Leaf vegetables

Leaf vegetables are harvested from plants with well developed leaves. The colors of the leaves can vary from green to yellow. However, some leaf vegetables have red leaves because of the anthocyanin they hold.

Most leafy greens are suitable to eat raw or cooked. Leaf vegetables consist for large percentage of water, which causes the vegetables to shrink considerably during the cooking process. Leafy vegetables are a rich source of vitamins, minerals, fiber and a host of other health beneficial ingredients. These vegetables strengthen the immune system and increase performance abilities.

Since the processing of leafy vegetables can be carried out in various ways, only the most common processes will be discussed.

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, the 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 the leaf vegetables. The 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 the 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. The leaf vegetables are moved with around with, for example, rotating blades. The washer can also be equipped with an air injection system, through which with turbulence wash water can be obtained. 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 vegetables, 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 the leaf vegetables are sterilized later, no cooling step will take place.

Dehydration

Before the 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 the 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 grants the cans time to dry, thus preventing corrosion.

Storing

The packages are labeled and packed in boxes, after which they can be stored for a longer period of time outside of the refrigerator.

Food Safety & Hygienic Design

Blanching is primarily intended to destroy the proteins responsible for enzymatic degradation of the cell walls of the vegetables. Blanching also kills off a large numbers of microorganisms, but is not equivalent to sterilization. The product leaving the blancher still contains vegetative cells and a large number of microbial spores.

Vegetables are generally not sufficiently acidic (pH> 4.6), in order to prevent the development of microbial spores after blanching. There are two ways to create vegetables with a longer shelf life:

• Stopping microbial growth by freezing (and frozen distribution)
• Killing of all microorganisms, including spores with sterilization - preferably in the package (canned or bottled)

The production equipment and machinery must be of a GMP-class. This means that the machinery and equipment must be visibly clean before use. In case that the machinery cannot be emptied fully (and is clean and set aside dry), the machinery has to be thoroughly cleaned and disinfected before the production process can start again. Cleanable to a microbial level (hygienic design) is only required after the blanching step if the vegetables are frozen and will not undergo any heat treatment later (ready-to-eat). If the vegetables will definitely undergo a heat treatment, for example, after packing or at home by the consumer (ready-to-heat), GMP-class is enough. A light microbial contamination should not be a problem.

Even though the vegetables undergo a killing heat treatment during blanching, this is, however, not a license to be less careful during the pre-processing phase or cleaning the processing equipment less frequently or not fully, or extending the production run. Large amounts of heat-stable toxins, coming from microorganisms, may still be toxic after sterilization and will pose a threat to the health of the consumer.