A large variety of fruits can be used to obtain by the appropriate technological process different kinds of semi – finished product starting from fresh fruits just harvested from field or stored in controlled atmosphere warehouses. The ripeness degree, the freshness and the cleanness of the raw materials are very important to obtain a high quality juice.
A fruit juice can be considered a suspension of water, sugars (manly: glucose – fructose – sucrose), acids, essences, vitamins, proteins, minerals, pigments and pectic-substances.
The most common semi – finished products are:
- Puree’ single strength or concentrate
- Cloudy or pulpy juices single strength or concentrate (which contain insoluble solid particles)
- Clear juice single strength or concentrate (without the insoluble substances)
The technological cycle has to be developed in order to preserve as much as possible the organoleptic properties, the colour and aromas of the fresh fruit. So especially the thermal treatments, which are necessary either to get a god yield in some steps of the process, either to have microbial stability, must be carefully evaluated just to avoid browning phenomena and vitamins and proteins degradation. This treatment is very depending from the fruit PH. The presence of organic acids such as citric, malic, and tartaric acid, depending on the kind of fruit, is responsible for the low PH value (1,5-4,5). The thermal cycle anyway is always a combination of temperature value and holding time.
The contact with the air can be dangerous for fruits sensitive to oxidation, so, in some cases, it could be necessary to carry out some steps of the process in controlled atmosphere or provide deaeration system before the product packaging.
Pectic substances are also very important.
For cloudy juices or fruit puree it is essential that the original content of pectins shall be retained throughout the production. In this case fruit enzymes must be inactivated during processing as soon as possible by an heating process.
For clear, filtered juices a breakdown of pectins is needed to enable a good yield and high degree of juice concentration, and this is done by adding specific pectolytic enzymes which hydrolise the pectic substances
To obtain clear juices the enzymation is not the only steps required, but after that other phases such us sedimentation with fining agents (gelatin – bentonite..), and filtration (by vacuum and ultrafiltration) are required.
As we said above all the semi-finished product can be single strength or concentrated.
With the evaporation process, taking out part of the water from the fruit, the juice concentrates can be considered as concentrated sugar-acid solution. Considering this and the low PH of the fruits the puree and juices concentrates are not a suitable substrates for bacteria potentially dangerous for human health.
Basically there two system for the concentration:
The puree and pulpy juices are concentrated by a forced circulation evaporators where the product is forced by appropriate centrifugal circulation pumps to go through tubular heat exchangers to be heated in order to release in a evaporation chamber the water vapors.
The evaporator is a under vacuum multiple effect system where the vapors generated in one effect are used in the next one the heath the product and making new vapors, this happen starting from the highest temperature allowed for the product going towards the lowest which is normally around 42-45°C. The system has also the advantage of a considerable reduction of the live steam consumption.
For clear juice the evaporator is always a multiple effect but the so called “falling film type” where the product flow rapidly in a thin film along the wall of vertical heat exchanger heated in the shell by the vapors coming from the previous effect. The advantage is that the heating is very quick and the residence time in the evaporator is shorter than the one in the forced circulation type, so that there is a better preserving of the product quality.
Connected with the evaporation process there is the possibility of retaining the fruit volatile aromas which otherwise would be lost in the condensed water.
Only juice coming from fresh, not rotten fruit will give a good aroma concentrate. If rotten fruits are used the amount of ethanol in aroma will rapidly increase with consequent degradation of the quality.
To obtain the aromas, the secondary condensates are recovered from the evaporator and sent to the aroma recovery plant.
This plant consists of two main parts: The boiler where the condensates coming from the evaporator are heated to produce vapors , and the fractioning column which split the vapors in aroma concentrate and a poor in aroma stripped liquor. The system concentrates the aroma in a ratio of 1:150—1:200 compared to the feed of condensates. The aroma storage has to be arranged at low temperature.
The storage of the semi-finished fruits products can be done in bag in box of 200 lt. volume by the following procedure:
The product is sterilized before packaging by a tube in tube heater for puree, and pulpy products ,or plate heat exchanger in case of clear or cloudy juices providing that the fibers content is below 5%.
The heating is done via superheated water instead of direct live steam to avoid thermal stress. The tube in tube technology provides uniform and reasonably quick heating because the product flows in a thick film allowing the thermal exchange on “two sides”.
After cooling the sterilized product is sent to a dedicated filler at one or two filling heads depending from the needed output to fill in multiple layer bags in a controlled atmosphere of live steam to avoid any contamination from the ambient.
The fruit process obviously depends from the row material and the required final product .
Here below some flow diagrams as examples.