In the food industry many end products are mixtures of solid substances. Examples of such products are dessert mixtures, sugar blends, instant drinks, dried soups and sauces, dietetic foods and spice mixtures. The great variety of constituents, such as milk powder, cream powder, sugar, cocoa, flour, vegetables, fungi, spices, fats (liquid and solid), places great demands on the batch mixing process.
The mixing process includes powder handling and preparation of the starting components (logistics), composition of the recipe (weighing process), the actual mixing process (mixing time, mixing intensity) and subsequent treatment of the product through to final packaging or downstream processing. Four aspects that are particularly important for the food industry are discussed in more detail.
- Quality of mixture – homogeneity
- Selection of the mixing principle
- Food Safety (traceability, cleaning validation etc.)
- Explosion protection
The goal of the mixing process is to distribute the components evenly, initially in the mixing chamber and then, most importantly, in the consumer packaging. In many cases products in a similar concentration range (e.g. 50:50 ratio) are mixed; a more difficult technical problem is the distribution of microcomponents (vitamins, active ingredients and flavouring substances) in a substrate substance. The optimum distribution of the microcomponents is determined not only by the choice of the solids mixer but also, most importantly, the physical size (particle size) of the solids is the decisive factor.
The quality of the product is primarily determined by the upstream weighing process; every error in metering appears as a deviation in the actual concentration from the desired final value in the end product. The homogeneity of powdered mixtures is frequently evaluated by empirical methods.
The homogeneity is evaluated as part of product monitoring by laboratory analysis or visual examination of product samples. This is best done with the end product. For example, the deviation range of the actual concentration (random sample variation) of an active ingredient or flavouring in a set of samples is used to estimate the mixing quality or the homogeneity. The smaller the variation in concentration and thus the variance the better the mixture.
The accuracy of the estimate depends on the number of samples and the sampling procedure. It is less wellknown that the variance (as a measure of the mixing quality or homogeneity) decreases as the sample size increases. With the same mixture 10 g samples show a greater variance than 1 g samples. The producer must therefore define two items when monitoring the mixing process and the product quality: the size of the samples to be taken and the sampling procedure. In general, the ideal situation is to keep the sampling procedure as random as possible.
The optimum mixing principle?
A wide range of different mixing principles are applied for mixing solid substances. There is no such thing as the universal mixer that can handle all mixing tasks. Tests are helpful when selecting the right mixer. One example is two horizontal mixers, which are quite different in their geometry: the widely used GMS double shaft mixer (Fig. 1) and the newly developed Gericke GBM single shaft mixer. Both types use paddles (also known as baker's shovels) as mixing tools.
The Gericke GMS multiflux mixer is fitted with two counter-rotating, overlapping mixing shafts. The paddles are installed opposite the horizontal plane for fast axial mixing in addition to the radial mixing. The rotation frequency of the paddles influences the mixing time, homogeneity and stress of the product. The Froude number 'Fr” describes the ratio of the circumferential acceleration to the gravitational acceleration and is ultimately a dimensionless rotary frequency.
If the Froude number equals 1 the weight force is equal to the centrifugal force. In the multiflux mixer the Froude number is 1.15 and is therefore high enough to generate a fluid bed mechanically with high axial and radial movement and as a result a high mixing speed. The double- shaft principle allows the mixing quality to be reached with a shorter length and a very short time compared to a singleshaft mixer of similar size. This mixer is installed where the requirement for mixing quality and gentle handling of the product is very high. It is also lower than the geometrically simpler GBM mixer (1 mixer shaft).