Spinach and leaf

SPINACH AND LEAF VEGETABLES

 

Leaf vegetables such as spinach, kale, leeks, endive and chervil are freshly available during a short season. Leaf vegetables can be processed into cans, glass jars, or they can be frozen.

 

Processing leaf vegetables

 

Harvesting

 

The cultivation method differs per kind of leaf vegetable. Harvesting leaf vegetables is done when the vegetables are still in an early stage of growth, this prevents the soil from being contaminated (rot). In addition, leaf vegetables are harvested in the morning, when it is still cool outside.

 

Transporting

 

After harvesting, transportation to the factory has to take place as soon as possible, in order to retain most of the organoleptic properties of leaf vegetables. Leaf vegetables must be free from roots, seeds, vermin and other undesirable parts. In addition, the leaves should have the desired color and be totally undamaged. In order to prevent the growth of thermophilic bacteria, the storage temperature of leaf vegetables should be kept low. They are not stored over a long period of time, since the quality will deteriorate rapidly.

 

Washing

 

A leaf vegetable washer consists in general, of a number of in in series placed washers. Through the use of different heights, the water flows from tone tank to another. The counter-flow principle is applied, ensuring that the product comes into contact with clean water each time. Leaf vegetables are moved with around with, for example, rotating blades. The washer can also be equipped with an air injection system. As a result, the washing effect is increased without damaging the product.

 

Reducing/ cutting

 

Size reduction of leaf vegetables will take place, depending on the kind of leaf vegetable, before or after blanching and dewatering. Spinach is reduced in size, for example, only after blanching.

When cutting before blanching, a leaf vegetable cutter can be used. For example, in the leaf vegetable cutter, endive is fed to the cutting machine on a conveyor belt, where the vegetables are first flattened with a drag roller. Then, the vegetables are cut into strips of about 2 cm wide with in parallel placed circular knives, after which a chopping knife cuts the pieces into even shorter pieces.

When cutting after blanching and dehydration, often, a wolf is used. In a grinder the leaf vegetables are pressed, through multiple perforated plates with a decreasing perforation diameter, with a screw. In front of each plate, a cross-shaped knife is placed. The resulting cut is determined by the combination of perforated plates, knives and the variation in rotational speed. Additionally, spinach can also be reduced with a passing machine. This passing machine consists of a cylinder-shaped sieve screen drum, in which the spinach is pressed through the sieve screen using multiple plates. In this way, the stems and hard pieces can be separated simultaneously. This process also creates a product with a more crème like texture.

 

Blanching

 

Blanching can both be done in a water- and a steam blancher. When blanching in water, losses through leaching occur, whereas with the steam blancher, larger lumps could form, due to irregular heating. Usually, a belt blancher is used. In case of water blanching, the leaf vegetables are guided between two conveyor belts through hot water. Because the upper conveyor belt keeps the vegetables submerged in water, the result is a constant and even heat treatment.

Depending on the leaf-vegetable, blanching is done at a temperature of from 90 to 98°C for about 60 seconds. Blanching causes leaf vegetables to shrink considerably, making grinding and filling easier. Moreover, enzymes are rendered inactive, causing the quality of the deep-frozen leaf vegetables less likely to deteriorate during long-term storage.

 

Cooling

 

After blanching, the vegetables that will be frozen later, are cooled down directly or indirectly to prevent microbiological spoilage during the following processing steps. The vegetables are in this way already significantly reduced in temperature.

Direct cooling can be done by pumping the leaf vegetables around in cold water or by spraying them with cold water. Indirect cooling, by means of a tubular heat exchanger is, however, preferred, since there will be no further leaching this way, and there is a smaller chance of contamination. This is especially important for the deep-frozen leaf vegetables, since deep-freezing will hardly kill any microorganisms. When leaf vegetables are sterilized later, no cooling step will take place.

Dehydration

 

Before leaf vegetables are frozen, they are led by a dehydration drum. The dehydration process can also take place by means of pressing. This method is generally used when leaf vegetables will be sterilized.

 

Freezing

 

Using a volume filling machine, the blanched leaf vegetables can be filled into cardboard cartons. Sauce may be added to products such as spinach and endive, before the product is filled into containers.

Leaf vegetables should be frozen as soon as possible to ensure that as many ice crystals as possible are formed and that the ice crystals do not get time to grow. Small ice crystals cause less tissue damage to the product.

Freezing can be done using a plate freezer, a freezing tunnel or a spiral freezer. Freezing times depend on the product size and can take up to several hours. In a plate freezer, products are cooled by means of a coolant. In a freezing tunnel or spiral freezer, freezing is done through circulation of air at -35°C with an air velocity of between 3 and 5 m/s.

Sterilizing

 

The blanched leaf vegetables are hot (> 70°C) filled into glass jars or lacquered cans. Subsequently, depending on the leaf vegetable, a 1 to 1.5% saline solution can be added as brine or syrup, and the package is closed. The sterilization takes place at a temperature of about 121°C in an autoclave. The sterilization time varies per product. Spinach takes up to 80 to 90 minutes, whereas for endive 60 to 70 minutes is preferable.

 

Cooling

 

After sterilization, the packages are cooled down to a temperature of about 30°C. This procedure grants the cans time to dry, thus preventing corrosion.

 

 

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