Lipid oxidation in herring fillets (Clupea harengus) during ice storage measured by a commercial hybrid gas-sensor array system

Volatile compounds released from herring fillets (Clupea harengus) during 15 days of storage on ice have been measured with a commercial hybrid gas-sensor array system. Using partial least-squares regression modeling, the sensor responses were correlated with data from chemical analyses (lipid oxidation products and antioxidants) and sensory analyses (odor). Eight of the 16 sensors proved significant in the correlation studies: 6 metal oxide semiconductor field effect transistor (MOSFET) sensors and 2 Taguchi type sensors. Correlation coefficients for chemical and sensory data ranged from 0.9 to 0.98 and from 0.49 to 0.92, respectively, with 0.92 referring to both "sharp/acrid" and "rancid" odors. Prediction errors ranged from 8 to 14% and from 11 to 25% for the chemical and sensory measures, respectively. That the prediction errors for oxidation product formation (5-9%) were close to the analytical errors of the chemical reference methods indicated close to "optimum" performance of the gas-sensor system. The sensor system predicted the storage time of the herring with a 1-day error. Results illustrate high potential of the gas-sensor technology in rapid nondestructive quality determination of ice-stored herring.     

Lipid oxidation in fillets of herring (Clupea harengus) during frozen storage. Influence of prefreezing storage.

Fillets of herring (Clupea harengus) were kept on ice for 0, 3, 6, and 9 days prior to storage at -18 degrees C for 0, 21, 42, 63, and 84 days. At each storage point, peroxide value (PV), absorbance at 268 nm (A(268)), fluorescent products (FP), alpha-tocopherol, glutathione peroxidase (GSH-px) activity, and ascorbic acid were measured. As shown by regression analyses, samples held for 6 days on ice formed oxidation products at the highest rate during frozen storage, followed by, for PV and FP, the 9-day samples. These data indicate that severe changes that negatively affect the oxidation process took place in the herring muscle between 3 and 6 days after catch. Both the initial antioxidant levels and the rate of antioxidant loss at -18 degrees C decreased with increased prefreezing holding time, the latter being most obvious for GSH-px activity and ascorbic acid. alpha-Tocopherol showed the largest losses and had disappeared entirely from the 6- and 9-day samples at the end of the frozen storage. Partial least-squares regression analysis of the data showed that ice storage had a greater effect than frozen storage on changes in PV, A(268), FP, alpha-tocopherol, and ascorbic acid. For GSH-px activity, frozen storage had the greatest effect.     

Preventing lipid oxidation during recovery of functional proteins from herring (Clupea harengus) fillets by an acid solubilization process

It has previously been found that a process based on solubilization at pH 2.7 gives high yields of herring muscle proteins with good functionality. In this study, the development of lipid oxidation during acid processing of herring mince was studied. It was tested how modifications of the process conditions and/or additions of antioxidants could prevent lipid oxidation during the actual process and then during ice storage of the protein isolates. Processing parameters evaluated were prewash of the mince, exposure time to pH 2.7, inclusion or exclusion of a high-speed centrifugation, and addition of antioxidants. Antioxidants tested were erythorbate (0.2%, 9.3 mM), sodium tripolyphosphate (STPP; 0.2%, 5.4 mM), ethylenediaminetetraacetic acid (EDTA; 0.044%, 1.5 mM), and milk proteins (4%). The first three antioxidants were added in the prewash or during the homogenization step, whereas milk proteins were added to the final precipitate. At time 0, all isolates were analyzed for pH, moisture content, and thiobarbituric reactive substances (TBARS). Selected isolates were also analyzed for lipid and protein content. Stability during ice storage was followed in terms of odor, TBARS, and color (a/b values). Extensive lipid oxidation took place using the "control" process without high-speed centrifugation. This was not significantly (p < or = 0.05) affected by a prewash or varied exposure time to pH 2.7. Including high-speed centrifugation (20 min, 10,000g) significantly (p < or = 0.05) reduced TBARS values, total lipids, a values and b values. Erythorbate alone, or in combination with STPP/EDTA, significantly (p < or = 0.05) reduced lipid oxidation during processing if added in the prewash or homogenization step. During ice storage, better stability was gained when antioxidants were added in both of these steps and when EDTA was used instead of STPP.     

Characterization of oxidative changes in salted herring (Clupea harengus) during ripening

Salted herring were prepared in barrels according to a traditional recipe. The biochemical changes in the fish and in the brine were monitored during a prolonged ripening period (12 months). The process was followed by measuring pH, protein, salt, dry matter, free fatty acids, and lipid content in the brine and in the fish according to standard protocols. The results showed that most of the biochemical changes occurred at an early stage in the ripening process. Lipid oxidation was followed in the fish muscle using spectroscopic determination for lipid hydroperoxide (PV) and by GC-MS for determination of secondary oxidation products. Protein oxidation was determined using spectrophotometric determination of protein carbonyl groups. To follow protein degradation (proteolysis) and protein oxidation SDS-PAGE and immunoblotting for protein carbonyl were performed on both brine and fish during the ripening period. Results revealed that no lipid oxidation occurred in fish muscle during ripening but a significant level of protein oxidation was detected. Finally, iron alpha-tocopherol, and 3-methylbutanal levels were also measured. Alpha-tocopherol levels decreased during ripening, further supporting that oxidative reactions took place. Peroxidase activity was demonstrated in the brine, suggesting that hemoglobin might be a crucial parameter, which might trigger protein oxidation. This indicates that protein oxidation might be important for the development of the characteristic organoleptic properties of salted herring.     

Effect of milk protein concentrate on lipid oxidation and formation of fishy volatiles in herring mince (Clupea harengus) during frozen storage

The effect of milk protein concentrate (MPC) at 0, 2, 4, and 6% on lipid oxidation and volatile formation in frozen stored herring mince (-18 degrees C) was evaluated by analyzing samples at 0, 2, and 4 months for fatty acid composition, volatiles, and thiobarbituric acid reactive substances (TBARS). Sensory evaluation was also conducted to assess the intensity of fishy odor, and the volatiles were analyzed using static headspace gas chromatography-mass spectrometry (SHGC-MS). The addition of 4 and 6% MPC to herring mince resulted in a 33% and 50% reduction of TBARS, respectively, at month 4 and lessened the intensity of fishy odor throughout storage. However, MPC did not protect fatty acids from enzymatic degradation unless it was added immediately after mincing. Volatile analysis using SHGC-MS showed that 4% MPC was able to reduce headspace volatiles associated with fishy odor. MPC is most effective for reducing 4-heptenal, 3-methyl-1-butanol, 2-hexenal, and 1-penten-3-ol, which are known to be potent odorants associated with lipid oxidation.     

Fatty acids and fat-soluble vitamins in salted herring (Clupea harengus) products

The fatty acid composition and contents of fat and fat-soluble vitamins of three salted products prepared from Icelandic herring were analyzed. The effects of storage on the products over their shelf life, 6 or 12 months, were investigated. The average oil content of salted, gutted herring and salted fillets in vacuum remained constant, 17 and 12% of wet weight, respectively. In the pickled product the oil content decreased during the 12 months of storage from 13 to 12%. The composition of the products was typical for herring, the most abundant fatty acids being oleic (18:1n-9), palmitic (16:0), cetoleic (22:1n-11), and gadoleic (20:1n-9) acids. Monounsaturated acids constituted clearly the main group with a proportion of >50% of all fatty acids. Eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) comprised together >12% of all fatty acids. During storage, some hydrolysis of triacylglycerol (TAG) occurred, causing a slight reduction in practically all esterified fatty acids. In none of the three products was the loss of polyunsaturated fatty acids from TAG greater than the loss of saturated ones, indicating that the loss of EPA and DHA was not due to oxidation. After packing, the average content of vitamins A, D, and E in the products varied between 27 and 87 microg/100 g (wet weight), between 17-28 microg/100 g (wet weight), and between 77-120 microg/100 g (wet weight), respectively. During storage, the level of vitamin A decreased significantly, whereas no loss of vitamin D was observed. The content of vitamin E was low in all products and showed wide variation. When compared to the recommended daily intake, it could be concluded that the products investigated were good and stable sources of long-chain n-3 fatty acids (EPA, DHA) and vitamin D.     

Changes in the antioxidative property of herring (Clupea harengus) press juice during a simulated gastrointestinal digestion

The aqueous fraction (press juice, PJ) from herring muscle was recently shown to inhibit hemoglobin-mediated oxidation of washed fish mince lipids during ice storage. As a first step to evaluate potential in vivo antioxidative effects from herring PJ, the aim of this study was to investigate whether herring PJ retains its antioxidative capacity during a simulated gastrointestinal (GI) digestion. Press juice from whole muscle (WMPJ) and light muscle (LMPJ) was mixed with pepsin solution followed by stepwise pH adjustments and additions of pancreatin and bile solutions. Digestive enzymes were removed from samples by ultrafiltration (10 kDa). Before, during, and after digestion, samples were analyzed for their peptide content and for antioxidative properties with the oxygen radical absorbance capacity (ORAC) and the low-density lipoprotein (LDL) oxidation assays. From 0 to 165 min of digestion, the content of <10 kDa peptides in WMPJ and LMPJ samples increased 12- and 7-fold, respectively. Further, both samples got approximately 12.5 times higher ORAC values and gave rise to approximately 1.3-fold increased lag phase in Cu2+-induced LDL oxidation. The largest changes in peptide content, ORAC values, and LDL oxidation inhibition occurred between 30 and 75 min of digestion, indicating that these parameters might be interrelated. When comparing analytical data obtained after 165 min of digestion with data obtained from analyses of native nondigested PJs, it was found that the data on peptide content, ORAC, and LDL oxidation from digested PJs were 64-69%, 121-161%, and 112-115%, respectively, of those of nondigested PJs. The study thus showed that enzymatic breakdown of PJ proteins under GI-like conditions increases the peroxyl radical scavenging activity and the potential to inhibit LDL oxidation of herring PJs. These data provide a solid basis for further studies of uptake and in vivo activities of herring-derived aqueous antioxidants.     

Antioxidative properties of press juice from herring (Clupea harengus) against hemoglobin (Hb) mediated oxidation of washed cod mince

The antioxidative effect of herring (Clupea harengus) light muscle press juice (PJ) against hemoglobin-(Hb-) mediated oxidation of washed cod mince during ice storage was tested. The PJ was fractionated into low-molecular-weight (LMW; <1 [corrected] kDa) and high-molecular-weight (HMW; >1, >3.5, and > 50 kDa) fractions; it was preheated (10 min, 100 degrees C) and tested with or without removing heat coagulated proteins. Its antioxidative effect was compared with that given by endogenous levels of two tentative antioxidant candidates: ascorbic acid and uric acid. Oxidation was followed by determining rancid odor, peroxide value, and redness. Whole herring PJ and the LMW-PJ fraction significantly (p < 0.001) extended the oxidation lag phase of controls, from 2 up to 8 and 7 days, respectively. The HWM-PJ fractions were significantly (p < 0.05) less efficient than the whole and LMW-PJ samples, giving only 3.5-4.5 days of lag phase. Heat-treated PJ, with and without the heat-coagulated proteins, gave 7 and 5 days of oxidation lag phase, respectively. Heating different batches of the LMW-PJ fraction grouped the results into two categories: one where heating almost fully destroyed the antioxidative activity (fractions prepared from spring-caught herring) and another where heating had no or a minor effect (fractions prepared from fall-caught herring). The spring LMW-PJ had low ascorbic acid levels (18-42.6 microM), and 50-100% were destroyed by the heating. In fall LMW-PJ, the levels were 76.2-137.6 microM, and only 43-51% were destroyed. Ascorbic acid fortification of heated spring LMW-PJ to reach the levels found in the corresponding unheated spring LMW-PJ sample and the heated fall LMW-PJ gave back most of the antioxidative activity, which proved an important role of ascorbic acid for the antioxidative activity of LMW-herring PJ. This conclusion is drawn despite the fact that pure solutions with endogenous levels of ascorbic acid (giving 8.4-19.6 microM in final model) only very slightly delayed Hb-mediated oxidation of the washed cod mince.     

Properties of protein powders from arrowtooth flounder (Atheresthes stomias) and herring (Clupea harengus) byproducts

Functional, nutritional, and thermal properties of freeze-dried protein powders (FPP) from whole herring (WHP), herring body (HBP), herring head (HHP), herring gonad (HGP), and arrowtooth flounder fillets (AFP) were evaluated. The FPP samples have desirable nutritional and functional properties and contained 63-81.4% protein. All FPP samples had desirable essential amino acid profiles and mineral contents. The emulsifying and fat adsorption capacities of all FPP samples were higher than those of soy protein concentrate. The emulsifying stability of WHP was lower than that of egg albumin but greater than that of soy protein concentrate. Thermal stability of the FPP samples is in the following order: HGP > HBP > WHP > HHP > AFP.     

Consistency and solubility changes in herring (Clupea harengus) light muscle homogenates as a function of pH

Fish muscle proteins can be isolated from a variety of low-value raw materials by solubilization in either acid or base. If the consistency of the resulting solution is sufficiently low, it is possible to recover most of the solubilized proteins and remove most of the lipids by centrifugation. Lipid removal should greatly stabilize the isolated proteins. In a previous investigation into the use of herring for production of these protein isolates, it was observed that this species had particularly high consistency values when the proteins were solubilized. This study was undertaken to determine the consistencies obtained with herring light muscle tissue over the pH range covered by the two processes, from about pH 2.7 to 10.8. Protein solubility was compared to consistency of the resultant solutions. Maximum consistencies of the homogenates, approximately 220 and approximately 175 mPa.s, were obtained at pH values of approximately 3.5 and 10.5, respectively. Consistency began to increase approximately when solubilization began. Storage of homogenates at pH 2.7 decreased the consistency over a 10 min time period. The magnitude of the consistency peaks at both acid and alkaline pH values increased when using ice-stored as well as frozen-stored herring, especially in the acid range. Protein solubility at pH <4 and pH >/=10.8 slightly decreased after post-mortem storage of the herring muscle. It is suggested that the observed changes in consistency result from the expansion and solvation of protein aggregates which eventually dissociate into smaller units, perhaps even monomers.     

Effect of hot smoking on the content of selected polychlorinated biphenyl congeners in herring (Clupea harengus) slices

Herring ( Clupea harengus) (as well as sprat and mackerel) is a fish species intensively harvested by the Polish fishing fleet. Its relatively low price and the habits of Polish consumers contribute to its high economic importance in Poland. Among 125.6 thousand tons of marine fish harvested in 2006, herring constituted 22.2 thousand tons (including 7.6 thousand tons harvested from the North Sea). Fish are usually subjected to various culinary treatments, which can influence the concentrations of contaminants in final products. Therefore, the present study aimed at determining the effect of three non- ortho-polychlorinated biphenyl (PCB) congeners (PCB 77, PCB 126, and PCB 169) and three mono- ortho-PCB congeners (PCB 114, PCB 156, and PCB 157) in the tissues of herring slices. Concentrations of dry matter and lipids in the final product were increased by 15.9 and 20.5%, respectively. Brining, which preceded the process of smoking, did not significantly influence concentrations of PCB congeners in herring slices. However, smoking resulted in significant ( p < 0.05) changes in concentrations of three non- -ortho PCBs. The biggest dynamics of losses in relation to smoking duration was observed for PCB 77 (30.2%). For PCB 126, the biggest losses occurred after 1 h of drying, followed by slight changes after 1.5 and 2 h of proper smoking and another increase of losses in the final product after 2.5 h. The duration of smoking did not significantly influence PCB concentrations in wet weigh, as their losses in lipids were much bigger. The observation is derived from strong negative correlations between concentrations of the congeners in lipids and lipid concentrations in herring slices during hot smoking ( r ranging from -0.824 to -0.950). Although the lipid concentration in the smoked fish increased, the PCB content diminished. It shows that during drying of the fish slices, PCBs codistilled with water vapor, settling on the walls of the smoking chamber and smoking trolleys. During the proper smoking, the compounds released with the smoke back to the smoking chamber and settled on the surface of smoked fish together with the disperse phase of the smoke (aerosol). Hot smoking contributed to a reduction of toxic equivalents (TEQs) by 22.7% (converted into lipids 41.9%), and in the final product, it amounted to 0.0188 +/- 0.0023 ng TEQ/kg of wet weight (0.0977 +/- 0.0064 ng TEQ/kg of lipids). The reduction was significantly higher than resulting from hot smoking of mackerel slices, when TEQs were reduced by 17.9% wet weight (converted into lipids 31.7%).     

Recovery of functional proteins from herring (Clupea harengus) light muscle by an acid or alkaline solubilization process

Proteins from herring (Clupea harengus) light muscle were extracted using acidic or alkaline solubilization; 92 and 89% of the initial muscle proteins were solubilized at pH 2.7 and 10.8, respectively, of which 96 and 94% were recovered during precipitation at pH 5.5. Consistency of the pH-adjusted muscle homogenates increased with increased raw material age and homogenization intensity; it declined following holding on ice. Some hydrolytic myofibrillar protein degradation occurred during cold storage of the acidified (pH 2.7) homogenates. With alkalized homogenates, hydrolysis was negligible. The total lipid content changed from 0.13 g/g of protein in the muscle to 0.04 g/g of protein in both the acid- and alkali-produced protein isolates. Corresponding values for the phospholipid content were from 0.037 to 0.02 g/g of proteins. Acid- and alkali-produced proteins made gels with equal strain and color. Stress values were equal or lower in acid- versus alkali-produced protein gels. When ice-stored raw material was used, strain and stress values of gels were reduced.     

Effects of season and processing on oil content and fatty acids of baltic herring (Clupea harengus membras)

Fatty acid composition, oil content, free fatty acid content, and peroxide value of Baltic herring (Clupea harengus membras) and two processed products (fried fillets and fish burgers) were investigated. The highest oil content of the fillets was found in autumn (10%), at the time when the free fatty acids had their minimum (1.4%). The main fatty acids were oleic (18-23%), palmitic (17%), palmitoleic (8-12%), and docosahexaeneoic (8-10%) acids. The proportion of saturated fatty acids was a constant 23% all year around, whereas mono- and polyunsaturated acids varied from 34 to 39% and 33 to 37%, respectively. During processing the oil content doubled and the fatty acid composition changed to the pattern of the rapeseed oil used for frying. Oleic acid was a major fatty acid in the products comprising over 40% of the total fatty acids. The proportion of n-3 acids decreased during processing but the total amount of polyunsaturated acids remained fairly constant.     

Determination of semivolatile compounds in Baltic herring (Clupea harengus membras) by supercritical fluid extraction-supercritical fluid chromatography-gas chromatography-mass spectrometry

The on-line supercritical fluid extraction-supercritical fluid chromatography-gas chromatography method was applied to the determination of volatile compounds of raw and baked Baltic herring (Clupea harengus membras). The analytes were extracted with supercritical carbon dioxide at 45 degrees C and 10 MPa pressure. After extraction, the volatiles and coeluted lipids were separated on-line using supercritical fluid chromatography and the volatile fraction was introduced directly into a gas chromatograph. In all, 30 compounds were identified from fish samples with mass spectrometry. The most abundant compounds in the fresh Baltic herrings were heptadecane and 1-heptadecene. When the fish were stored for 3-6 days at 6 degrees C, the total peak area of the volatiles began to increase and the proportions of short chain acids (acetic, propanoic, 2-methylpropanoic, and 3-methylbutanoic) also increased. After 8-9 days of storage, 3-methylbutanoic acid comprised about 36 and 40% of all volatiles in raw and baked herring, respectively.     

Hemoglobin-mediated lipid oxidation and compositional characteristics of washed fish mince model systems made from cod (Gadus morhua), herring (Clupea harengus), and salmon (Salmo salar) muscle

The use of washed cod light muscle minces in mechanistic studies of hemoglobin (Hb)-mediated fish lipid oxidation has largely increased in the past 5 years. Although cod light muscle has a low level of intrinsic lipid oxidation catalysts, a prerequisite for a good oxidation model system, we believe it cannot fully mimic the oxidation kinetics taking place in other fish species being more susceptible to lipid oxidation. The aim of this study was to systematically investigate whether washed mince model systems useful in Hb-mediated oxidation studies could be prepared also from herring (Clupea harengus) and salmon (Salmo salar) light muscles. The kinetics of oxidation in the washed models was measured during ice storage (+/-Hb), and the results were related to compositional differences. Minces from cod, herring, and salmon light muscles were washed 3 times with 3 volumes of water and buffer. A 20 microM portion of Hb and 200 ppm streptomycin was then added, followed by adjustment of pH and moisture to 6.3 and 86%, respectively. Samples with or without Hb were then stored on ice, and oxidation was followed as peroxide value (PV), rancid odor, redness (a*) loss and yellowness (b*). Prior to storage, all minces and models were also analyzed for total lipids, fatty acids, alpha-tocopherol, proteins, Hb, Fe, Cu, and Zn. Hb-mediated lipid oxidation appeared within 2 days on ice in all models. Small differences in the oxidation rates ranked the models as herring > cod > salmon. These differences were ascribed to more preformed peroxides and trace elements in the herring model, and more antioxidants in the salmon model. Controls, without Hb, stayed stable in all cases except herring, where a very slight oxidation appeared, especially if the herring raw material had been prefrozen. In conclusion, fattier fish like dark muscle species and salmonoids are useful for making washed mince model systems and would be a better choice than cod if there is an interest in the oxidation kinetics of such species.     

Inhibitory effect of known antioxidants and of press juice from herring (Clupea harengus) light muscle on the generation of free radicals in human monocytes

Reactive oxygen species (ROS) can cause oxidative stress, which has been linked to various diseases. It has been suggested that antioxidant-rich foods can reduce such oxidative stress. However, the lack of suitable model systems to screen for in vivo effects of food-derived antioxidants has prevented a clear consensus in this area. In this study, the aim was to use a single-cell model system (human monocyte) to evaluate whether certain pure antioxidants and complex muscle extracts (herring light muscle press juice, PJ) could prevent ROS formation under in vivo like conditions. ROS were excreted from the monocytes upon stimulation with phorbol myristate acetate and were then detected as isoluminol-enhanced chemiluminescence (CL). Adding 2000 units of catalase and 50 units of superoxide dismutase to the monocytes model lowered the CL response by 35 and 86%, respectively. Ascorbate (14.1 mM) lowered the response by 99%, alpha-tocoperhol (188 microM) by 37%, and Trolox (50 microM) by almost 100%. Crude herring PJ gave a dose-dependent reduction in the CL response. At 10, 100, and 1000 times dilution, the PJ reduced the CL signal by 93, 60.5, and 10.6%. PJ fractionated into low molecular weight (LMW) (<1000 Da) and high molecular weight (>3500 Da) fractions decreased the CL response by 52.9 and 71.4%, respectively, at a 100-fold dilution. Evaluation of the PJ samples in the oxygen radical absorbance capacity test indicated that proteins may be the primary radical scavenging compounds of PJ, whereas the ROS-preventing effect obtained from the LMW fraction may also be attributed to other mechanisms. Thus, this study proved that the monocyte assay can be a useful tool for studying whether food-derived antioxidants can limit ROS production under physiologically relevant conditions. It also showed that herring contains numerous aqueous compounds demonstrating antioxidative effects in the monocyte model system.     

Changes in Salt Solubility and Microstructure of Proteins from Herring ( Clupea harengus ) after pH-Shift Processing

Salt solubility of pH-shift isolated herring ( Clupea harengus ) muscle proteins was studied in relation to pH exposure and microstructure using transmission electron microscopy (TEM). Using protein solubilization at pH 11.2 with subsequent precipitation at pH 5.5, salt solubility of the proteins decreased from 78 to 17%. By precipitating the alkali-solubilized proteins at the pH of native herring muscle, 6.5, salt solubility only decreased to 59%, proving that pH values between 6.5 and 5.5 affected protein salt solubility more than the pH cycle 6.5 → 11.2 → 6.5. Precipitation at pH 5.5 resulted in hydrogen bonds, hydrophobic interactions, and S-S bridges, whereas precipitation at pH 6.5 resulted only in the formation of hydrophobic interactions. The alkaline pH-shift isolation process severely rearranged the protein microstructure, with precipitation at pH 6.5 forming a finer, more homogeneous network than precipitation at pH 5.5. The former protein isolate also contained less lipid oxidation products and formed more deformable gels, without affecting protein yield.     

Protein and lipid oxidation during frozen storage of rainbow trout (Oncorhynchus mykiss)

This study aimed at investigating protein and lipid oxidation during frozen storage of rainbow trout. Rainbow trout fillets were stored for 13 months at -20, -30, or -80 degrees C, and samples were analyzed at regular intervals for lipid and protein oxidation markers. Lipid oxidation was followed by measuring lipid hydroperoxides (PV), as well as secondary oxidation products (volatiles) using dynamic headspace GC-MS. Free fatty acids (FFA) were measured as an estimation of lipolysis. Protein oxidation was followed using the spectrophotometric determination of protein carbonyls and immunoblotting. Significant oxidation was observed in samples stored at -20 degrees C, and at this temperature lipid and protein oxidation seemed to develop simultaneously. FFA, PV, and carbonyls increased significantly for the fish stored at -20 degrees C, whereas the fish stored at -30 and -80 degrees C did not show any increase in oxidation during the entire storage period when these methods were used. In contrast, the more sensitive GC-MS method used for measurement of the volatiles showed that the fish stored at -30 degrees C oxidized more quickly than those stored at -80 degrees C. Detection of protein oxidation using immunoblotting revealed that high molecular weight proteins were oxidized already at t = 0 and that no new protein oxidized during storage irrespective of the storage time and temperature. The results emphasize the need for the development of more sensitive and reliable methods to study protein oxidation in order to gain more explicit knowledge about the significance of protein oxidation for food quality and, especially, to correlate protein oxidation with physical and functional properties of foods.     

Sterol oxidation in ready-to-eat infant foods during storage

The effect of storage on sterol oxidation of ready-to-eat infant foods was evaluated. Two different liquid infant foods (honey or fruits flavors), prepared with milk and cereals, were stored for 0, 2, 4, 7 and 9 months at 25 degrees C. Sterol oxidation products (SOP) were isolated by cold saponification, purified by silica solid-phase extraction, and analyzed by gas chromatography (GC) and GC-mass spectrometry. beta-Sitosterol was the most representative sterol, followed by cholesterol and campesterol. No significant differences in the total and single SOP content (0.8-1 mg/kg of product) were observed with respect to storage time and type of sample; the main SOP found was 7-ketositosterol (<0.2 mg/kg of product). The extent of stigmasterol oxidation (2.9%) was higher than that of cholesterol (1.9%) and beta-sitosterol (1.4%). The type and quality of raw materials, as well as the processing conditions, seem to greatly influence SOP formation and accumulation in infant foods.