QUANTITATIVE QUALITY TESTS FOR FISH MEAL. II. AN INVESTIGATION OF THE QUALITY OF SOUTH AFRICAN FISH MEALS AND THE VALIDITY OF A NUMBER OF CHEMICAL QUALITY INDICES

INTRODUCTION

Fish meal is produced in South Africa from several species of fish unsuitable for human consumption. These fish include anchovy (Engraulis capensis), red-eye (Etrumeus whiteheadi), maasbanker (Trachurus trachurus), lantern fish (Lampanyctodes hectoris) and pilchard (Sardinops ocellata), the latter largely in the form of cannery waste. The meal is produced by a heating, pressing and drying process and normally consists of about 60–70% protein, 10–12% residual lipid and 8–10% moisture, with the rest consisting of bones and salt (ash). After heating, the fish is pressed, yielding a solid presscake and press liquor. The press liquor is centrifuged and separated into an aqueous phase (stickwater) and fish oil. The stickwater is then subjected to a three-stage evaporation process and added back to the presscake, which is dried in either flame (direct) dryers, or steam (indirect) dryers. Ethoxyquin (1,2-dihydro-6-ethoxy-2,2,4-trimethylquinoline, EQ) is normally added in quantities of about 400 mg/kg immediately after drying to prevent oxidation of the valuable polyunsaturated fatty acids (PUFA). The price of the meal in South Africa is solely determined by its protein content (% nitrogen×6.25) and no account is taken of protein quality. The quality of the meal depends on the freshness of the fish from which it was produced and on processing conditions. Normally, high quality meals are the result of extremely fresh fish and low temperature drying conditions, while spoiled fish inevitably yields poor quality meal.Citation[1] Enzymatic and bacterial spoilage of the fish leads to the formation of volatile nitrogenous bases (TVBN) from the proteinCitation[2] and free fatty acids (FFA) from the lipidsCitation[3] thus these quality indices are frequently used to express meal quality.

The TVBN content of a meal also depends upon drying conditions, i.e., temperature and duration, as some of the bases are volatilised during the drying process. Valid comparisons of TVBN content can therefore only be made on meals that were dried under identical conditions.Citation[2] The FFA content of a meal is also not a reliable index because FFA generated enzymatically increase to a maximum and then decrease.Citation[4] This is most likely due to long chain FFA being degraded into short chain volatile and water soluble FFA, which escape detection when titrating the meal lipids for FFA content. As an addition to these two quality indices the phospholipid (PL) level of a meal has been suggested.Citation[3] Fresh fish have a PL content of 0.5–0.8% (wet fish)Citation[5] which decreases rapidly with spoilage.Citation[3], Citation[6] In addition, PL remain almost exclusively in the meal and very little ends up in the press oil,Citation[3] so the meal’s PL level should be a good index of the original freshness of the fish. This index also differs from the TVBN and the FFA contents in that it does not determine the level of spoilage products which are volatile, as in the case of TVBN, or vulnerable to further breakdown, as in the case of FFA, but rather the remaining quantity of a reasonably constant amount of starting material.Citation[3], Citation[6]

Another factor that affects the TVBN content of a meal is the variable addition of concentrated stickwater to the presscake prior to drying. This addition inevitably increases its TVBN level giving a false estimate of the underlying quality of the meal. This stickwater also contains some PL but its contribution to the PL level of a meal is only in the order of 1–6%, i.e., negligible.Citation[7] In short, the PL level of a meal should be included in expressing the quality of a meal as it accurately measures the quality of the dried presscake apart from the concentrated stickwater.

The quality of a meal may be also adversely affected by drying. This quality deterioration is usually referred to as heat abuse (burning or scorching) and is distinct from the deterioration due to spoilage of the fish prior to processing. Heat abuse destroys the PUFA of the meal lipids and can therefore be readily detected by analysing the meal for PUFA content. The lipids of a properly dried anchovy, pilchard and red-eye meal have PUFA contents of 40–45%, while a meal that has suffered heat damage has lower values.Citation[8]

Meals which have no added EQ inevitably show low PUFA levels that rapidly decrease with time.Citation[9] Such meals, which seldom occurred, have been deliberately excluded from this study. Steam (indirect) dryers are reported to cause more heat abuse than flame (direct) dryers.Citation[10] If so, then this is probably due to the fact that the residence times in steam dryers are much longer than in flame dryers (60–120 min vs. 10–20 min), respectively.Citation[10]

The present report represents a survey of 243 fish meals from seven different South African factories. These meals were evaluated for pre-processing fish spoilage by analysing for TVBN, FFA and PL levels. In addition, 215 meals made from anchovy, pilchard and red-eye (selected from the original 243 by excluding meals such as maasbanker and lantern fish) were evaluated for heat damage by determining PUFA levels. The survey extended over four fishing seasons which were conveniently divided into three: the combined 1996/1997, the 1998 and 1999 season. In addition, the survey critically evaluates the performance of the seven factories, and the validity of the different quality indices. A preliminary report of the results of the 1996/1997 fishing seasons has been issued.Citation[11]