1. Introduction

All species of fish, when properly chilled, will stay fresh for longer periods than those that are not preserved in any way. The use of chilling techniques such as ice, therefore, effectively prolongs the length of time available for fishing trips and makes it possible to increase the catch with economic benefits for the vessel and crew. Products brought to market in a well-preserved condition will generally command higher prices, both at wholesale and retail levels, and thus give better returns to the fishing operation.

Given the above, it might be assumed that all types and sizes of fishing vessels would benefit from the use of ice for catch preservation. However, in practice there are limitations. On the smallest types of vessels, such as small rafts and the smallest dugout canoes, there is no space to keep ice until it is needed. However, this may not be a problem as the fishery undertaken by these very small craft usually only lasts a few hours and fish is consumed or sold on a daily basis. In some of these very small fishing craft, owners are aware of the problems of catch deterioration and often use wet sacking or palm leaves to cover the catch, lower the temperature and so reduce spoilage.

Many larger vessels capable of spending a day or more in fishing operations will benefit from the use of some form of on-board preservation, such as ice or chilled seawater (CSW). This category might include artisanal fishing vessels, such as larger dugout canoes, outboard-motor-powered launches and larger inboard-engine-powered vessels up to 20 m long.

With increasing demand for good-quality fresh fish, globalization of the market for these products and increasing awareness of fishermen, the use of ice on board boats is growing. Increase in the use of ice creates a need to ensure that it is used efficiently. Ice production consumes a lot of energy, so unnecessary waste is to be avoided. The most economic way of reducing this waste on board fishing vessels is by using proper storage, such as adequately insulated ice boxes, containers and fish holds where ice is stored and used to preserve the catch.

On small boats portable insulated boxes made of various materials are often used to carry ice to the fishing grounds. Ice is then transferred to the catch in suitable ratios until either all the ice is used, or there is no more space aboard for more fish. Larger boats are able to carry more ice, which allows them to make longer fishing trips, generally with better economic returns for the vessel and crew.

With advances in refrigeration, in particular the advent of compact and relatively lightweight ice-making machines suitable for on-board installation, it is now possible to install ice machines of various types on quite small vessels. This gives a certain measure of independence in fishing operations where trip length is no longer limited by the quantity of ice loaded in port or by how long it will last in the ice hold.

The beneficial effects of using ice can be apparent for a wide range of fishery activities, both small and large scale, and for virtually all species. Ice raises both the quality, and thus the value, of practically all species of fish. This promotes sustainable use of these renewable resources because the harvesting sector is able to preserve catches for longer periods and therefore reduce post-harvest losses.

1.1 Chilling versus freezing of fish

This publication is particularly concerned with chilling in fishing operations. However, there are other means of preserving fish that enable it to be stored for periods of time before marketing. One of the methods closely allied to chilling is freezing. There are many factors to be taken into account when considering the differences between chilling and freezing of fish products for various markets. Both chilling and freezing operations can produce stable products and the choice of one or the other depends on many factors.

Table 1.1 lists some of the advantages and disadvantages of the two methods. It can be used to help decide whether freezing or chilling is the option most appropriate to a particular situation.

1.2 The preservative effects of chilling fish

The use of temperature reduction as a means of preserving fish and fishery products is very important worldwide both for local and export markets. This publication specifically examines the preservative effects and use of ice on board small fishing vessels.

For the purpose of this publication, the definition of chilling is as follows:

Chilling is the process of cooling fish or fish products to a temperature approaching that of melting ice.

The purpose of chilling is to prolong the shelf-life of fish, which it does by slowing the action of enzymes and bacteria, and the chemical and physical processes that can affect quality. Fresh fish is an extremely perishable food and deteriorates very rapidly at normal temperatures. Reducing the temperature at which the fish is kept lowers the rate of deterioration. During chilling the temperature is reduced to that of melting ice, 0 °C/32 °F.

TABLE 1.1
Advantages and disadvantages of chilling and freezing

The most common means of chilling is by the use of ice. Other means are chilled water, ice slurries (of both seawater and freshwater), and refrigerated seawater (RSW). For the full benefits of chilling to be realized, it is essential to maintain chill temperatures throughout the different fish-handling operations.

Although ice can preserve fish for some time, it is still a relatively short-term means of preservation when compared to freezing, canning, salting or drying, for instance. When used properly it can keep fish fresh so that it is attractive in the market place.

The use of ice for preserving fish and fishery products has proved to be an effective handling method on board fishing vessels for the following reasons:

• Ice is available in many fishing areas or ports.

• Purchasing patterns can be varied according to need (e.g. block ice of different sizes is frequently manufactured, and crushed, small or fragmentary ice ready for use is sold by weight).

• Ice has a very high cooling capacity.

• Ice is harmless, and in general relatively cheap.

• Ice can maintain a very definite temperature.

• Ice can keep fish moist and as it melts it can wash surface bacteria from the fish.

• Ice can be moved from place to place and its refrigeration effect can be taken to wherever it is needed.

• Ice can be made on shore and used at sea.

However, packing fish in ice on board small fishing vessels, whether in boxes, shelves or pounds, is a labour-intensive task and other methods have been introduced to reduce the time and labour required. Among these, the most widely used are RSW and CSW. RSW is labour saving and an acceptable chilling method, but requires onboard mechanical refrigeration, pumping and filtering systems. It is also a relatively costly system. In CSW systems, sufficient ice is carried on a fishing voyage and mixed first with seawater before fish are added to the ice and water slurry.

Both these systems offer the advantages of quick chilling, reduced physical damage to the fish and quicker handling with less labour. However, they require more specialized installations on board and have usually only been found suitable where large volumes of fish need to be handled in a short time period, for instance when handling small pelagics on board purse seine vessels.

A typical comparison of temperature profiles for a medium-size round fish chilled in crushed ice, RSW and ice slurry is shown in Figure 1.1. According to these data, the fastest and most efficient chilling medium is ice slurry followed by RSW. The ice chilling rate is the lowest due to reduced contact of ice with the fish (an air layer surrounding the fish was created during ice meltage). To ensure maximum contact of ice with the fish, proper selection of the size of ice particles and good stowage practices are needed. The rate of chilling is governed by:

• the size, shape and thickness of fish;

• the method of stowage;

• adequate mixing of ice, water and fish (in ice slurries);

• adequate contact of ice with the fish;

• the size of the ice particles.

FIGURE 1.1 Comparison of temperature profiles for whole fish (1.6 kg/piece) chilled in crushed ice, RSW and ice slurry

1.3 Factors affecting the rate of spoilage in fish

The main factors that affect the rate of spoilage in chilled fish are:

• temperature

• physical damage

• intrinsic factors

1.3.1 Temperature

It is well known that high temperatures increase the rate of fish spoilage and low temperatures slow it down. Therefore, if the temperature of fresh fish is low, then quality is lost slowly. The faster a lower temperature is attained during fish chilling, the more effectively the spoilage activity is inhibited. Generally, the rate at which fish loses quality when stored in ice (0 °C) is used as the baseline when comparisons are made regarding shelf-life at different storage temperatures. The relationship between the shelf-life of fish at 0 °C and at t °C is known as the relative rate of spoilage at t °C (RRS) and is defined below:

Further information on spoilage rates can be found in FAO Fisheries Technical Paper No. 348, Quality and quality changes in fresh fish (FAO, 1995a).

1.3.2 Physical damage

Fish is soft and easily damaged, therefore rough handling and bruising result in contamination of fish flesh with bacteria and allow releases of enzymes, speeding up the rate of spoilage. In addition, careless handling can burst the guts and spread the contents into the fish flesh.

1.3.3 Intrinsic factors

The intrinsic factors affecting the spoilage rate of chilled fish are shown in Table 1.2.

1.4 Shelf-life of fish in ice

Chilling of fish can slow down the spoilage process, but it cannot stop it. Therefore, it is a race against time and fish should be moved as quickly as possible.

The main question for fishermen, traders and consumers is how long fish will keep in ice. As discussed previously, shelf-life will depend on several factors. However, the fish spoilage pattern is similar for all species, with four phases of spoilage as outlined in Table 1.3.

There have been many research studies regarding the shelf-life of fish stored in ice. Based on these studies, it is generally accepted that some tropical fish species can keep for longer periods in comparison to fish from temperate or colder waters. This can be attributed to differences in the bacterial growth rates, with a 1-2 week slow growth phase (or period of adaptation to chilled temperatures) in tropical fish stored in ice. However, due to differences in the criteria used to define the limit of shelf-life, and methodologies used, comparison between shelf-life of fish from tropical and temperate waters is still difficult. Tables 1.4 and 1.5 show shelf-life of several fish species stored in ice and RSW.

TABLE 1.2
Intrinsic factors affecting the spoilage rate of chilled fish

Source: FAO, 1995a.

TABLE 1.3
The four phases of fish spoilage

TABLE 1.4
Shelf-life of some marine and freshwater fish species stored in ice

¹ Fat content and shelf-life are subject to seasonal variations.

Source: FAO, 1995a.

TABLE 1.5
Comparison of shelf-life of various fish species stored in ice, RSW and RSW with added CO₂

Source: FAO, 1995a.