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Mayonnaise & Dressings
|Plate heat exchanger|
|Tubular heat exchanger|
|Scraped surface heat exchanger|
|Direct steam injection|
Mayonnaise is an oil-in-water emulsion. Small droplets of oil are covered with an emulsifier (in most case egg yolk) and dispersed in a water-phase (with sugar, salt and vinegar). The high oil content (sometimes up to 83%) gives the stiffness to the product.
To manufacture mayonnaise there are three processes possible:
- batch process
- continuous process
- batch-continuous process
In a batch process the water-phase is prepared by just adding the water-soluble ingredients to the water (the water-phase) and stir it for a while. Next the egg (yolk) is added, and mixed slowly in the water-phase. Finally the oil is added to the water-egg-phase. The oil is mixed in with help of a high speed mixer. When the oil is added too quickly the emulsion could inverse and end with small water-droplets in oil.
The advantages are the simplicity (and investment) of the process and the extreme flexibility. The minimum quantity is the batch size of the vessel with high speed stirrer.
In a continuous mayonnaise process the water-phase is prepared separately. A multi head piston pump doses the three phases (water-phase, egg and oil) together in a pin stirrer. The pin stirrer creates a coarse emulsion, which is directly homogenized in the colloid mill.
Working first with a coarse emulsion which can later homogenized in finer emulsion in a second piece of equipment gives a better (more narrow) droplet size distribution, and so a saving in oil of 1 to 5%.
The disadvantage is that the start up costs time. The process is only of interest with larger volumes (and batch sizes)
The batch-continuous process is a mix of both processes. The coarse emulsion is made batch-wise with a special designed high speed stirrer for a very short time. After transferring to a buffer the coarse emulsion is homogenized in a continuous process with the help of a homogenizer.
The advantage is the flexibility. The batch size of the coarse emulsion is the minimum production run. There are no further start up losses, except of the content of the piping to the colloid mill.
Dressings are oil-in-water emulsions where a special water-phase with a thickener is added. Mostly the oil-content is 25%, but 0% is also possible.
The special water-phase with the thickener is mostly a hot-swollen starch. The mixture is prepared in a vessel with a stirrer. The mixture has to be heated to about 90ºC to let the starch swell and is later mixed with the mayonnaise (80% oil-in-water emulsion).
The heat transfer can be taken place by a number of different processes:
- jacketed vessel
- plate heat exchanger
- tubular heat exchanger
- scraped surface heat exchanger
- direct steam injection
The jacketed vessel is the most common used process to heat up the starch solution. The vessel has a scraper that scrapes the surfaces clean to avoid that the swollen starch burns on the surface. The temperature difference in the vessel is sometimes 2ºC, and that is too much for a smooth starch solution made with natural starches. For the more heat stable modified starches the jacketed vessel is a good option.
Due to the maximum pressure drop and the narrow gap the plate heat exchangers can only be used for thin starch solutions without particles (like vegetables).
Better is the more expensive tubular heat exchanger. The heat transfer can be increased by using static mixer elements. Not all static mixer elements can be used when particles are present.
Scraped surface heat exchanger
The scraped surface heat exchanger is a tubular heat exchanger with a knife that scrapes the wall clean. It has the best heat transfer coefficient, and has less hold up and requires less space. Depending of the gap between the axis, knife and wall large particles can pass without blockage.
Direct steam injection is possible when the recipe allows to take some water out to be injected later as steam or superheated (high pressure) water. Native starches can be cracked when the temperature difference is too big. A good (double acting) control system is required, not only to reach the right temperature, but also the right quantity in one step. The advantage is that this process can be started and stopped with significant losses.
The mayonnaise (80% oil-in-water emulsion) and the (swollen) starch-phase is mixed together to form the final dressing.
The mixing can be :
- batch wise
The two phases are added is a slow stirring vessel, and mixed together for a while and transferred to the filler.
The two phases are transferred to a pin stirrer or a static mixer and mixed together. The advantage of the static mixer above the pin stirrer is the low cost, but is less flexible to large changes in the dosing ratio. The disadvantages of both continuous mixing processes are the start up loss and the more complex control of the dosing compared with the batch wise of mixing.