|After grapes have been destemmed and crushed, the juice is not protected by the skin anymore and is exposed to air and everything which occurs on the grape skin. Air consists amongst other of oxygen, which can cause the browning of the juice. Different microorganisms that are not visually detectable occur on the outside of the
berry. These micro-organisms can be yeasts or bacteria; however, we do not know whether they are good or bad. Once the berries are broken, the juice flavours can evaporate during the processing of the crushed grapes. One of the ways to prevent or minimise the abovementioned problems, is by cooling the crushed grapes as soon as possible after crushing. It is however practically difficult because the crushed grapes consist of juice, skins, seeds and stem particles. The skins, seeds and stem particles can easily block the cooling equipment and therefore a special grape and juice cooler, known as a mash cooler is used. To effect the cooling as soon as possible after the
crushing of the grapes, the grape mash cooler is always installed as close as possible to the crusher.
A mash cooler is sometimes also called a tube cooler, because it consists of double jacket stainless steel tubes. To save space the tubes are horizontally mounted and connected by U-couplings. The quantity of grapes, which can be cooled, is determined by the total length and diameter of the tubes. The way the cooler works can be described according to Figure 2. The crushed grapes (“must or water”) is pumped through the inner tube of the cooler and the cold water or brine (”refrigerant”) is pumped in the opposite direction through the outer tube of the cooler. Thermometers on the inlet and outlet sides of the tubes can be used to monitor the cooling efficiency. The flow
rates of the crushed grapes and the water or brine and its respective temperatures will determine how cold the crushed grapes will be after cooling. The sleeve of the inner tube separates the crushed grapes from the cold water or brine, and the contact between the two tubes causes the decrease in the temperature of the crushed grapes. Whilst
the temperature of the crushed grapes decreases, that of the water or brine increases. Thus the heated water or brine is pumped to the cooling plant, where it is cooled for recycling.
Two practical problems usually occur with these coolers. Firstly it can block easily and secondly cream of tartar can precipitate on the inside of the tube where the mash is transferred. Blocking can occur if pruning shears, trellising wire or other foreign material are delivered in the grape loads and move through the grape crusher to the mash cooler. It usually blocks the U-couplings, which must then be loosened to remove the foreign material. Cream of tartar occurs naturally in grapes, but when the temperature
decreases cream of tartar crystals, which precipitate on the surface of equipment are formed. These crystals are sharp and look like glass particles. They can be seen on the inside of grape crushers, mash coolers and tanks. When they precipitate on the tubes of the mash cooler the cooling of the mash is not so effective. The precipitated cream of tartar must therefore be removed regularly. This is usually done at the end of the day by rinsing the tubes with hot water and thus dissolving the cream of tartar and by cleaning the tubes weekly with a caustic solution. If the latter is used the tubes must afterwards be rinsed with a citric acid and water solution, before the mash cooler is used for crushed grapes again.