Fat melting

Although this section describes fat melting, the reader’s attention is drawn to the fact that this activity is similar in many ways to rendering and some of the equipment described is identical for both processes. The feedstocks differ and consequently the conditions for separating the fat, water and solids fractions vary accordingly. The product of fat melting is generally for food use, so feedstocks are required to be fresh and consequently cause less odour problems during storage and processing.

Three methods of fat melting have been reported: batch wet fat melting, batch dry fat melting and continuous wet fat melting. The method used affects the quality of fat produced. The most important quality demands are: low content of free fatty acids (FFA); low water content; good keeping qualities; low peroxide value; neutral taste, flavour and colour and high solidification point. Extended storage and processing times adversely affect both quality and environmental standards. Unfresh raw materials may cause odour problems and add to the waste water pollution burden.

Under certain conditions, fat undergoes two important chemical changes, i.e. hydrolysis and oxidation. Hydrolysis is a chemical reaction between fat and water, through which free glycerides and FFAs are formed. The compounds formed during oxidation give the product a rancid taste.

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RHC/EIPPCB/SA_BREF_FINAL Version November 2003 47 The handling and storage of the raw material before processing and the type of processing carried out determine the FFA and the peroxide value.

To achieve a low water content the fat may be purified in a separator. The water content of the fatty raw material normally ranges between 6 - 25 %.

The FFA content increases with the duration of storage and treatment, especially any time at raised temperatures. To avoid this, the constituents need to be separated quickly.

Batch wet fat melting

Batch wet fat melting is the oldest method of fat melting. An autoclave is filled with precut raw material and is then closed. Live steam is then injected into the raw material at a pressure corresponding to a saturation temperature of about 140 ºC. To minimise the heating time, the cooking vessels are not too large and are equipped with agitators.

Since the method is wet a good separator needs to be used, to ensure the final water content is below the required level.

After a heat treatment lasting 3 - 4 hours, depending on the size of the autoclave and the nature of the raw material, the pressure is slowly reduced to atmospheric pressure, in order to avoid emulsification.

After a settling period, the free fat is drained from the autoclave to an intermediate tank, and the moist greaves are pressed and then dried. The fat collected is allowed to settle or is separated.

The batch wet fat melting/rendering process is illustrated in Figure 2.3.

Figure 2.3: The batch wet fat melting system with autoclaves [145, Filstrup P., 1976]

Batch dry fat melting

Batch dry fat melting employs indirect heating. The melting vessel is steam-jacketed and often also has a steam-heated agitator. The operating pressure can vary from slightly above atmospheric to a vacuum. The latter will require a shorter processing time because the water is evaporated at a lower temperature.

The material is agitated during heat treatment and good heat transfer is obtained. It is, therefore, possible to use lower temperatures than for wet fat melting and to still liberate the fat in one batch within a shorter period of 1.5 - 2 hours. The water in the raw material is removed by

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48 Version November 2003 RHC/EIPPCB/SA_BREF_FINAL evaporation. It is important that the heating process is stopped before all the water is evaporated,

otherwise the product will be discoloured.

After discharge into a percolator, the free fat is drained to an intermediate tank. The greaves are then pressed or centrifuged, and then milled.

As with wet fat melting, the liberated fat is either allowed to settle or is centrifugally separated before packaging.

Dry fat melting requires less space and time than wet fat melting. Due to the process being dry, the fat yielded will not be equally neutral in taste, flavour, or colour, even if the cooking cycle has been carried out correctly. The slightly roasted taste of the fat is a desirable feature in some countries. Compared to wet processing, an advantage is that all of the water is removed by evaporation and there is less waste water contamination, because water is not added to the raw material which would then need to be subsequently removed. The evaporated water will, however, contain some volatile substances released during the drying process. There is a disadvantage, in that certain discolouring elements, which would otherwise have been extracted with the water, now remain in the fat.

The batch dry fat melting/rendering process is illustrated in Figure 2.4.

Figure 2.4: The conventional batch dry fat melting/rendering method [145, Filstrup P., 1976]

Continuous wet fat melting

Continuous processing combines mechanical and thermal treatments in order to minimise the processing time.

The raw material is first passed through a mincer. It is then conveyed to an airtight section, where it is heated in two stages to about 60 ºC and 90 ºC respectively, in a matter of a few minutes.

The heated material is then separated in a decanter centrifuge, especially designed for the continuous removal of solids from a liquid. The solids, known as greaves, leave the plant at this point. The liquid, now consisting mainly of fat, but also containing some water and fines, is

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RHC/EIPPCB/SA_BREF_FINAL Version November 2003 49 subjected to a final purification. In this stage, it is separated into three phases. The “fines” are automatically discharged from the purifier at preset intervals.

The final purified fat flows through a plate heat-exchanger, in which it is cooled down to about 40 ºC before leaving the plant.

The direct injection of steam makes the processing time very short, displaces the air and minimises oxidation of the product. It has been reported that there is no rise in the FFA content or peroxide value in fat processed by continuous wet fat melting.

Continuous wet fat melting takes less time and space than either batch wet fat melting or batch dry fat melting. The fat yield is, however, lower than the batch methods because the effluent water and the greaves contain more of the fat.

The continuous wet fat melting process is illustrated in Figure 2.5.

Figure 2.5: The continuous low temperature wet fat melting system [145, Filstrup P., 1976]

The combinations of techniques in use are numerous. Some further examples also used in rendering are described briefly in Section 2.2.2 and Table 2.5. Their relationship with the quality of fat produced is shown in Table 2.2.

System Quality of fat produced Batch dry rendering/fat melting

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50 Version November 2003 RHC/EIPPCB/SA_BREF_FINAL Continuous drying in recycled fat Low quality fat

Wet pressing or other separation,

evaporation, drying in fat, pressing Fair quality fat Separation, evaporation,

drying in natural fat

Drying difficult with pressure cooking before

Cooking and drying in added fat in

multiple-effect evaporator, pressing Fair quality fat Wet pressing, separation,

multiple-effect evaporation, drying without fat Good quality fat

Table 2.2: The relationship between rendering/fat melting system and the quality of fat produced [289, EFPRA, 2002]