray-dried WMP may be increased by using smaller nozzles and higher nozzle pressures. Hansen and Hansen (1990) saw an effect on chocolate viscosity between WMP atomized from a nozzle at different pressure. Higher nozzle pressure gave lower viscosity, most likely due to the higher free fat content. Lower drying capacities (temperature difference between product and air) also led to higher chocolate viscosity (Hansen and Hansen, 1990) in direct correlation to the measured free fat content (higher free fat with lower temperature difference). Another important factor that affects free fat levels is the degree of lactose crystallinity (Haylock, 1995; Twomey and Keogh, 1998), since the crystalline lactose (as opposed to amorphous lactose) causes the milk fat to be expressed from the droplet. Some manufacturers mimic a high free milk fat powder by blending AMF with skim milk powder (SMP). This provides 100% free fat to the chocolate but does not have the desired flavor characteristics (perhaps due to the lack of heating of the AMF). Thus, this alternative is not widely used (Campbell and Pavlasek, 1987).
The effects of free fat content in milk powder on milk chocolate rheology have been studied to some extent (Campbell and Pavlasek, 1987; Haylock, 1995; Twomey and Keogh, 1998). What is less clear are the effects of these powder properties on further processing requirements and the ultimate product characteristics of milk chocolate. The different structures produced in milk chocolates made with different milk powders potentially lead to differences in tempering requirements, different physical characteristics of the finished chocolate, and differences in stability to bloom formation. Finally, the flavor of milk chocolate also can be influenced by choice of powder ingredient.
Tempering of chocolate involves crystallization of the cocoa butter into the proper number of crystals of small size and desired polymorph. The temperature-time profile during tempering is regulated, depending on the nature of the chocolate, to produce this desired crystalline structure. The addition of milk fat is widely known to result in lower tempering temperatures due to the inhibitory effect on crystallization of cocoa butter (Hartel, 1998). Thus, powders with different free fat levels require different tempering conditions to attain the same degree of crystallization.
Hardness of chocolate is governed by a combination of the crystallized lipid phase and the solid dispersed phase (sugar crystals, cocoa solids, and milk solids). Addition of milk fat causes a softening effect on cocoa butter and results in softer chocolates. Thus, higher free fat levels from milk powder ingredients would be expected to lead to slightly softer chocolates. The packing arrangement of the dispersed phases in chocolate may also determine the mechanical properties of the solidified product (factors such as hardness, snap, etc.). Markov and Tscheuschner (1989) and Tscheuchner and Markov (1989) documented the effects of various additives on the physical properties of chocolate. Heathcock (1985) shows electron micrographs of different structures of chocolate based on type of milk powder used in the formulation.
The formation of fat bloom in chocolate is governed by many factors (Hartel, 1998), some of which are also influenced by choice of milk powder source. The amount of milk fat available from the powder source may influence the inhibition effect of milk fat on bloom formation since, in general, the higher the milk fat level, the greater the level of bloom inhibition (Hartel, 1996). A study by Bricknell and Hartel (1998) has shown that the shape and nature of the dispersed phase (sugar particles) influences the rate of bloom formation in chocolate. The packing arrangement of the particles and/or the surface shape/characteristics affect the rate of bloom formation. Thus, it is likely that choice of milk powder also influences the rate of bloom formation in milk chocolates, although no work has been done to verify this hypothesis.
The objective of this work was to evaluate the use of various milk powders in milk chocolate. The effects of different milk powders on processing condition requirements, physical properties, and sensory characteristics of the final milk chocolate products were studied. Chocolate properties evaluated include rheological properties of the chocolate melt, temperature conditions needed to produce well-tempered chocolate, hardness, bloom stability, and sensory properties.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Milk Powder Properties
Certain physical characteristics of milk powders influence the properties of the chocolate made from them.
Fat content.
Total milk fat in each powder was determined by AOAC official method 932.06. Values ranged from 1.0% for LSN to 29.4% for RDW. The amount of fat easily extracted from a powder is often called "free" fat (Buma, 1971; Aguilar and Ziegler, 1994), since it is likely that this fat can interact with the matrix of the product to which that powder is applied. In this study, 2.5 g of milk powder was mixed with 40 ml of petroleum ether under gentle agitation for 1 h. After the extract was filtered, the solvent was evaporated and the amount of fat determined gravimetrically.
Lactose crystallinity.
A Phillips PW 1729 x-ray diffractometer (Almelo, The Netherlands) was used to quantify the extent of lactose crystallization in each powder. Mixtures of pure 
-lactose monohydrate and LSN (a completely amorphous powder) were used to generate a standard curve. A scan of 2
between 15 and 30° with a step size of 0.05° and scan time of 10 s was used. A characteristic peak for lactose at 19.6° was used for reference purposes to estimate the percentage of lactose crystallinity in the milk powder samples based on the standard curve.
Density.
Both apparent and true density of each milk powder were measured at room temperature. Apparent density (
a) was measured by the volume displacement of sunflower oil. True density (t) was measured by using a pycnometer (AccuPyc 1330, Micromeritics Equipment Corp., Norcross, GA) with He gas at 236 kPa. Mean value and standard deviation of 10 measurements were calculated. The vacuole volume, or the void space within the powder particle, was calculated from the two densities as
 | (1) |
Particle size and shape.
Photomicrographs of each powder were taken with a Nikon Optiphot optical microscope (Garden City, NY) after dispersion of the powder in mineral oil. At least 10 randomly selected fields of view were used to accumulate over 800 particles for each distribution. Particle size distribution, based on the equivalent circular diameter of the projected area, was obtained by performing image analysis with Optimas 6.1 (Bothell, WA) software and a custom program for generating distribution statistics. Note that this measurement method gives a number-based size distribution, whereas light scattering devices generally give a volume-based distribution. Thus, mean sizes reported here are lower than those typically reported for chocolate particle size measured by light scattering.
The characteristics of each milk powder used in this study are shown in Table 2.
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Refining.
A laboratory-scale, three-roll refiner (model 4 x 8, Day, Cincinnati, OH) was used to reduce particle size of the mixture of sugar, cocoa liquor, cocoa butter (only about 2/3 of the total cocoa butter was added at this step), and milk powder. The roller gaps were set at 100 and 30 µm, respectively. Two passes through the refiner were used to ensure good particle size reduction.
Conching.
A modified Hobart mixer (A-120T, 8 L, Troy, OH) was used to conch each chocolate. A plastic scraping blade was attached to the Hobart paddle mixer, and a heating mantle was placed around the Hobart bowl. The remainder of the cocoa butter and one-half of the lecithin were added at the beginning of the conch (60°C for 24 h). The remainder of the lecithin was added after about 6 h of conching.
Tempering and molding.
A cyclothermic tempering procedure (Kleinert, 1970) was used to temper the chocolates. This process involves two sequential cooling and heating steps to promote formation of stable cocoa butter crystals. The best temperature conditions for attaining a tempered chocolate must be determined to some extent by trial and error, although measurement of viscosity changes (torque on a stirrer at constant rpm) after each change in temperature takes some of the guesswork out of this procedure. The cyclothermic tempering procedure is useful for ensuring excellent chocolate temper even for chocolates with different tempering requirements. A custom-built temper meter was used to measure proper crystallization during tempering, based on the shapes of the cooling curves in the temper meter. Chocolates were poured into plastic molds (50 mm diameter and 5 mm depth) or aluminum molds (20 mm diameter and 40 mm height) for hardness measurements and cooled in a 5°C room with gentle air movement. Proper temper was further verified by the visual appearance of the solidified chocolate products. The chocolates had good glossy surface, were not bloomed or dulled, and had excellent snap upon breaking.
Chocolate Analysis
Several analytical tests were performed on the chocolates, including particle size, rheological properties, hardness, and bloom stability. In addition, a sensory study was done to assess the effect of these milk powders on milk chocolate characteristics.
Particle size.
There are numerous methods for measuring particle size in chocolate. Probably the most accepted method is laser light scattering. However, no suitable light scattering unit was available to the researchers at the time of this study, so an optical microscope technique was adopted. Melted chocolate was dispersed in mineral oil to create a dilute suspension of particles (sugar crystals, cocoa powder, and milk powder particles). A drop of this dispersion was placed on a microscope slide and observed with the Nikon Optophot microscope used for characterizing the milk powders. A magnification of 200x was selected to give a compromise between detecting the smaller particles (minimum detectable size was estimated at 0.6 µm) and not missing larger ones. Because most particles in chocolate are between 0.5 and 50 µm, the vast majority of particles are visualized under these conditions. Multiple frames (about 10) of each chocolate dispersion were analyzed to ensure random sampling and to minimize loss of larger particles due to the small frame size at this magnification. Automated image analysis using Optimas 6.1 software and custom-written software was performed to determine the particle size distribution based on the equivalent circular diameter of the projected area of each particle. At least 800 particles were counted for each distribution, and the population-based mean size, L1,0, was calculated along with the standard deviation of the distribution.
Melt rheology.
Rheological properties of the chocolate mass at 40°C were characterized by use of a Brookfield DV-1 HATD viscometer (Stoughton, MA) with a small sample adapter (SC4-13R) and spindle (SC4-21), according to the international guidelines (Office International du Cocao et du Chocolat, 1973). The chocolate mass was stabilized for 10 min in the temperature-controlled cup and presheared at 20 RPM for 5 min prior to measurement. Ascending (0.5 to 100 rpm) and descending (100 to 0.5 rpm) tests were performed for each sample. Torque readings at each shear rate were recorded after 30 s of shearing for duplicate samples. The data were analyzed according to the modified Casson model of fluid rheology (Steiner, 1958)
 | (2) |
where 
is shear stress (obtained from torque data) and 
is shear rate (obtained from rpm data). The two parameters used to fit the Casson model are c, the Casson yield value, and 
c, the Casson plastic viscosity.
Hardness.
The molded chocolates were analyzed for hardness by using a Texture Analyzer (model TA-XT2, Haslemere, England) at 20°C. Test samples were penetrated by a 2-mm stainless steel cylindrical probe at 0.2 mm/s to a depth of 5 mm. Maximum force for penetration was determined as well as the work required for penetration (area under the force curve).
Bloom stability.
Chocolate discs were stored either in a temperature-controlled cabinet with temperatures cycling between 19 and 29°C every 6 h to accelerate bloom formation or at room temperature. A Hunter color meter (Color QUEST, Hunter Associates, Reston, VA) was used to measure whiteness of the unmolded side of the chocolate disc (Bricknell and Hartel, 1998). An average of 4 readings on each disc (after 90° turn) was taken, and 8 different discs were used for each chocolate sample. Mean whiteness values and standard deviation are reported. A rough visual score of relative bloom formation was also used to evaluate each sample. Bloom level for each disk at each storage condition was rated on a scale from 0 (no bloom) to 5 (severe bloom) by the experimenter.
Sensory analysis.
A descriptive panel, conducted according to IFT protocols through the Sensory Laboratory in the Department of Food Science, was used to judge a variety of attributes of milk chocolates. An experienced panel (n = 35) rated each chocolate on a 7-point scale for 11 attributes. These included brown color intensity, rate of melt down, textural smoothness, chocolate flavor, rate of chocolate flavor release, milk flavor intensity, milk powder flavor intensity, butter flavor intensity, mouth coating sensation, off-flavor intensity, and overall acceptability. The ballots with coded values for the descriptive attributes from the panel session were subjected to analysis of variance by use of SAS (Cary, NC) statistical software package. For each sensory attribute, statistical analysis provided the mean scores for each sample, the F-value for all samples, and the least significant difference (LSD) for making sample comparisons. The LSD value computed for a 5% level of significance was used for comparison of the paired means.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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). Optical microscope images of the powders are seen in Figure 1. The free fat content of each milk powder varied from 0% for the spray-dried skim milk powder to 24.9% for the roller-dried whole milk powder. Initially, each of the milk powders, with the exception of LSN, contained 28.5% milk fat. Thus, different amounts of milk fat were easily removed from the powders with petroleum ether. Interestingly, the powder made by drying cream and skim milk powder together (HFW) had lower free fat content than the RDW. When used in chocolates, the "free" milk fat content in the chocolate varied from 3.9% for LSN (anhydrous milk fat added) to 0.25% for the spray-dried whole milk powder. The chocolates made with RDW and LSN (with 3.9% AMF addition) had approximately the same "free" milk fat content in the final chocolates. The values of free milk fat in the chocolate are only calculated and do not necessarily represent the actual amount of milk fat available to mix with the cocoa butter in chocolate. During refining and conching, for example, breakdown of particles may result in release of additional fat that may interact with the cocoa butter in the finished chocolate. The numbers reported here simply represent free milk fat based on the amount of milk fat easily extracted from the powders.
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Particle size distributions and shape of the particles in these milk powders also were different. The spray-dried skim milk powder had the smallest particles and narrowest distribution. The largest particle size, expressed as equivalent circular diameter for the projected area of each particle, was found in the roller-dried powder; however, the particles were flat, irregularly shaped, two-dimensional particles that were sized based on their largest area surface (Figure 1). This flat shape is likely to have different effects on the physical properties of chocolate, especially compared to the more uniform and spherical particles formed by spray drying. The powder made by drying cream and skim milk powder appeared as agglomerated particles of the original skim milk powder. Thus, the mean size of HFW was significantly higher than for the spray-dried powders.
Properties of Milk Chocolates
Particle size.
The particle size of the chocolate impacts both the economic aspects of chocolate as well as the sensory aspects. The increased surface area that results from the formation of many small particles leads to higher costs since more cocoa butter must be added to reduce viscosity. Smaller particle size, however, generally leads to a smoother chocolate, unless particle size is reduced too much. Too many particles of very small size (less than 1 to 2 µm) can lead to a sensation of greasiness or slipperiness in the final chocolate. Thus, fragmentation of the particles into fines during refining can lead to increased cost as well as to an undesirable sensory attribute.
The different physical aspects of the milk powders may lead to differences in fragmentation during refining. For example, a powder with higher vacuole volume may break into many smaller pieces than a more solid particle. An unusual shape, like the flat plates of the roller-dried sample, may also influence fragmentation during refining. Table 3 shows the particle size distribution statistics for each of the chocolates. In general, the size distributions of all chocolates were not dissimilar despite the differences in milk powder characteristics. This should not be surprising since the milk powder makes up only a small portion of the dispersed phase volume (along with sucrose and cocoa powder) and thus, does not have such a major effect on particle size distribution in the chocolate. One point of difference among the chocolates, however, is the relatively large size of the largest particles (up to 46 µm) in the chocolate made with the RDW. The particles of RDW were flat and large (Figure 1) and could easily slip through the roller gaps sideways without being fully broken down.
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c) represents the interactions among components of the fluid that impede motion when the fluid is at rest. Once the fluid is moving, the forces among the components of the fluid under shear conditions give rise to a viscosity value (c). Because chocolate has a dispersion with a high dispersed phase volume (55 to 65%), the particulate nature of a chocolate can have significant impact on the rheological properties. The size distribution of particles influences how easily they move across one another during shearing, as can the shape and surface characteristics. A decrease in mean size generally results in an increase in both plastic viscosity and yield value, as shown by Chevalley (1988). In addition, the dispersed phase volume itself is an important parameter that influences chocolate rheology. The presence of free fat (fat that is allowed to interact with the cocoa butter) in the milk powder, acts to effectively dilute the dispersed phase volume, leading to a reduction in both yield stress and plastic viscosity (Chevalley, 1988).
In this study, both Casson yield stress and plastic viscosity were influenced by the nature of the milk powders used to make the chocolates. Both yield stress and plastic viscosity decreased as the free fat in each powder increased (Figure 2). The chocolates with the lowest yield value and plastic viscosity were those that had the highest free fat level. In the case of LSN, 3.9% AMF was added to the formulation, which led to essentially all free milk fat for this chocolate. The chocolate made with the RDW had essentially the same level of free milk fat, based on the extraction method, which led to essentially the same yield stress and plastic viscosity. The chocolate made with LSW had essentially zero free milk fat, based on the negligible amount of milk fat easily extracted from the powder particles, and this chocolate had the highest yield value and plastic viscosity. A chocolate manufacturer would have to add additional cocoa butter to this chocolate to reduce viscosity to the desired specifications, a practice that would lead to higher costs.
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The approximate conditions needed for proper tempering of each chocolate (based on temper meter curves and visual observation of the finished chocolates) are shown in Table 4. Several trends among milk powders were observed, including the initial temperature for tempering and the torque required to maintain constant RPM during stirring. The chocolates with high free milk fat required slightly lower temperatures in the first step of tempering. The milk fat inhibited cocoa butter crystallization so lower temperatures were required in the first stage to promote the desired nucleation of cocoa butter. In addition, the chocolates made with roller-dried milk powder required lower torque (M) than the other chocolates to maintain constant RPM of the stirrer during tempering. Because torque is generally directly related to viscosity, it is surprising that the melt viscosity of this chocolate is not lower than the viscosity of the chocolate made with skim milk powder supplemented with AMF (LSN) with the same free fat level (Figure 2). However, as cocoa butter crystals form and interact with the solid particles, the rheological properties of the melt may change.
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Figure 3 shows that an increase in free milk fat content in chocolate generally led to a decrease in hardness, as might be expected based on the dilution of cocoa butter mentioned above. This result was seen for both the maximum force of penetration as well as the total penetration work (area under the force curve). However, the chocolate made with the spray-dried skim milk powder supplemented with AMF (LSN) had higher hardness than expected based on free milk fat content. Apparently, the nature of the milk powder particles also influences the hardness of chocolate, although the exact mechanism for this effect is not clear.
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Figure 4 shows the formation of bloom in milk chocolates, made with different milk powders, after storage for 9 wk with the temperature fluctuating from 19 to 29°C on a 6-h cycle. Figure 5 shows the results for visual observation of bloom formation under the same conditions. In both measurements, the chocolate made with LSN and supplemented with AMF was quite resistant to bloom formation, having essentially the same appearance (and little change in whiteness index; see Figure 6) as the original chocolates. The chocolate made with HFW also had good bloom stability, showing just dulling under these accelerated storage conditions. Although the whiteness index for the chocolate made with HFW increased after 3 wk of storage, after 9 wk it was back to a low value matching the visual observation. In contrast to these stable chocolates, the chocolates made with RDW and LSW had bloomed significantly (both in whiteness index and in visual evaluation) during this time period of storage.
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shows the development of bloom during storage of these chocolates at both cycling temperature conditions and room temperature. The chocolate made with LSN (with added AMF) had the least bloom under both storage conditions.
Sensory analysis.
An experienced sensory panel was used to evaluate 11 characteristics of the milk chocolates made with different milk powders. Results of the sensory analysis are shown in Table 5. Only slight (but still statistically significant at P < 0.5) differences were observed among the chocolate samples based on the properties of the milk powders used. There was no clear correlation between free milk fat levels and brown color intensity, meltdown rate, milk flavor, and milk powder flavor. Chocolate flavor, chocolate flavor release, and mouth coating sensation also were not influenced significantly by the different milk powder types. It is likely that the level of milk powder used in this study was too low to clearly demonstrate the sensory differences expected from the different milk powders, even though this level was sufficient to observe differences in physical properties.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Received for publication August 16, 2001. Accepted for publication July 24, 2003.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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