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Making Fish Blocks

INTRODUCTION

The block liner, also known as a Fish Block Carton, in which your fish is packed is of vital importance to the quality of the finished product. The relationship between the size of the liner and freezing frame has a direct and major influence on the correct block size and shape.

When switching from one product to another e.g. from fillets to mince, a number of changes to production methods will become necessary.

In general terms there are five main areas which most affect production techniques and subsequent fish block quality.

These notes comment on one of the most common problems encountered in Fish Block Production - the creation of voids. Suggestions are offered as to causes which may most likely be examined to effect remedies.

VOIDS [Also called Pits or Holes]

A void is a 'Hole' or 'Pit', containing air or ice, within a fish block. Generally they are seen in the surface of the block . Voids are undesirable because they affect the final weight and appearance of a portion produced from the block. A void containing ice may not affect a fish finger which is crumbed but will be detrimental to the quality of a battered or cooked product

An example, although not necessarily typical, block specification may require,

"no more than three voids per block ; approx. void size 1.9mm dia. x 0.3mm deep ".

Voids may be up to 30 mm dia.[1"] and 5 - 10mm [0.2" -0.4"] deep in which case a reject situation occurs. When packing fillets [as opposed to mince, pieces etc.,] voids may be elongated rather than round. The incidence of voids must be contained to the lowest possible level and will ideally be zero.

VARIABLES

Many variable conditions affect the incidence of voids in, and the overall quality of, a fish block. The main parameters to be taken into account in our quest to eliminate voids include the following:-

Index

  1. Different species
  2. Product Configuration
  3. Weight to Volume Ratio
  4. Packing Methods
  5. Overfill
  6. Temperatures
  7. Standtime
  8. Plate pressure and Freezing temperatures
  9. Frames

CONSIDERATION OF THE VARIABLES

1. Different Species

Different species of fish have varying firmness of flesh and muscle tissues. Their weight / volume ratios vary with the flesh density. Therefore their 'packing densities' also vary. In other words you can pack more weight of one fish into a given space, than you can of another. Back to Index.

2,Product Configuration

Depending on product availability and market demand you may be packing Fillets, fish pieces or mince, or a combination of these.

Blocks made only from fillets tend to have more voids in their surface than those containing mince or smaller fish pieces.

Fillets placed randomly ["jumbled"] are likely to produce a block with less voids than when the fillets are neatly placed in the liner. Back to Index.

3. Weight to Volume Ratio

The density of fish flesh [weight:volume ratio] may vary between species. Differences also occur seasonally and geographically within a particular species. Many fish show a marked variance in physiological condition between pre- and post-spawning periods.

The fresher the product, the firmer and more elastic will be it's cell structure. In other words it becomes harder to 'squeeze' the same weight of fish into a block. Thus a higher than 'normal' plate pressure may be needed to press it into an acceptable homogeneous block, but probably with less overfill.

Older fillets, or product, may require less pressure because of their loss in cell elasticity but may exude more fluids.

This 'packing density' is also affected by the elapsed time since the fish was caught; the temperature of the flesh at packing; and whether the fish has been previously frozen.

Frozen fish that is semi-, or completely, thawed before packing into blocks will exhibit different characteristics from fresh fish. Even the means and speed of thawing can have an effect on the overall block production methods and final results.

The initial freezing will have an impact on the thawed fish characteristics and final block quality. [e.g. frozen immediately or some time after catching; taken slowly or quickly through the critical zone; to what initial core temperature; held at what storage temperatures and subject to what temperature fluctuations.] Back to Index.

4. Packing and Handling Procedures

Blocks should contain only the product - i.e. no polythene or other packaging materials; foreign objects, added water, etc.

Ensure that all product is thoroughly drained before packing. Sufficient draining time should be allowed on the filleting line so that there is no surface moisture on the flesh when it reaches the packing area. If polyphosphate is being used, the flesh should have no visible surface moisture when the coating or injection is complete.

The liner surface must be dry before packing commences. If it is wet on the outside it will transfer moisture to the freezer plates. Water droplets on the inside will encourage the formation of voids. Back to Index.

5.Overfill

If there is too little product to properly fill the liner, when under pressure, voids will occur.

The most experienced block producers are said to use only 40g [1.4 ozs] overfill. However many block producers are using up to 100g [3.5ozs]. In some situations 100g -200g [3.5 - 7 ozs.] is quite acceptable to produce a block of the required standard. As an initial guide, trial with an average of 50 - 100g [1.75 - 3.5ozs].

The degree of overfill will depend on many factors, most of which have been mentioned elsewhere in these Guidelines. In summary these factors include; type of product [mince, fillets, pieces]; product quality; the actual species; fresh or pre-frozen fish; stand time before freezing; and to some extent to the methods of processing and packing.

You may find an overfill of 150g [ 5ozs.] necessary with layered fillets, whereas 50g [1.75ozs.] might be sufficient with the same product when 'random' or 'jumble' packed. Back to Index.

6. Temperatures

Product temperature will ideally be about 0 - 4c [35.5F -39F] when the fish is packed. As with all fish or food processing it is important that the product stays below temperatures at which bacteria easily multiply. [e.g. in some specifications you may find that product temperatures exceeding 8 c [46.5F] during processing, and prior to freezing, constitute a reject situation]

The room temperature has a direct effect on a product's temperature as it moves through the process chain. This is no doubt the reason most premises are moving to temperature controlled processing and packing rooms. Back to Index.

7. Standtime

The amount of stand time you allow will vary with the room and product temperatures; the type and quality of product; and the product wetness or moisture content. Standtime may vary quite significantly from one product to another and should be one of the first parameters checked when changing product or when voids start to occur.

Blocks awaiting freezing should be left to stand outside the plate freezer, unless the freezer plates are all above freezing temperature. If product is loaded directly into the freezer, which is switched off but the plates are still below 0 c [32F], surface freezing of the block will start. This allows the slow freezing of liquids and product on the surface of the block, encouraging the formation of voids.

Do not start to freeze blocks too soon after packing. There must be sufficient time for the proper absorption of fluids by the liner. This function is important as it is this absorption which ultimately helps protect the block against de-hydration and freezer burn. It also helps complete and easy removal of the liner from the block.

Conversely if the standtime is too long the liner may become soft and weakened by excessive fluid up-take.

To avoid an unacceptable increase in product temperature it may be desirable to stand product, awaiting freezing, in a chilled area rather than in the packing room or a non-refrigerated lobby.

Typically, blocks may stand for 20 minutes to 2 hours [depending on the operation] between packing and the start of freezing. If for only a short time, the loaded frames can be stacked, about 4 to 6 frames high. This helps consolidate the block, expel air, etc. If the packed blocks are to stand for a longer period [say 1 hours] it is better not to stack them but place them individually on racks. This will help to prevent excessive fluid loss, due to compression.

If the loaded liners / frames have stood for only a short time try applying plate pressure for five minutes before starting the freeze cycle. Back to Index.

8. Plate Freezers and Freezing

Fish blocks are most commonly frozen in horizontal plate freezers and may be handled manually or by automatic systems.

Typical freezer settings are:- Evaporating temperature -38 C to -40 C [-36F to -40F ] giving a plate surface temp of -34 C [-29F] or lower. Freezing blocks to a core temperature, on break out, of -18c [0F] or lower should take 1 - 2 hours given ice-free plates and good contact on all surfaces. If blocks are removed from the freezer prematurely further changes in block shape may occur during sub-zero storage.

Plates must be free of ice and any other objects before loading and freezing otherwise uneven pressures and block temperatures occur.

A steady plate pressure is ideal and the use of pulse-operating plate freezers [especially in the early stages of freezing] is not recommended.

Insufficient pressure exerted by the plates allows the formation of ice, voids, and swelling etc. within the block. If the block bulges too much one remedy is to increase plate pressure.

Plate pressure which is too high may cause excessive loss of fluids and damage to the flesh structure. It may also cause deformation of the freezing frames.

Pressures are often expressed in two ways, i.e. the 'system pressure' or 'plate pressure'. System pressure relates to the operating pressure within the freezer's hydraulic system.

Plate pressure normally relates to the downwards vertical force exerted by a plate on to the product below it.

Sometimes plates are operated between 0.25 - 0.5 bar [3.7 - 7.4 psi.]. We generally work on a plate pressure of 1 bar [15 psi.] which translates to a system pressure of 70 -75 bar [1000 - 1100 psi ]. A system pressure of 900 - 1200 psi. seems to be a good working range. Variations will occur in the actual pressure exerted on a block. This is influenced by the number of blocks per station; the number of stations; the hydraulic ram size and hydraulic pump capacity.

A practial example is a 10 station horizontal plate freezer [i.e. 11 plates] with plate size 1500mm x 1100 mm. System pressure has been increased to 102 bar [1500 psi] from about 1050 psi with a consequent reduction in numbers of voids. [1 bar = 14.7 p.s.i.] Back to Index.

9. Frames And Block Dimensions

Frames may be singles or doubles, with fixed or loose bottoms, in stainless steel or aluminium [or a combination of both ]. Some processors use sheet plastic bases which are removed from the plate freezer once the frame is in position, and before freezing starts. This allows maximum contact between the block liner and the plates, thus minimising freezing time.

It is important that frames are clean, square and flat. If they are not flat the freezer plates will create uneven pressures on the blocks and other frames. Back to Index.

Standard block dimensions are listed separately.