IntroductionActive fishing of common sprat (Clupeonella cultriventris caspia Svetovidov, 1941), anchovy sprat (Clupeonella engrauliformi Borodin, 1904) and bigeye sprat (Clupeonella grimmi Kessler, 1877) in the Caspian Sea contributes to the development of technologies for processing of these fish species taking into account species characteristics [1]. Significantly, increased volumes of harvested sprat and high quality of raw materials make it possible to produce a wide range of food fish products. At the same time, despite the observance of technological regimes fishing organizations are faced with a number of defects: in the process of storage in compliance with the rules and regimes do ISS 32744-2014 “Small frozen fish establish found surface and subcutaneous yellowing of the surface of fish within the shelf life. Technical conditions” [2]. In this regard, the preservation of high quality of frozen raw materials with an active enzyme system, which include Caspian Sea sprat, is extremely important. It is generally believed that the quality of frozen fish during storage is affected by desiccation, which causes hardness and dryness of the fish surface, and fat oxidation, which contributes to the acquisition of rancid taste and unpleasant odor. During storage, the lipids of raw materials are subject to hydrolytic and oxidative processes. If storage conditions are not met, the process of hydrolysis is accelerated and, because of the action of molecular oxygen and biochemical oxidation of unsaturated fatty acids of lipids due to the action of lipoxygenase enzymes, there are deep and irreversible changes in raw materials that reduce quality. Oxidative changes in fats lead to the loss of lipophilic vitamins (A, D, E, K), essential fatty acids (linoleic, linolenic, arachidonic acids) and other biologically active substances [3]. The intensity of these processes depends on a variety of factors, including the initial state of the raw material to be processed, the packaging used, temperature and conditions of further storage. Modern studies indicate that when choosing methods of fish processing it is necessary to take into account the period of extraction (catch) [4].  In accordance with the currently valid ISS 32744-2014 “Frozen small fish. Technical conditions” shelf life of frozen sprat without division into types and catching periods is recommended not more than 6 months [5]. The quality of small frozen fish is determined organoleptic: by determining the characteristic odor, taste, and the presence of surface and subcutaneous yellowing. However, surface yellowing of frozen sprat surface is often noted at the beginning of frozen storage.Earlier studies conducted by scientists on anchovy sprat from the Caspian Sea indicated that subcutaneous yellowing of frozen sprat with a shelf life of 3 months is due to the presence of the pigment taraxanthin. The studies indicated that the skin of freshly caught ancho sprat contained 1.35-1.5% mg of caratinoid substances, while the fat extracted from muscle tissue contained 0.2 to 0.3% mg of caratinoids. After a short-term freezing process of 6 hours at a temperature of –18 °C, the skin contains from 0.2 to 0.3% mg of caratinoid substances, while in tissue fat their amount is from 1.3 to 1.4% mg, which indicates the migration of these substances from the skin to muscle tissue during freezing and storage [3]. Accordingly, surface yellowing, which is detected at the beginning of storage, is not associated with oxidative processes and is not a factor indicating a decrease in fish quality. Earlier studies were based on the technological characteristics of the anchovy sprat population, as this object was the basis of the fishery. At present, the main share of catches is common sprat. The maximum production of common sprat by a multi-depth trawl was reached in 2023 and amounted to 31.8 thousand tons, which is higher than ever in the last 20 years. In this connection, the research on establishing the causes of yellowing was carried out for the common sprat with the establishment of shelf life of frozen food products from it.The aim of the work is to identify the causes of yellowing of frozen sprat and to propose ways to prevent their occurrence. Objects and methods of researchFrozen common sprat from the spring, fall and winter periods of the marine commercial fishery (trawl catches of the vessel “Sailor's star”) was used as the objects of the study. Sampling was carried out in accordance with GOST 31339-2006 [6]. Organoleptic characteristics were investigated by the method of sensory analysis with subsequent construction of profilograms. The intensity of change in organoleptic parameters (odor, consistency, subcutaneous yellowing, surface yellowing, and gloss) during storage at a temperatureof –18 °C for 6 months was evaluated on a five-point scale (Table 1). Table 1 Organoleptic evaluation of frozen sprat quality in the process of storageMarkCharacteristicsSmellConsistencySubdermal yellowingSuperficial yellowingSurface lustre5NeutralElastic, denseVividly expressedover the entire surface of the fishVividly expressed over the entire surface of the muscle tissueVividly expressed, surface clean, without spots4Slightlypronounced,no foreign odourDensePresent over the entire surface of the fishPresent over the entire surface of the muscle tissueLustre present, surface clean3Moderatelypronounced,no foreign odourNot smearing, weakenedPresent in fish bellyand head areaPresent in fish belly and head areaLustre partially present2Moderatelypronounced, foreign odour presentNot significantly weakenedPresent on the bellyof the fishPresent on the entire surfaceNot homogeneous1Distinctlypronounced, foreign odour presentWeakenedSlight presence on the surface of the fish bodySlight presence in the abdomen area of the fishFish surface tarnished0Unpleasant, foreign odour presentSignificantly weakenedAbsent over the entire surface of the fishAbsent over the entire surface of the fishAbsent  Fat content in muscle tissue was determined according to ISS 7636-85 [7]. Determination of fat quality indicators (acid, aldehyde number) of frozen sprat was carried out on the fifth (control), thirtieth day and subsequently every month during 6 months of storage at a temperature of –18 °C in accordance with methodological guidelines 4.2.1847-04 “Sanitary-epidemiological evaluation of justification of shelf life and storage conditions of food products” [8]. Preparation of samples for determination of fatty acid composition of fat was carried out according to ISS 31665-12 [9], fatty acids content was determined according to ISS 31663-2012 on chromatograph Kristall 5000 [10].  Acid number of fat was determined by titrimetric method according to ISS 7636-85 [7]. Aldehyde number was determined on a photocolorimeter.  Microbiological parameters (QMAFAnM, BLCP, S.aureus, sulfite-reducing clostridia, listeria and V. rarapoliticus) were determined according to ISS 10444.15-94, ISS 31746-2012, ISS 31747-2012 [11-13].  Mathematical processing of the obtained results was carried out in Microsoft Excel by calculating correlation dependencies using the nonlinear regression method. Results and discussion The study of changes in organoleptic parameters of common sprat stored for 180 days at a temperature of –18 °C, without glazing, packed in a polyethylene bag, harvested in the spring, autumn and winter fishing periods showed that the surface of the studied frozen blocks during the first month of storage is frequent, without signs of oxidation and foreign odor. Surface yellowing of common sprat appears in the first month of storage and is distributed over the entire surface of the fish body, foreign odor is absent, and consistency is elastic. After four months of storage at a temperature of –18 °C, surface and subcutaneous yellowing was observed in common sprat from the fall and winter fishing periods (Fig. 1).                    а                                                                                                                          b  Fig. 1. Increase in yellowing of the muscle tissue of common sprat during storage:a – storage for 120 days; b – storage for 180 days During storage of common sprat at –18 °C, subcutaneous yellowing of muscle tissue is not uniform: it starts from the abdomen and gradually increases to the tail part of the body (Fig. 1). The process of surface and subcutaneous  yellowing  in the common sprat caught in different periods of prey proceeds with different intensity. The change in the organoleptic profile based on the organoleptic quality assessment score of frozen common sprat (Table 1) from the spring catch period during storage is shown in Fig. 2.  Fig. 2. Changes in the organoleptic profile of common sprat duringthe spring fishing period at a temperature of –18 ºC  The fat content in the muscle tissue of common sprat from the spring fishing period is 4.88 % (± 0.05). Organoleptic characteristics begin to change insignificantly after 180 days of storage at –18 °C. Accordingly, the common sprat caught in the spring fishing period during 6 months keeps the quality indicators established in ISS 32744-2014 “Small frozen  fish.  Techni-cal conditions” [5].Common sprat caught in the fall fishery contains twice as much fat in muscle tissue 8.2 % (± 0.05) compared to spring-caught common sprat, which is reflected in its organoleptic characteristics during frozen storage (Fig. 3).  Fig. 3. Changes in the organoleptic profile of common sprat duringthe autumn fishing period at a temperature of –18 ºC  Common sprat of autumn fishery retains organoleptic characteristics during the shelf life established by ISS 32774-2014 “Fish small frozen terms. TC” [5]. At the same time, after 120 days of storage luster of fish surface is lost, slight yellowing of fish surface is observed. The consistency of muscle tissue elastic, the smell of fish is weakly expressed. After 150 days of storage, deterioration of organoleptic characteristics is observed: surface and  subcutaneous  yellowing,  the  consistency of fish becomes weaker.More intense surface and subcutaneous yellowing is found in the common sprat from the winter period of catch, the fat content in the muscle tissue of which is at the level of 14.5% (±0.03). After 3 months of storage at a temperature of –18 °C yellowing is found on the surface of fish with further penetration of yellowing into the subcutaneous layers of muscle, with the formation of odor characteristic of oxidative processes of fat. Oxidative processes are more active in fish in the superficial layers of the block. Changes in the organoleptic profile of common sprat from the winter period of fishing at a temperature of –18 °C are shown in Fig. 4.Fig. 4. Change in the organoleptic profile of common sprat during the winter fishing period at a temperature of –18 ºC  Oxidative and hydrolytic processes during storage of frozen common sprat show different changes in organoleptic parameters depending on the fatty acid composition. The data presented in Table 2 indicate changes in the fatty acid composition of common sprat by season of fishing, physiological processes cause this fact: the spring period is characterized by maturation of gonads; nutrients are spent on the formation and maturation of sexual products [14].  Table 2 Fatty acid composition of common sprats, %AcidCommon sprat from the marine fisheryspringwinterSaturated fatty acids (SFA)Myristic acid (С14:0)3.82.56Palmitic acid (С16:0)22.719.08Stearic acid (С18:0)5.14.94Arachinoic acid (С20:0)0.20.2Behenoic acid (С22:0)0.20.18Total saturated fatty acids32.026.78Monounsaturated fatty acids (MNFA)Miristoleic acid (С14:1)0.30.8Palmitoleic acid (С16:1)5.812.1Oleic acid (С18:1)33.632.3Elaidinic acid (С18:1t)0.1–Erucic acid (С22:1)0.10.15Total  monounsaturated acids39.945.35Polyunsaturated fatty acids (PUFA)Linoleic acid (С18:2)1.63.04linolenic acid (С18:3)0.20.11Alpha-Linolenic acid (C18:3)1.81.5Eicosatrienoic acid (С20:3n6c)0.10.16Arachidonic acid (С20:4)0.60.62Eicosapentaenoic acid  (С20:5)5.310.25Docosahexaenoic acid (С22:6)14.514.24Total polyunsaturated fatty acids24.129.92Correlation SFA: MNFA: NFA1.3 : 1.6 : 11 : 1.7 : 1.1  In the studied samples of sprat ordinary, the presence of 17 fatty acids was revealed. The spectrum of higher fatty acids is presented from myristic S14:0 to docosahexaenoic S22:6. According to the data obtained, MNFA and saturated fatty cysts constitute the main group of fatty acids of sprat lipids. The results obtained indicate a rather high level of PUFA content in the muscle tissue of sprat (24.1 in spring and 29.92 in winter, in %), while this group of fatty acids has a high sensitivity to oxidation due to the presence of unsaturated double bonds. Accordingly, the greater the PUFA content, the deeper the oxidative processes. In connection with the above, the  accumulation  of tissue fat hydrolysis products, to which free fatty acids belong was studied based on the change in the acid number of sprat fat of various production (catch) seasons during storage for 180 days at a temperatureof –18 °C (Fig. 5). Fig. 5. Changes in the acid number of common sprat fat during storage  The increase in the acid number (Fig. 5) characterizes the amount of primary lipid oxidation products (peroxides and hydroperoxides). An intensive increase in the acid number occurs in the first 60 days, after which the intensity of the process decreases. At the same time, this trend does not depend on the period of catch of sprat.As you know, the process of oxidation of fish fats begins with unsaturated fatty acids with the formation of free radicals and peroxides; they contribute to the destruction of fish pigments, a change in the organoleptic properties of the product during storage. Carriers of a bitter taste and smell are volatile products of the breakdown of peroxides, mainly aldehydes. In this regard, during the studies, a change in the aldehyde number of sprat fat of ordinary storage for 180 days at a temperature of –18 °C was found (Fig. 6).  Fig. 6. Changes in the aldehyde number of common sprats during storage  Results of aldehyde number accumulation studies (Fig. 6) show a smooth increase in fat oxidation products throughout the storage period. In the first 60 days of storage, the accumulation of aldehydes occurs smoothly, with a sharp increase after 60 days of storage. The accumulation of oxidation products occurs faster in the raw materials of winter fishing. The data obtained are consistent with the results of organoleptic assessment: superficial and subcutaneous yellowing is visually detected at the third month of storage of raw materials. Previously published studies for a similar type of raw material indicate that an increase in the aldehyde number of ≈ 15 ‰ has a significant effect on the formation of superficial yellowing of anchovy sprat [15]. Studies and calculation of the correlation of the growth of aldehyde and acid chi-villages during storage on the appearance of surface and subcutaneous yellowing by non-linear regression indicate the same level of influence of these oxidative processes of fat (Fig. 7).  Fig. 7. Assessment of the degree of influence of oxidative fat processes on the appearanceof surface yellowing during storage of common sprats: a – effect of aldehyde number increase on surface yellowing;b – effect of formation of acid number on formation of surface yellowing  Accordingly, the formation of superficial and subcutaneous yellowing during chilling is influenced not only by an increase in the acid number, but also by the aldehyde number, which must be taken into account when monitoring the quality of sprat when setting the timing of chilling. The quality of sprats during storage was also assessed by microbiological parameters. No yeast, molds, coliform bacterias, pathogenic microorganisms (including salmonella), S. aureus, sulfite-reducing clostridia, V. paranoliticus, Listeria were detected in the test samples during 180 days of storage. The number of mesophilic aerobic and facultative anaerobic microorganisms, CFU/cm3, g, does not exceed the limits established by technical regulations. During the time set in ISS 32744-2014 “Small frozen fish. ТC” [5] shelf life of sprat is safe in terms of microbiological indicators, regardless of the period of production (catch). To preserve the quality of ordinary sprats frozen in the 32744-2014 established by ISS “Small frozen fish. ТC” [5] shelf life should be taken into account for the period of production (catch), modern methods of treatment should be introduced into the technological process, which can slow down oxidative processes (ultra-high pressure treatment, exposure to  ultrasound, use of protective coatings). ConclusionsPreservation of sprat quality during storage is due to a combination of biochemical, chemical and physical factors. Studies have shown that when the sprat is frozen, processes similar to those that occur when the sprat is frozen by anchovy and bigeye: the formation of yellowing of the top of the fish is observed. Superficial yellowing, which is detected immediately after the process of fermentation of sprat, is due to the transition of the taraxanthin pigment from the skin to muscle tissue. The reason for the appearance of surface yellowing, which is found during the storage of sprats in frozen form after several months of storage, depends on the period of production (catch) and is a consequence of qualitative changes in fats. The fat content in sprat is subject to significant fluctuations over the seasons. Sprat, common, caught in the spring and autumn, belongs to medium-fat fish species, in winter – to fatty ones. The qualitative composition of sprat fat is also changing: in the spring, when the costs of the fish organism for the formation of sexual products occur, the level of MNFA and PUFA content decreases. The largest amount of PUFA was recorded in the oil of sprats of the ordinary winter period of production (catch), which must be taken into account when choosing processing methods and setting shelf life. When monitoring the quality of sprat during storage, it is necessary to take into account not only the change in the level of the acid number of fat, as previously thought, but also the change in the level of the aldehyde number of fat. The studies carried out indicate the need for further work on the establishment of methods for extending the shelf life of frozen sprat of autumn and winter washing by examining preservatives that slow down the processes of yellowing of ordinary sprat.