Description of the technological process for producing whole peeled and diced tomatoes
Another important consideration is that peeled and/or diced tomato processing must always take place alongside the production of tomato paste.
It is not convenient to produce only peeled tomatoes as the waste products of the grading and sizing operations (which may be as high as 30%) together with the waste obtained from the peeling process (about 10%) can be recovered and used for the production of tomato paste. This is why tomato peeling and paste lines always work side by side.
The way fresh tomatoes are delivered to the factory is especially important in this case as the fruit for processing must be perfectly preserved, otherwise the peeling line’s yield will be extremely low. Tomatoes should be transported in plastic boxes of 350 – 470 kg rather than loose in the truck.
PEELED TOMATOES: Fresh tomatoes are unloaded into a flume (a hydraulic duct) which transports the fruit to a grading station where staff removes the cracked, immature or excessively small tomatoes. This process eliminates about 5% of the incoming raw material.Suitable tomatoes shall be divided according to size so that the peeling machine can be fed with fruit which is as homogenous as possible. This process eliminates about 15% of the incoming raw material.
The sized fruit is placed into a “scalder” (or boiling water rotating tank) to facilitate skin removal in the subsequent peeling stage.
The tomatoes are now ready for the peeling phase, which can be carried out using three different methods: (a) with caustic soda, (b) mechanically and (c) with steam (known as thermo-physical peeling).
FENCO suggests that only the mechanical and thermo-physical methods should be used as the caustic soda process produces a finished product of inferior quality (loss of colour and flavour) and increases pollution.
Mechanical peeling, also known as Savi from the name of the company which invented this procedure, is most suitable for small productions (up to 10-15 tons/hr of raw material). It is economically profitable due to reduced machinery costs, a total recovery of waste product and a finished product with extremely high quality features (colour, flavour, etc.). The disadvantages lie in the fact that the fruit must be more carefully sized and that the mechanical parts must be constantly serviced, even if this maintenance procedure is not particularly expensive as these devices are quite simple. During the mechanical peeling process the scalded fruit (rapid skin heating) must be channelled, then rotating blades make an incision on the skin surface. Mechanical “hands”, manufactured in special rubber, literally take hold of the tomato and, by exerting a light pressure, retain the skin while letting the whole tomato slip out. The strips of skin remaining on the peeled tomato are then removed by separators, positioned immediately after the mechanical peeling machine.
On the other hand, the thermo physical steam peeling method is recommended for large quantities, (i.e. over 15tons/hr), to amortize higher machinery costs. There are basically two main advantages associated with this method: large quantities of product can be processed using only one machine (even up to 50 tons/hr); the machine can also be used for peeling tuber vegetables, such as potatoes, carrots, etc. In the steam peeling method the pre-heated fruit first undergoes an extremely rapid increase in temperature, up to about 115°C in a high pressure chamber, and then an equally rapid cooling process under vacuum conditions to produce a sort of skin explosion, whereby the skin comes away from the pulp. The skin is not completely detached from the fruit inside the peeling machine, but by the skin-removal units, positioned after the peeling unit. In this case the effective fruit peeling takes place in the skin-removal units, which must be large enough to undertake this load. Furthermore, thermal stress at 115°C under high pressure also causes a waste of the pulp lying immediately under the skin, which is the best part of the fruit; this does not take place when using a mechanical peeling machine.
The completely peeled tomato then undergoes thorough inspection on special conveyors to ensure that only perfect fruit (completely peeled and whole) is sent for packaging. This inspection can be carried out manually by specialized personnel or by an electronic device, called “optical selector”, which recognizes the unsuitable fruit and removes it using electrical-pneumatic devices. Obviously these electronic devices can be justified only for large workloads and in countries where manual labour costs are high.
The peeled and selected tomatoes can now be sent for packaging in metal cans with a capacity ranging from ½ kg to10 kg. Thanks to telescopic fillers, this process is carried out automatically. The empty space inside the can, which represents about 35% – 40% of its total weight, must be filled with natural tomato juice or semi-concentrated juice at about 7° Brix. This is carried out automatically using a juicer. The filled can may now be sealed.
The processing procedure is, however, not yet finished. The containers and contents must now be sterilized to provide a stable bacteriological level and to ensure a long product preservation. The filled cans are placed inside large pasteurising machines, equipped with a can rotating device, at atmospheric pressure. These machines heat the product up to 98°C for a pre-set time which varies according to the can’s dimensions. The can’s core must reach at least 95°C to ensure a perfect product pasteurisation. It is essential to use can rotating pasteurising machines in this type of process as the heating times would otherwise be excessively long, thereby reducing the quality of the finished product. This pasteurising machine also cools and dries the filled cans at a temperature of 35 – 40°C so that they are ready to be automatically packaged in cardboard boxes.
It is extremely difficult to draw conclusions about output when dealing with peeled tomatoes, as many technological process variables must be taken into account, although it is clear that the quality of the raw material is a decisive factor as regards the end product yield.
However, statistics from Italy (the largest peeled tomato producer in the world) show that every 100 kg of fresh tomatoes on the processing line produces 60 kg of peeled tomatoes for canning and 40 kg of total processing waste (grading rejects, calibration, peeling). About 30 kg of these rejects are used to produce concentrate or additional juice for peeled tomatoes. Therefore, the effective, unusable waste is only 10 kg. It must be remembered that these are average values.