Introduction

This note defines the term hot smoking, gives a general description of the process and the equipment used, and then gives specific recipes for all the hot smoked fish products encountered in the UK industry. Advice is also given on the packing, storage and keeping quality of the finished products.

Definition of hot smoking

There are two methods of making smoked fish in the UK, by cold smoking and by hot smoking. Cold smoking means curing fish by smoking at an air temperature not higher than 33°C to avoid cooking the flesh or coagulating the protein; with the one exception of smoked salmon, which is eaten raw, all cold smoked products are cooked before they are eaten. Hot smoking means curing fish by smoking at a temperature of 70-80°C at some stage in the process in order to cook the flesh; hot smoked fish products do not require further cooking before consumption.

The hot smoking process

Raw material

Chilled wet fish or thawed frozen fish of good quality should be used for making hot smoked products; stale raw material makes poor smoked fish.

Whole fish should first be washed to remove loose scales and slime, then gutted and if required beheaded. The belly cavity should be cleaned to remove traces of blood, and any black bellywall lining removed. The fish should be washed again before brining them. If fillets are cut, they should be trimmed and be reasonably free from blemishes.

Brining

All fish for hot smoking are brined to give them flavour. The recommended brine strength for most products is 80°; a stronger brine reduces the immersion time but has the disadvantage that, after the fish are dried, salt can crystallize on the surface of the skin in unattractive white patches. Salt is absorbed more uniformly by fish in brine weaker than 80°, but residence time is longer; an 80° brine is a practical compromise.

Brine strength

brineometer degrees	weight of salt g/litre brine
10	26·4
20	52·8
30	79·2
40	105·6
50	132·0
60	158·4
70	184·8
80	211·2
90	237·6
100	264·0

Immersion time varies with size, thickness and fat content of the fish.

Brining of hot smoked products is critical on grounds of safety. The salt concentration in the water of the finished product should be high enough to inhibit the growth of any food poisoning organisms present, particularly Clostridium botulinum, without making the product unpleasantly salty to eat; a minimum concentration of 3 per cent has been found to be effective for hot smoked fish, particularly mackerel and trout.

Salt concentration should be checked from time to time by taking a sample of flesh from the thickest part of a fish one day after smoking, and having the salt and water contents measured in g/100 g flesh. These values can then be used in the following expression to give salt concentration.

The kiln

All the recipes in this note are for application in a Torry mechanical smoking kiln.

Hot smoked products traditional in the UK probably originated in Germany and Holland, where mainly fatty fish were processed in small brick kilns capable of reaching a high temperature and of holding the heat; hardwood logs damped with wet sawdust gave a lot of heat and intense smoke. The heat and humidity gave a cooked product that had a golden brown colour and a silky sheen on the skin.

The Torry kiln, designed originally for cold smoking, has been successfully adapted to hot smoking by provision of additional heaters and improved smoke producers, so that most traditional products can be reproduced with adequate colour and uniform weight loss. Full details of construction and operation are given in 'Fish smoking: a Torry kiln operator's handbook', which should be read in conjunction with this note.

The kiln heaters must be capable of quickly reaching and maintaining a temperature of 80°C with a full load of fish. In a kiln with one trolley the heaters are normally all in the top duct but, in a kiln with two or more trolleys, heaters should be located both in the top duct and in the smoking chamber between trolleys.

Trolleys for hot smoking should have support rails on all four sides so that fish can be presented edge on to the smoke flow, no matter in what manner they are hung. There should be a drip tray in the bottom of a trolley for hot smoking, since the fish are usually loaded immediately after brining and not left to drain first, as is the practice in cold smoking. Some of the brine and juices from the cooking fish will drip during smoking; it is simpler to collect this liquor than to allow it to drop on the floor of the kiln where, once it has hardened, it is extremely difficult to remove.

Sawdust for smoking should contain a high proportion of hardwood to impart a pleasant flavour to the fish. The sawdust should be clean and free from wood preservatives or saw lubricants like paraffin. Sawdust containing particles of plastics laminates should never be used for smoking fish, since the fumes can be toxic.

Most modern Torry kilns are equipped with automatic smoke producers that can produce dense smoke from a mixture of hardwood and softwood sawdust; consumption is about 13 kg an hour for a kiln of 375 kg capacity. Large kilns may require two smoke producers in operation at once to give a sufficiently dense smoke for hot smoking.

Smoking and drying

Processing time in the kiln is usually in three stages, a preliminary drying period at 30°C, during which the skin is toughened to prevent subsequent breakage, a smoking and partial cooking period at 50°C and a final cooking period at 80°C. The total time, and the proportion spent at each stage, will depend on the species, its size and fat content, and the kind of product required.

During hot smoking the kiln is controlled in the following manner. For the preliminary drying stage, both the main and chimney fans are switched on, and the thermostat set at 30°C. The air inlet is set half to threequarters open. With the smoke producer giving maximum smoke, the smoke flow is balanced by opening the porthole, and closing the recirculation damper so that smoke begins to billow out of the porthole. The damper is then slowly reopened until smoke just stops escaping. The kiln is then operating under a slight suction so that smoke leakage is minimal. The drying period usually takes 30-60 minutes.

For the second stage the thermostat is reset at 50°C, the air inlet reduced to quarter open, and the smoke flow rebalanced as before. When the temperature reaches 50°C, typically after 30-45 minutes, the smoke is directed up the chimney and the main fan is switched off, leaving the chimney fan to clear the smoke. Trolleys are then turned round and interchanged.

For the third stage the thermostat is reset at 80°C, the air inlet reduced to 2-3 cm, and the smoke flow rebalanced. The fish are left until they are cooked before removing them; small fish and fillets are usually almost ready by the time the kiln warms to the final temperature but large fish may require another 30 minutes at 80°C before the flesh of the thickest part at the shoulders is no longer jellylike but opaque and cooked.

Cooling and packing

Hot smoked fish must be allowed to cool to at least room temperature, and preferably to about 0°C in a chillroom, before packing them. The exception is fish that are to be vacuum packed; if they are taken from a chillroom and packed, condensation will form inside the pack and, when the pack is frozen, the resultant ice crystals may give the impression that the pack has been thawed and refrozen at some stage. Fish should be put into vacuum packs at room temperature.

Fish packed too warm will have a reduced shelf life, and moulds will readily grow on them.

Storage and keeping quality

Hot smoked products made from white fish generally keep better than those made from fatty fish, although shelf life will vary considerably, depending on the amounts of salt and smoke present, the degree of drying, and the storage temperature. At a chill temperature of about 3°C fatty products will keep in good condition for about 6 days and white fish products for about 8 days; at 10°C the shelf life is reduced to 2-3 days for fatty fish and 4-5 days for white fish.

Hot smoked products can be frozen and kept in cold store at -30°C for at least 6 months, and for longer when vacuum packed. Products with a high fat content are inclined to have a soft texture after freezing and thawing. Vacuum packing excludes oxygen and thus retards the onset of rancidity in fatty fish products, but has no further advantage over other packing methods; the shelf life of the product is still dependent on the time and temperature of storage. Vacuum packing, particularly for chilled products, can sometimes mask true quality by yielding a product of good appearance after the time when it is no longer good to eat.

Recipes for hot smoked products

Methods for specific white fish, fatty fish and shellfish products are given in alphabetical order for ease of reference. Some may differ slightly in detail from ones published earlier in other advisory notes or the kiln operator's handbook; those given here should be followed since they incorporate more recent knowledge and experience.

Buckling

This herring product originated in Germany and is popular on the Continent. In Germany the product can be made from whole ungutted fish, from nobbed fish or even from block fillets, but in the UK the herring are normally first nobbed by hand. A downward cut is made behind the head, just clear of the lug bones, but the knife blade is pushed towards the head before it passes right through the gut cavity, thus forcing off the head and the attached long gut; any roe or milt is left in. The nobbed fish are washed to remove blood from the cut surface.

The nobbed herring are immersed for 1 hour in 80° brine and are stirred from time to time; the proportion of fish to brine should be not greater than one to two. The fish are speated on stainless steel rods in one of two ways for smoking. A rod is pushed either through the thick shoulder, in which case the rods are hung on the trolley across the smokestream, or through from the gut cavity and out at the back, when the filled rods are hung parallel to the stream. The latter method has the advantage that only one hole is made in the skin, as opposed to two in the former.

Filled trolleys are loaded straight into the kiln without a dripping period, the first trolley to be filled being positioned downstream and the last upstream, so that the wettest fish are nearest the inlet end. Smoking is in three stages, first a period of 45-60 minutes at 30°C with the air inlet half to threequarters open, during which time surplus water dries off and the fish skins begin to set, then a second period in which the temperature is raised to 50°C and the air inlet reduced to quarter open. During this period the flesh begins to cook, and the higher humidity reduces further weight loss. Once the temperature reaches 50°C it is held at that for a further 30 minutes, after which time the skin of the fish feels firm, and colour starts to develop. Before the third stage, trolleys are reversed and interchanged or, in a single trolley kiln, the trolley is reversed to ensure uniform processing. The thermostat is reset at 80°C, the air inlet almost closed and, by the time the new temperature is reached, small fish will be cooked; larger ones may require up to half an hour longer. Smoke flow should be balanced, using the recirculation damper, at each stage in the manner described earlier in the general description of the hot smoking process. The whole smoking process for buckling takes roughly 3 hours. The finished product should have a dark to golden brown colour, and the skin should be dry with a silky sheen.

The product can be partially cooled in the kiln by diverting the smoke flow, turning off the heaters and drawing cold air through the kiln for about 20 minutes before removing the trolleys and leaving them to stand until the fish are down to room temperature. Better practice still is to cool the full trolleys further in a chillroom. The finished product can be packed in cardboard boxes, polyethylene bags or in vacuum packs. The full flavour does not develop until the buckling have been packed for 24 hours.

Dogfish flaps

Some UK processors have tried to make and export hot smoked dogfish flaps to Germany where, under the name Schillerlocken, they are extremely popular. The flaps, or belly walls, are skinned and washed. After 4 minutes in 80° brine they are speated; since the flaps curl up to some degree the direction of the speats on the trolley is unimportant. Kiln temperature is set at 90°C from the start, and trolleys put in as soon as the kiln is up to the temperature. The air inlet is set 2-3 cm open, and with maximum smoke production the flaps are left in the kiln for 1 hour, by which time they are cooked and dark golden brown in colour. German production is chilled for local distribution, but for export the finished product should be packed in cardboard boxes lined with greaseproof paper or plastics film, frozen and cold stored.

Eels

Eels in the size range 450-700 g are the most popular for hot smoking, but small ones weighing 100-150 g are sometimes used for serving whole. Eels are usually kept alive until just before they are required for processing, when they are killed either by dousing them in salt or immersing them in a dilute ammonia solution; electrical methods are difficult to operate safely. Alternatively thawed frozen eels can be used for smoking. The dead eels are washed thoroughly in clean cold water and the skin scraped to remove all traces of slime. The eels are gutted, and the belly is slit beyond the vent in order to remove the kidney. All traces of blood are removed, and the eels rinsed again. The head is left on. Killing and cleaning are described in more detail in Advisory Note 37.

______________

If you have any enquiries, write, 'phone, or call at any of the addresses below:

	The Director,	The Officer in Charge,
	Torry Research Station,	Humber Laboratory,
	PO Box 31,	Wassand Street,
	135 Abbey Road,	Hull,
	Aberdeen AB9 8DG	HU3 4AR
	Tel. 0224 877071	Tel. 0482 27879

Other recent Notes in this series, which are available free of charge in the UK from the above addresses are:

61 Gaping of fillets, by R. M. LOVE.
62 The freezing time of fish, by F. J. NICHOLSON.
63 Fishing ports in the UK, by J. J. WATERMAN.
64 Fish silage, by I. TATTERSON and M. L. WINDSOR.
65 Fishworking machinery, by S. MAIR.
66 Handling and processing mackerel, by J. N. KEAY.
67 The haddock, by J. J. WATERMAN.
68 Icemaking plant, by J. GRAHAM.
69 Cook-freeze fish products, by J. N. KEAY.
70 Advice for the fish industry; who does what, by J. J. WATERMAN.
71 Processing cod; the influence of season and fishing ground, by R. M. LOVE.
72 Reducing odour in fish meal production.
73 Stowage of fish in chilled sea water, by J. H. KELMAN.
74 Handling and processing rainbow trout, by A. MILLS.
75 Freezing small pelagic fish, by I. MCDONALD.
76 Dark colour in white fish flesh, by R. M. LOVE.
77 Squid, by G. D. STROUD.
78 Health hazards of handling industrial fish, by A. WARD.
79 Minced fish, by J. N. KEAY.
80 Round worms in fish, by R. WOOTTEN and D. C. CANN.
81 Handling and processing blue whiting.
82 Hot smoking of fish, by A. MCK. BANNERMAN.
83 Fish smoking: a dictionary, by J. J. WATERMAN.
84 Handling and processing oysters, by G. D. STROUD.
85 Chilled and frozen fish: a dictionary, by J. J. WATERMAN.
86 Shopping for fish: advice on quality, by A. CRAIG.
87 Composition and quality of fish: a dictionary, by J. J. WATERMAN.

60 Key to Advisory Notes 1-59, by J. J. WATERMAN.

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