Although today some fishes and seafood are also eaten raw, for example in the form of sushi or sashimi, most products are still eaten cooked. In addition to enzymatic cooking techniques such as salting, cold smoking or cold marinating, thermal cooking techniques such as pasteurisation, hot smoking, frying and cooking are of particular significance. Thermal techniques are used just as much in private households as in canteen kitchens or in industrial fish processing. Probably 70 to 80% of all industrially produced seafood products throughout the world have undergone thermal treatment. This treatment renders the raw products edible, enjoyable and also more easily digestible. At the same time, depending on the cooking temperature, heating also affects the appearance and consistency of the products. Usually it improves their smell and flavour, and alters their water and fat content. Cooked products look more appetizing, they can be more easily chewed, and they are more enjoyable. Whilst beef, pork and other kinds of meat from warm-blooded farm animals are not fully cooked until a temperature of between 75 and 85°C is reached, most fish species are already cooked between 60 and 65°C. These slightly lower cooking temperatures mean that a lot of heat-sensitive nutrients are optimally preserved which gives seafood products their high nutritional value as food.
An energy input is necessary for thermal cooking. This usually happens through the transfer of heat energy but can also be in the form of high frequency energy in a microwave oven. In order to transfer the heat energy effectively to the product a contact medium is needed – usually water (steam), fat or air. But thermal cooking is also possible without direct contact, for example through radiation energy during grilling. The allocation of some industrial cooking methods to individual products is in the meantime sometimes difficult because some technical terms can be associated with different meanings. Cooking a fish product by submerging it in a pan of hot oil should correctly be referred to as deep-frying. In practice, however, the simple term ‘frying’ is often used. The term blanching is often used for cooking fish in a water bath or in hot steam.
Preserving and increasing enjoyment
Most thermal cooking techniques can be divided into two groups depending on the type of heat transfer and the necessary processing temperature. The group of “wet” techniques includes steaming, boiling, stewing and simmering in water, as well as cooking in a high frequency electric field. The “dry” methods include mainly frying and deep-frying, baking, grilling and roasting. In his book “Fischverarbeitung” [Fish processing] (1994) Manfred Tülsner distinguishes between the following thermal cooking techniques:
|Cooking technique||Heat transfer medium||Temperature and process control|
|Pasteurisation)||Water||75 – 95°C, constant or gradually falling from about 100°C to 75°C|
|Boiling||Water (steam)||100°C, constant|
|Pressure cooking (sterilization)||Water||Up to about 120°C with 3 temperature phases: rising, maintaining, and falling|
|Stewing||Water, fat and steam||100°C, constant|
|Steaming||Steam or steam and air||75 – 98°C, constant|
|Pressure steaming||Steam||Up to about 120°C with 3 temperature phases: rising, maintaining and cooling|
|High-frequency cooking||without||Variable up to about 250°C|
|Frying||Fat (-film)||About 200°C, sometimes falling to 100°C|
180 – 200°C, constant,
Cooking in oil 120 – 140°C
|Baking in the air||Air||Mostly 120 – 250°C, to max. 300°C|
|Grilling||Infrared radiation and air||Up to 400°C, about 200°C at a distance of 1 cm from the surface of the product|
|Roasting||Roasting dish||Roasting dish up to 350°C, surface of the product at a constant temperature of about 200°C|
Although the main aim of thermal cooking is the development of the desired product attributes it is above all the enhancement of enjoyment that is the objective of the various cooking techniques. Indeed, some products are even only consumable after previous heating. However, heating foods also has a preserving effect. The degree of preservation is strongly dependent on the temperature and duration of heat impact. Most simply, cooking at relatively low temperatures already kills numerous vegetative microorganisms and inactivates enzymes within the product. This means that the products remain stable and enjoyable for a slightly longer time, but the preservative effect of the thermal cooking process is overall much lower than that achieved for example by pasteurisation or sterilisation that takes place in airtight containers. These processes specifically kill spoilage microbes and enable the shelf-life of individual products to be increased by several years (e.g. canned fish).
Pasteurisation and sterilisation
All of the thermal cooking processes that are used in fish processing today fully inactivate the enzymes that the raw materials contain. In order to kill pathogenic microbes effectively, however, certain temperature conditions are necessary. At high temperatures that are still below 100°C vegetative bacteria, mould and yeast cells are killed in a relatively short time. Some spores, however, are very resistant to high temperatures and can survive under these conditions for a longer time. A measure for the resistance of these microbes is the D-value (destruction value). This value indicates the time that is necessary to reduce the number of microbes in a product at a constant temperature by one decimal place, i.e. by 90%. At defined temperatures the microbes are not killed suddenly but in exponential order. Higher initial counts of microbes thus demand correspondingly longer times, and absolute sterility is practically not possible. Under practical conditions, a reduction in the bacterial count of 6 decimal places for preserved products is mostly required, i.e. a reduction from 1 million microbes to 10. This is the only way to rule out health risks for consumers to an adequate extent. Techniques that fulfil these requirements are termed 6D processes. Because microorganisms of the species Clostridium botulinum produce particularly dangerous toxins thermal preservation techniques are often geared to the certain killing of these microbes. Because thermally cooked products are often seen as ready-to-eat they also have to take into account other germs, above all Listeria monocytogenes, which is considered particularly heat-tolerant.
During the heating process it is not only the temperature that is decisive for the pathogen killing effect but also the temperature curve. After a phase in which the temperature rises until the desired temperature is reached the sterilisation temperature has to be kept at a constant level for a certain time before it can be reduced again. The lethal effect thus does not depend on the overall duration of heat application but – and above all – on the duration of the phase during which the sterilisation temperature is kept constant. How long this has to be depends on the F-value. The F value indicates how many minutes a concrete temperature has to be maintained to kill a certain microbe type as reliably as would be the case at 121.1°C. Depending on the microbe species the F values are usually between 2 and 10 minutes.
Documentation of cooking processes for full traceability
Because a lot of seafood species have a relatively low fat content but a very high water content they are often cooked for too long or too strongly. This “overcooking” reduces product quality since it dries the product out, the flesh loses its tenderness, and a lot of flavour components are destroyed. This particularly applies to low-fat fish species such as cod and saithe but also to some shellfish and crustaceans such as scallops, shrimps and lobster. Like overcooking, undercooking is a serious quality defect, too, because it can lead to the premature spoilage of the product and constitute a health risk if pathogens are not killed sufficiently. Already for these reasons all process stages during thermal cooking of seafood products have to be fully documented. This applies in particular to products that are traded internationally. In order to guarantee their full traceability not only the absolute temperatures but also the temperature curves are documented.
The obligation to document cooking processes enables processing plants not only a minimum level of security in the event of complaints but can also help them to optimize processing stages. Using the measurements taken they can adapt temperatures, cooking times and other processing conditions optimally to the products concerned and improve their quality or render it reproducible. This is of particular significance in the case of brand products which – irrespective of the size and condition of the raw materials – always have to have constant quality features. When cooking crustaceans a lot of producers aim at a core temperature of 83°C during cooking, for example, that is then maintained for one minute so that the meat of lobster, shrimps or crayfish is fully cooked but does not become too hard and has the right bite. Of course, the size of the crustaceans is also of significance, and whether they are fresh or frozen, are processed with or without the shell, and which cooking method is used. Fishes and seafood are natural products that can have different qualities (size, fat content, consistency) depending on their age and the season. Independent of the initial product conditions the end result, the final product, should always be as similar as possible.
Industrial cooking facilities today offer a very large number of possibilities for achieving consistent, reproducible results even from fluctuating raw material qualities. One striking feature of industrial plants is their size, otherwise they basically work much the same as the appliances we know from household kitchens, and the actual cooking processes are certainly comparable. A salient feature of industrial cooking processes is above all the larger product throughput and the plant’s capacity which has to be in line with the desired requirements in everyday routine operation. If a facility is too large there will frequently be standstill with resulting unnecessary waste of energy. If, on the other hand, the capacity is insufficient it might be difficult to reach and maintain the necessary temperatures. A lot of cooking processes can be carried out in continuous throughput mode in cooking lines. The products pass through the plant on a conveyor belt and are cooked in steam, boiling water, sizzling fat or in the hot air. The time the product spends in the production line is equal to the cooking time and this can be controlled by the speed of the conveyor belt. Such continuous cooking lines are often very long, however, and for that reason alone cannot be installed simply anywhere. Producers with less space available can use discontinuous systems which work in batches and thus mostly require considerably less space.
Program controls for all the important process parameters
Programmable controls and continuous process controls are in the meantime almost the usual standard for industrial cooking plants. Some controls do not only offer cooking programmes for different products but also fully automatic modes. In the automatic mode it is often enough to enter the raw material name, the cooking category and the desired product. All subsequent process stages are regulated independently by the system. After sensors have identified characteristic features of the raw material that are important for the cooking process and the system has determined the batch size the computer controls can work out the optimal cooking time, the right temperature and any other processing conditions. Over the course of the cooking process the parameters are checked constantly and adapted as necessary. The automatic cooking option guarantees consistent processing results, independent of fluctuating raw material qualities. Apart from that, the operating staff have less work to do and the error rate sinks. However, anyone who wants their products to have a “personal note” can also often cook “traditionally” with modern cooking lines... Or least with those systems that have a manual mode. This mode enables the operator to control and alter all key processing conditions at any time (e.g. cooking temperature, humidity, core temperature). However, a lot of experience is required to achieve consistent results with this mode.
That is why most processing facilities use the programme mode to guarantee reproducibility, success and high productivity. The typical cooking routines are often already programmed so that the operating staff only have to press the button with the desired recipe. In addition, a lot of systems can also be programmed freely, however, to offer users all options for their own creativity. The cooking programmes can often be saved on USB sticks or CDs and then sent directly by e-mail to other users. In this way the same cooking techniques and quality standards can be achieved in different company locations.
The range of industrial cooking plants of different size and with different equipment is immense, extending from simple tiltable frying pans for big kitchens to complete frying lines for industrial usage purposes, from water boilers to whole cooking lines. For blanching, potential users can, for example, choose between a blanching tunnel or a rotating drum blancher, a belt blancher or a spiral blancher. The range of pasteurisation systems is similarly extensive, from individual appliances to complete lines. There are also hot air ovens and steam tunnels that usually work on a throughput basis. Autoclaves and other pressure cooking based machinery, on the other hand, have to be used in batches. It must also be taken into consideration that the performance of the peripheral equipment, e.g. dosing machines and mixers, portioners, weighing systems and packaging machinery have to be geared to the capacity of the cooking facilities. Otherwise bottlenecks could arise within the in-company product flow.
Cooking machinery and systems should not only be user-friendly, i.e. easy to operate and ergonomically designed to enable convenient loading/ unloading, but should also be easy to clean. Only then will it be possible to meet all hygiene requirements at all times in everyday operation. Efficient heating technologies and good insulation which prevent unnecessary heat loss are key prerequisites for energy-saving work processes.