Chocolate production, methods and effects of refining

fats (blotting paper); the resulting mass will be more compact and have the consistency of mud, so that it can be more easily transferred on a conveyor belt to the refining operation. The refining machine features 5 rollers mounted almost vertically one on top of the other, and by means of cold water circulation, like the pre-refining machine. The distance between the rollers (gap) is accurately adjusted in order to obtain the highest refining fineness. The systems used for adjusting the “gap” can be: mechanical or pneumatic or in last-generation refining machines-electronic. From the first to the last roller, the rotating speed is progressively increased, so that while moving from the slower roller to the quicker one (flowing from bottom to top) the product is also crushed and stretched. The rotation speed is of approx. 0.75 m/sec for the first roller, 1.25 m/sec for the second, 1.80 m/sec for the third, 2.45 m/sec for the fourth, and 3.70 m/sec for the fifth. Unlike the rollers of the pre-refining machine, the rollers of the refining machine must be convex in order to compensate for cylinder flexion caused by the effort and for the congenital extension deriving from the heat produced by the friction, thus guaranteeing a uniform refining. Suitable blades, tangent to the whole length of the roller, will detach the chocolate film from the surface. In the first refining machines, usually with 3 rollers, the mass had to be submitted to this treatment two or three times to obtain the desired particle size. On the new 5 roller refiners, a fineness up to 18-20 micron can be achieved in a single process. In some cases, the particle size even reached smaller values, but there is no evidence that “the game is worth the candle”, because palate sensitivity does not allow the identification and distinction of this kind of fineness. It is a fact that human lingual papillae allow to detect particles measuring not less than 25 micron, which means that larger particles can be perceived as “grains” or “sand”. Apparently, hence, from a merely technological point of view, it should be guaranteed that chocolate particles have a maximum size of less than 25 micron.

Electronics to control roller pressure
However, gustative feeling is not determined only by particle size, but also by the quantities present in the different sizes. This means that coarseness is felt in presence of a considerable amount of particles larger than the specified threshold value. It should be reminded also that advanced grinding generate products that are “greedy for fat” (blotting paper effect) usually with reduced rheologic properties. Machine capacity depends both from roller length (presently Refining machines can be equipped with rollers measuring between 1,300 and 2,500 mm in length), and from the type of product. Usually production capacities range between 500 Kg/hour and 1,000 Kg/hour and more. Milk chocolate and similar products containing milk are more difficult to refine due to the presence of Lactose. The use of refining machines has always requested considerable experience and care; it should be reminded, in fact, that a single passage is made and this allows little margins of error. The recent introduction of electronics to control roller pressure led to reconsider the need of working with skilled operators; however the observance of several basic parameters should not be neglected: Cooling water flowing through the rollers shall never exceed the temperature of 30°C; no machine idling is allowed. If the chocolate mass piles up on one side of the roller, the “gap” between the relevant rollers must be adjusted; whereas, when some areas are not covered with chocolate, the cooling water flow shall be slowed down, because this problem is caused by excessive cooling. The scraping blade shall always be perfectly sharp to guarantee that the mass can be removed from the surface of the rollers. During refining, the product considerably changes its physical and flowing properties; in fact at the exit from the process, a mass is obtained that mainly consists of impalpable powder, which will be transferred to the next operation – usually by means of conveyors: conching. As already described, there are several different refining methods, although the most popular is the one described above. However, there is a variation to this roller refining system; in fact, sometimes powdered sugar is used instead of crystal sugar. This change, although apparently of little importance, becomes relevant if one takes into account that the pre-refining operation can be avoided. Years ago this processing technology was widely used, but for two reasons it has been partly abandoned: 1) Crystal sugar had to be ground on site, because the one available on the market, for its preservation, contains 4-5% of starch that may negatively affect the final product structure; 2) A higher need of fat (cocoa butter) 27-29% due to the high adsorption power of powdered sugar compared to crystal sugar. The most popular alternative to roll refining is represented by the use of Ball Mills. This processing method practically replaces the whole line described above; it is widely used for the production of anhydrous creams and surrogates, as well as – in many countries – for the production of chocolate and dried fruit, as well as for semi-finished products as nougat paste, etc. The ball mixers technology comes from the world of paints and inks, and was already in use in the 1920. In the ‘50s it was used with excellent results in the confectionery sector by the Dutch company Wiener for grinding roasted cocoa. The ball mill mainly consists of a double chamber roller used both for heating and cooling, inside which a considerable amount of steel balls are moved by a central shaft equipped with special arms or disks. A ball mill refining system practically replaces a conventional processing plant integrating mixing, pre-refining, roller refining and conching, although with capacities. The ingredients are mixed in a vertical or horizontal tank, and the highly liquid mass resulting from this process is then pumped into the ball mill. In order to obtain a fineness very similar to that obtained with roller refining, normally this process has to be repeated several times; not always with positive effects. In fact, some parameters, especially viscosity, flowability and formation of sub-fine particles, that notably affect the mass plasticity, can undergo to considerable changes. As already said, the ball mill refining system replaces the conching phase. Synthetically said, ball mill refining is an excellent alternative to roller refining technology, especially as far as the costs of the system and its management are concerned; albeit end results are not always compatible. For instance, if all goes well, the grain size reaches 22-25 micron; the temperature cannot always be effectively controlled; finished products often have a metal taste; products containing milk tend to over-heat, causing a degeneration of proteins and a modification of the structure; finally, for a good processability, the fat content should be of at least 30%.

by Tino Carrega