Impact of fat content during grinding on particle size distribution and flow properties of milk chocolate

Consumer acceptance depends on taste and mouth feeling. Processing can generate varying particle size distributions (PSD), which again influence flow parameters. Chocolate can be produced by roller refining and conching or alternatively by ball milling. Fat content during milling is an important parameter of both processes. The aim of this project was to elucidate relationship between this processing parameter, PSD and flow properties, so they can be controlled. Milk chocolate samples were produced in pilot scale and measured. The results showed that varying fat contents influences PSD for both processing types. Higher fat contents resulted in bimodal distributions for roller refining. Small particles fill voids between big ones, improve packing density and liberate fat for flowing. Nevertheless, fat content during refining should not be too high either, as downstream conching requires low initial content. Pre-dried raw materials were used for ball milling, since this process offers few possibilities to evaporate water and undesired volatiles, unlike conventional conching. Samples showed narrower, multi-modal PSDs. The lack of fine particles decreases packing density and results in higher viscosity at medium and high shear rates. Viscosity at low shear can be less than for roller refined products, which is a consequence of reduced specific surface. Unlike roller refiners, the PSDs became broader at lower fat contents. Almost bimodal PSDs were achieved in lab scale at very low fat contents. Under industrial conditions, the mass must be pumpable for milling. This is a restriction for influencing properties via PSD and needs further optimization.     

The functionality of milk powder and its relationship to chocolate mass processing, in particular the effect of milk powder manufacturing and composition on the physical properties of chocolate masses

Summary The functionality of twelve different milk powders that are used for chocolate mass processing was investigated. In two types of spray‐dried and one type of roller‐dried powder, the milk fat and milk fat fractions were integrated. Depending on the production process, the amount of free fat available in the milk powders varied greatly. A good correlation was found between the free fat content of the milk powder and the viscosity of the chocolate mass when comparable particle sizes were used. This study reports on the development of spray‐dried milk powders, which when used in chocolate processing produced low viscosities, comparable with those obtained by using roller‐dried milk powder. Calorimetric analysis showed that the shape of the milk powder particles has no influence on the calorimetric qualities of chocolate masses. Only when milk fat was added in a free form, was a higher ‘mixing effect’ in the crystallization peak of cocoa butter and milk fat observed.     

In-situ lecithination of dairy powders in spray-drying for confectionery applications

Powders are essential ingredients of chocolate. In particular for milk chocolate milk and whey powders are important, together with sucrose, lactose and cocoa solids. During processing to maintain a good flow of the molten chocolate mass, particles with hydrophilic surfaces, such as dairy powders and sugars, are coated with a surface-active compound. Only lecithin and polyglycerol polyricinoleate (PGPR) (at a limited level) are allowed in chocolate, and as these are expensive as little as possible is added, whilst maintaining rheological properties. Conventionally, lecithin is added during conching, and through the intense kneading of the chocolate mass it is distributed throughout the mass. Usually about 0.5% is added, although the level depends upon the composition of the chocolate. Here we present a new approach to lecithination of spray-dried milk and lactose powders, which we call in-situ lecithination. It has been found that the surface of a spray-dried powder is dominated by any surface-active species, and in a competitive situation, the most rapidly adsorbing species dominates. This behaviour is utilised when lecithin is added to the spray-dryer feed, and through the competitive adsorption of surface-active agents during the drying process, it dominates the powder surface composition as measured by X-ray photoelectron spectroscopy (XPS). This is also seen in differences in sedimentation rate when the powders are mixed with cocoa butter to assess the rheological properties of the powder dispersions. The effect was large for lactose powders, but smaller for skim milk powder and whey powder.     

Improving particle size distribution and flow properties of milk chocolate produced by ball mill and blending

Chocolate mass is produced by roller refining and conching or alternatively by ball milling. Grinding can generate varying particle size distributions (PSD), which again influence flow parameters. The latter are very important for downstream processing as well as mouthfeel and consumer acceptance of the final product. The objective of the work was to influence and control PSD and flow properties of ball mill chocolate. Various milk chocolate samples were produced in laboratory and pilot scale, and their PSD and flow properties were measured. First, it was tried to pre-grind milk and cocoa particles to very small size and then to produce coarser sugar particles in order to achieve a bimodal PSD. This should increase package density, which makes more fat available for the flow process. The attempt did not work very well for various practical reasons. Secondly, when instead fine and coarse fractions of the same particle composition were blended, it was possible to achieve bimodal PSDs as they are known from roller-refined products. The lowest values for flow parameters were achieved with a mix of 63 % coarse and 37 % fine particles as predicted in the literature. Thirdly, it was tested to blend roller-refined and ball-milled samples. It was possible to use one-third of the latter without considerably changing flow properties. This opens up various practical possibilities for the industry, e.g. line extension and productivity increase with relatively little effort and investment.     

Viscosity of Molten Milk Chocolate with Lactose from Spray-Dried Whole-Milk Powders

Dry whole-milk powders containing 0, 30, 50 and 70% nominal lactose prepared by spray-drying alone, or followed by post-drying crystallization, were incorporated into milk chocolate to give 0–50% substitution of lactose for sucrose. Increasing the concentration of amorphous lactose from spray-dried powders in the chocolates decreased viscosity, increased particle size of refined chocolate mass, and lowered the concentration of surface-active agents at which a minimum in Casson yield value was observed. Increasing the concentration of crystalline lactose from milk powders in the chocolates increased viscosity, decreased particle size, and increased the concentration of surface-active agents at which a minimum in Casson yield value was observed. Conditions which affect lactose crystallinity in milk powders, such as improper storage and handling prior to use in chocolate production, could be responsible for variations in chocolate viscosity noted sometimes by processors.     

Improvement of process parameters and evaluation of milk chocolates made by the new coarse conching process

Chocolate mass is produced by roller refining and downstream conching or alternatively by ball milling. The latter requires high fat contents; water and off-flavours cannot be volatilized as in classical dry conching. To overcome this, coarse conching is performed first, applying the high shear vortex chamber built into the kneading zone. It does not only intensifies energy and mass transfer, but also reduces particle size of crystal sugar. Finally, grinding is done downstream. Only some examples from the extensive initial development phase are shown, e.g. on the behaviour of skim and whole milk powder. During process optimization, three variables were combined with two full factorial experimental designs carried out using differently scaled ball mills. The resulting flow properties of the samples were combined in model equations. All results showed evidence that during grinding, the lecithin must be added as late as possible in order to achieve low viscosities over the entire flow curve. Using smaller grinding balls resulted in higher yield values, but lower viscosities at higher shear. Intensive extra shear after industrial-scale milling had little impact, but did resulted in lower viscosities at high shear after using the pilot-scale mill. After finding optimized settings, confirmation trials were run allowing reduced energy consumption by the vortex chamber. Evaluation of sensory perception showed cocoa taste to be more intense in samples from the coarse conching process when compared to the conventional process. Finally, the results allowed line set-ups for small scale as well as for continuous large-scale production.     

Pre-dried refiner flakes allow very short or even continuous conching of milk chocolate

Chocolate conching is usually still performed in a time consuming batch process. For milk chocolate, its main purpose is to remove water and to adjust flow properties. As the conch is not an efficient dryer and flow properties are mainly influenced by shear treatment, the objective of this study was to separate and optimise both processes. Prior to liquefaction, refiner flakes were dried to <0.6% of water in an oven, which allows to omit the dry phase of conching. Conventional conching of 5 h was then reduced to a liquefaction process taking 20 min. Alternatively, continuous processing was tested applying a high shear mixer REFLECTOR^®. Sample properties were very close to a standard. Only continuous liquefaction at very high throughput resulted in slightly increased viscosity. Sensory analyses just found negligible differences. Thus neither pre-drying nor short or continuous liquefaction had a negative impact on product quality.     

Sensory Characteristics of Milk Chocolate with Lactose from Spray-Dried Milk Powder

Dry whole-milk powders with 0-70% nominal lactose prepared by spray-drying alone (containing amorphous lactose) or combined with post-drying crystallization (containing crystalline lactose) were incorporated into milk chocolate to give 0-50% substitution of lactose for sucrose. No significant differences occurred in sweet, bitter, and thickness of melt (P > 0.10) based on milk powder preparation method or lactose concentration. As lactose increased, hardness and onset of melt increased regardless of milk powder preparation method; but, chocolates containing crystalline lactose were softer than those containing amorphous lactose. Higher chocolate, milk and caramel flavors were perceived in milk chocolates containing crystalline lactose. Graininess of milk chocolate increased at the highest level of added lactose.     

巧克力专用乳粉

研究了不同乳粉的颗粒大小和结构、游离脂肪含量对巧克力浆料粘度的关系。滚筒乳粉与喷雾乳粉之对比，滚筒干燥的全乳粉具有下列优点；游离脂肪含量高，可减少可脂的用量，赋与巧克力以更好的风味；可降低巧克力浆料的粘度。乳粉中羟甲基糠醛含量可增进巧克力风味、口感，提高喷雾乳粉中游离脂肪含量改进的工艺。     

Influence of formulation and processing variables on ball mill refining of milk chocolate

The influence of both formulation and processing variables on the physical characteristics of milk chocolate has been studied during milling with a pilot ball mill. Nine experimental millings were carried out varying the type of dried milk used (roller, high free fat and spray-dried milk) and the time of lecithin and dried milk addition during the refining process. For the samples collected during 2 h of milling, particle size, moisture, Casson plastic viscosity and yield value were evaluated. In all the experiments, the solid particle size was effectively reduced during the first 120 min of refining; after this time the particle size reduction assumed an asymptotic trend. The addition of lecithin at the beginning of the refining kept the viscosity low and constant throughout the process, thus allowing the addition of milk from the start of the process, regardless of the technology adopted for milk drying.     

Thickening of molten white chocolates during storage

When white chocolates are kept molten in storage tanks, problems can arise due to uncontrolled thickening and solidifying of the chocolate mass. The thickening of molten white chocolate was simulated on a laboratory scale using a rotational rheometer under static conditions, interrupted by short shear periods to measure the increasing viscosity. Several chocolates having different dairy components and fat contents were investigated for their tendency to thicken. In addition, sorption isotherms for white chocolates were obtained using Dynamic Vapour Sorption at different temperatures. The sorption isotherms showed the presence of amorphous lactose in all the chocolates that were manufactured from milk powders. Moisture that is released during the crystallisation of amorphous lactose causes stickiness and agglomeration of the neighbouring particles and starts the thickening process. This process is highly temperature-dependent. On elevating the temperature the lactose crystallisation occurs at lower relative humidities. In order to reduce the tendency of white chocolate to thicken, a high free fat level should be maintained, based on a high total fat content and on the use of high free fat milk powders, preferably roller-dried whole milk powders or the combination of skimmed milk powder and anhydrous milk fat.     

Thickening of molten white chocolates during storage

When white chocolates are kept molten in storage tanks, problems can arise due to uncontrolled thickening and solidifying of the chocolate mass. The thickening of molten white chocolate was simulated on a laboratory scale using a rotational rheometer under static conditions, interrupted by short shear periods to measure the increasing viscosity. Several chocolates having different dairy components and fat contents were investigated for their tendency to thicken. In addition, sorption isotherms for white chocolates were obtained using dynamic vapour sorption at different temperatures. The sorption isotherms showed the presence of amorphous lactose in all the chocolates that were manufactured from milk powders. Moisture that is released during the crystallization of amorphous lactose causes stickiness and agglomeration of the neighbouring particles and starts the thickening process. This process is highly temperature-dependent. On elevating the temperature the lactose crystallization occurs at lower relative humidities. In order to reduce the tendency of white chocolate to thicken, a high free-fat level should be maintained, based on a high total fat content and on the use of high free-fat milk powders, preferably roller-dried whole milk powders or the combination of skimmed milk powder and anhydrous milk fat.     

Fast conching for milk chocolate

The objective of the study was to shorten the conventional conching process for milk chocolate, which still takes many hours, in order to increase productivity. Due to recent findings and to progress made in raw material treatment, a hypothesis was proposed: that flavour development in the conch could be less important for milk chocolate than it certainly is for dark chocolate. Other functions of conching, like water removal and fat covering of particles, could be achieved faster by using drier raw materials and by machines applying more shear force. For the experiments, a conventional process (taking 5 hours) was condensed into 30–90 minutes. In screening trials various flake pre-drying techniques were also tested. Samples produced were analysed for particle size and flow properties, as well as by a trained sensory panel. Results from the experimental design showed best results at 60–90 minutes conching time. Probably the procedures for flake pre-drying had little or no influence under the conditions used. Verification trials compared the fast conching procedure to the standard process using two different recipes. No relevant differences in physical or sensory properties were found between samples from standard and 90-minute-fast-conching, proving that under favourable conditions conching times for milk chocolates can be drastically reduced without negative impact on product properties and perception.     

Optimisation of processing conditions and rheological properties using stephan mixer as conche in small‐scale chocolate processing

This study investigated the suitability of Stephan mixer for use as conche during small‐scale chocolate production in an attempt to reduce cost and processing time. Molten chocolate was processed using the Stephan mixer under different temperature and time combinations and Buhler Elk'Olino conche (as reference), and the flow properties (Casson plastic viscosity, Casson yield stress and thixotropy) were studied using a shear‐rate‐controlled rheometer. Optimum settings of 65 °C for 10 min at 443  g dry conching followed by 50 °C for 15 min at 443  g wet conching in the Stephan mixer resulted in similar flow properties as the reference sample. Increasing the blade rotary speed for both dry and wet conching resulted in a decrease in flow properties. The Stephan mixer proved to be suitable for conching small‐scale (approximately 1 kg) chocolate productions and could be explored as a fast and cost‐effective method for small‐scale chocolate conching processes.     

Effects of selected properties of ultrafiltered spray-dried milk powders on some properties of chocolate

The effects of selected properties of spray-dried milk fat powders on chocolate were determined. Milk powders produced from control or ultrafiltered (UF) milks with various levels of fat were blended with skim milk powder to give a standard 26 g fat 100 g⁻¹ powder. Particle size of the chocolate mixes after refining decreased as the fat content and free-fat content of the powders increased. Despite this, increasing fat and free-fat contents of powders reduced the Casson viscosity of the subsequent molten chocolates. Casson viscosities using powders from control or UF milks were similar, but decreased as the particle size of powders increased and particle size after refining the chocolate mix decreased. Casson yield value and hardness decreased as fat content of powders increased. Casson yield value increased with vacuole volume of powders. It is possible to alter important properties of chocolates using milk powders of varying fat contents, free-fat contents and particle sizes.     

Effects of milk powders in milk chocolate

The physical characteristics of milk powders used in chocolate can have significant impact on the processing conditions needed to make that chocolate and the physical and organoleptic properties of the finished product. Four milk powders with different particle characteristics (size, shape, density) and "free" milk fat levels (easily extracted with organic solvent) were evaluated for their effect on the processing conditions and characteristics of chocolates in which they were used. Many aspects of chocolate manufacture and storage (tempering conditions, melt rheology, hardness, bloom stability) were dependent on the level of free milk fat in the milk powder. However, particle characteristics of the milk powder also influenced the physical and sensory properties of the final products.     

GRINDING SPRAY-DRIED MILK POWDER NEAR the GLASS TRANSITION TEMPERATURE

The fine grinding of chocolate is typically accomplished on five‐roll mills. Chocolate manufacturers consider milk powder, a component of milk chocolate, difficult to grind. Spray‐dried milk powders comprise a glassy lactose matrix in which fat globules, air vacuoles and protein are entrapped. the glassy‐rubbery transition in commercial milk powders usually lies between 60–70C, depending on the moisture content. A mixture of 60% wt/wt commercial whole milk powder, T_g～ 60C, and 40% wt/wt cocoa butter was ground in a three‐roll refiner at temperatures of 40, 50, 60, 70 and 75C. Below T_g the particles exhibited brittle fracture, while above T_g plastic deformation was evident and particles became highly asymmetric. the amount of fat liberated from the lactose matrix, so‐called free fat, particle density, and mean particle size increased with grinding temperature. However, the Casson yield value and plastic viscosity of finished “white chocolate” coatings, manufactured to a constant free fat content, increased with grinding temperature, suggesting an influence of particle shape on flow behavior.     

The effect of high-pressure jet processing on cocoa stability in chocolate milk

Fat-free chocolate milk formulations containing skim milk, cocoa powder, and sugar were thermally treated and then processed using high-pressure jet (HPJ) technology from 125 to 500 MPa. The rheological properties and stability of HPJ-treated chocolate milks were compared with controls (no HPJ processing) prepared both with and without added κ-carrageenan. As expected, carrageenan-free chocolate milk exhibited immediate phase separation of the cocoa powder, whereas formulations containing κ-carrageenan were stable for 14 d. An increased stability was observed with increasing HPJ processing pressure, with a maximum observed when chocolate milk was processed at 500 MPa. The apparent viscosity at 50 s⁻¹ of HPJ-processed samples increased from ~3 mPa·s to ~9 mPa·s with increasing pressure, and shear-thinning behavior (n < 0.9) was observed for samples processed at HPJ pressures ≥250 MPa. We suggest that HPJ-induced structural changes in casein micelles and new casein-cocoa interactions increased cocoa stability in the chocolate milk. Because casein seemed to be the major component enhancing cocoa stability in HPJ-treated samples, a second study was conducted to determine the effect of additional micellar casein (1, 2, or 4%) and HPJ processing (0–500 MPa) on the stability of fat-free chocolate milk. Formulations with 4% micellar casein processed at 375 and 500 MPa showed no phase separation over a 14-d storage period at 4°C. The addition of micellar casein together with HPJ processing at 500 MPa resulted in a higher apparent viscosity (~17 mPa·s at 50s⁻¹) and more pronounced shear-thinning behavior (n ≤ 0.81) compared with that without added micellar casein. The use of HPJ technology to improve the dispersion stability of cocoa provides the industry with a processing alternative to produce clean-label, yet stable, chocolate milk.     

Using extra dry milk ingredients for accelerated conching of milk chocolate

Core objective of the work was to shorten conventional conching by means of omitting the dry conching phase and to replace it with a short or continuous liquefaction process. In order to achieve this for milk chocolate, it is necessary to dry milk powder below its equilibrium beforehand. Thus water removal by conching will no longer be necessary, but fat covering of particles and adjustment of flow properties still has to be done by using high shear force. In a first step a procedure was developed, in which skim milk powder is dried to <1% moisture and then coated and stabilised in fat. Milk chocolate flakes produced thereof contained <0.6% water. Secondly, conching time was gradually reduced. Flow properties of samples were very close to a standard, if enough shear was applied. Thirdly, scale up trials using industrial equipment gave similar results. A batch process now takes 15–30 min, however using a continuous high shear mixer is also feasible. Minor sensory differences between samples and standards were neither related to milk powder drying nor to fast liquefaction. Finally an outlook is given on other feasible drying methods and productivity benefits.     

Dairy cow feeding system alters the characteristics of low-heat skim milk powder and processability of reconstituted skim milk

Low-heat skim milk powder (LHSMP) was manufactured on 3 separate occasions in mid lactation (ML, July 4–20) and late lactation (LL, September 27 to October 7) from bulk milk of 3 spring-calving dairy herds on different feeding systems: grazing on perennial ryegrass (Lolium perenne L.) pasture (GRO), grazing on perennial ryegrass and white clover (Trifolium repens L.) pasture (GRC), and housed indoors and offered total mixed ration (TMR). The resultant powders (GRO-SMP, GRC-SMP, and TMR-SMP) were evaluated for composition and color and for the compositional, physicochemical, and processing characteristics of the reconstituted skim milk (RSM) prepared by dispersing the powders to 10% (wt/wt) in water. Feeding system significantly affected the contents of protein and lactose, the elemental composition, and the color of the LHSMP, as well as the rennet gelation properties of the RSM. The GRO and GRC powders had a higher protein content; lower levels of lactose, iodine, and selenium; and a more yellow-green color (lower a* and higher b* color coordinates) than TMR powder. On reconstitution, the GRO-RSM had higher concentrations of protein, casein, and ionic calcium, and lower concentrations of lactose and nonprotein nitrogen (% of total N). It also produced rennet gels with a higher storage modulus (G′) than the corresponding TMR-RSM. These effects were observed over the combined ML and LL period but varied somewhat during the separate ML and LL periods. Otherwise, feeding system had little or no effect on proportions of individual caseins, concentration of serum casein, casein micelle size, casein hydration, heat coagulation time, or ethanol stability of the RSM at pH 6.2 to 7.2, or on the water-holding capacity, viscosity, and flow behavior of stirred yogurt prepared by starter-induced acidification of RSM. The differences in the functionality of the LHSMP may be of greater or lesser importance depending on the application and the conditions applied during the processing of the RSM.