Tom A. Gill

Effects of salt and storage temperature on chemical, microbiological and sensory changes in cold-smoked salmon

Abstract Chemical, microbiological and sensory changes during storage of vacuum-packed cold-smoked salmon were studied using a factorial experimental design with two storage temperatures (5 and 10 °C) and two salt levels (2.2 and 4.6%). The spoilage characteristics were typical of microbiological activity in all treatments, but there was no relation between sensory changes and any of the microbiological numbers (total viable counts, total psychrotrophes, lactic acid bacteria or Enterobacteriaceae). Total viable counts typically reached 108 cfu/g weeks before sensory rejection. Acetic acid, hypoxanthine, trimethylamine and ethanol concentrations increased with storage time in all treatments. The increase in ethanol depended on salt concentration but not storage temperature. Absolute values of trimethylamine ranged from 2–8 mg TMA-N/100 g initially, to 10 mg TMA-N/100 g at sensory rejection. Acetic acid levels increased with temperature and decreased with salt concentration, but varied between 12–23 μmol g at rejection. Initial concentrations of hypoxanthine increased from 2–3 μmol g to maximum 8–9 μmol g , with values of 5–7 μmol g indicating the limit of sensory acceptability. Hypoxanthine was considered to be the best objective indicator for sensory quality of cold-smoked salmon.

Importance of autolysis and microbiological activity on quality of cold-smoked salmon

The relative importance of autolysis and microbiological activity on spoilage of cold-smoked salmon and the origin of the chemical compounds hypoxanthine, acetic acid, trimethylamine and total volatile bases were studied in a storage experiment of dry salted and injection-brined, vacuum-packed salmon with normal and reduced loads of microorganisms. Comparative studies of cold-smoked salmon with a reduced and normal load of microorganisms showed that microbiological activity caused production of the characteristic spoilage odours and flavours, while the autolytic enzymes from the fish tissue had major impact on the textural deterioration. Total volatile bases and hypoxanthine were produced in significantly higher levels in salmon with a normal bacterial load. Sixty-eight per cent of the hypoxanthine found in stored samples originated from microbiological conversion of inosine to hypoxanthine. The concentration of acetic acid only increased in samples with a normal load. At the onset of spoilage, the microflora in dry salted salmon was dominated by marine vibrio in contrast to the injection brined product where a mixture of Enterobacteriaceae and lactic acid bacteria were prevalent. The different ratios of hypoxanthine produced to trimethylamine also suggested that spoilage of salmon salted by the two methods was caused by different microorganisms.

Objective analysis of seafood quality

Abstract Canadian seafood quality has traditionally been examined by the subjective parameters of odor, color, flavor and appearance. Also, Canadian seafood products have most often been marketed on a one price system; products of high and mediocre quality being sold at the same price. A principal advantage of objective quality assessment is the ability to assign meaningful numerical scores to raw material and finished product, thus permitting adjustment of market prices and providing a cash incentive to fishermen, processors and retailers to maintain high quality standards. Development of rapid, inexpensive objective techniques for seafood quality evaluation has been the focus of research in our laboratories during the past several years. Rapid procedures for the analysis of amines, nucleotides and ammonia are presented and the applicability of each for the overall evaluation of seafood quality discussed.

Antibacterial effect of protamine in combination with EDTA and refrigeration

The antimicrobial effect of protamine (clupeine) on a range of gram-positive and gram-negative foodborne pathogens and spoilage bacteria, was evaluated using an agar dilution assay and a broth dilution assay with Alamar Blue as growth indicator. Protamine was tested alone at concentrations from 0 to 10,000 microg/ml, and in combination with EDTA (0.9 mM). Assays were performed at 5 degrees C, 10 degrees C, 18 degrees C and 30 degrees C to test the effect of temperature. Minimum inhibitory concentration (MIC) values ranged from 10 microg/ml for Brochothrix thermosphacta to no inhibition at 10,000 microg/ml for bacteria such as Aeromonas hydrophila, proteolytic strains of Clostridium botulinum, Hafnia alvei and Morganella morganii. The minimum bactericidal concentrations (MBCs) were generally higher than MICs. In combination with EDTA, MICs of protamine decreased for gram-negative test strains, whereas EDTA alone inhibited gram-positive strains. The effect of assay incubation temperature was variable and not clear for most strains. Concentrations of 100-750 microg/ml protamine inhibited the five non-proteolytic C. botulinum strains, while none of the eight proteolytic strains was inhibited, indicating the possible role of proteolytic enzymes in protecting cells from protamine. Clearing zones, indicative of proteolytic activity, were observed in the opaque TSB-agarose around colonies of some but not all protamine-resistant bacteria, suggesting that this is not the only resistance mechanism. Addition of 5% (w/v) gelatin to study the effect of an increased protein concentration in the agar dilution assay showed that electrostatic interactions between protamine and the protein decreased the antimicrobial efficacy of the peptide.

The dynamics of thermal denaturation of fish myosins

Abstract Thermal denaturation of fish myosins was studied by turbidity measurements, differential scanning calorimetry (DSC), circular dichroism (CD), intrinsic fluorescence and 8-anilino-1-naphthalene sulfonic acid binding studies. Results of DSC and turbidity studies showed that thermal aggregation of myosins from three fish species (cod, herring and silver hake) proceeded prior to the final stage of denaturation of the myosin molecules. CD and fluorescence measurements revealed that all fish myosins exhibited similar helix melting profiles, but the unfolded domains of each fish species displayed different surface hydrophobicities. Over a temperature range of 30–50°C, greater hydrophobic surface area was exposed by cod and silver hake myosins than by herring. The extent of myosin aggregation was species dependent; cod > silver hake > herring. Therefore, exposure of hydrophobic domain(s) is a prerequisite for the formation of large myosin aggregates. The effect of the sequence of unfolding of fish myosin molecules on thermal aggregation is discussed.

Evaluation of the in vitro antioxidant properties of a cod (Gadus morhua) protein hydrolysate and peptide fractions

Mechanically-deboned cod muscle proteins were sequentially hydrolysed using pepsin and a trypsin+chymotrypsin combination, which was followed by passing the digest through a 1 kDa equipped tangential flow filtration system; the permeate (<1 kDa peptides) was collected as the cod protein hydrolysate (CPH). Reversed-phase high performance liquid chromatography (RP-HPLC) was used to separate the CPH into four peptide fractions (CF1-CF4) and their in vitro antioxidant properties investigated. Results showed that most of the peptide fractions (CF2-CF4) displayed significantly higher (p<0.05) oxygen radical absorbance capacity values (698-942 μM Trolox equivalents, TE/g) and 2,2-diphenyl-1-picrylhydrazyl scavenging activities (17-32%) than those of CPH (613 μM TE/g and 19%, respectively). However, the unfractionated CPH displayed improved capability to scavenge superoxide and hydroxyl radicals as well as significantly higher (p<0.05) ferric iron reduction and chelation of iron than the RP-HPLC peptides. The CPH and peptide fractions displayed a dose-dependent inhibition of linoleic acid oxidation.

Effect of salt concentration and temperature on heat-induced aggregation and gelation of fish myosin

Abstract Thermal aggregation of myosins prepared from cod (Gadus morhua) and herring (Clupea harengus) was investigated by turbidimetric methods and by means of phase contrast microscopy. Turbidity measurements and microscopical studies showed that the extent of aggregation of cod myosin molecules increased with temperature (35–55°C) and was consistently higher than those for herring at all temperatures. Thermal aggregation of herring myosin showed little dependence on salt concentration (0·6 – 1·4 m NaCl, pH 6·5) but salt enhanced aggregation of cod myosin at heating temperatures ≥ 50°C. In addition, the effects of temperature and salt on the rheological properties of heated cod and herring surimi were examined and the relevance to the myosin system was discussed. Torsion tests revealed that cod surimi gels prepared at all temperatures (35–55°C) yielded higher shear strains at failure than those for herring, and gels from either species were not affected by salt levels ranging from 0·6 to 1·4 m NaCl. The present study indicates the importance of ordered intermolecular aggregation as well as clustering of myosin aggregates for the formation of superior surimi gels. Herring myosin formed smaller aggregates and inferior surimi gels compared to those of cod.

Solubility and antimicrobial efficacy of protamine on Listeria monocytogenes and Escherichia coli as influenced by pH.

The antimicrobial efficacy of protamine on Listeria monocytogenes and Escherichia coli was evaluated at concentrations from 50 to 10 000 μg ml−1 and pH levels from 5·5 to 8·0. The minimum inhibitory concentrations decreased with increasing pH. Protamine inhibited E. coli at all pH values while L. monocytogenes was inhibited at pH 6·5 and above. The antimicrobial efficacy of protamine decreased in the presence of negatively charged gelatine B but remained almost unchanged with addition of the positively charged gelatine A. Binding studies showed that the amount of protamine adsorbed to culture media components in tryptic soy broth and bacterial cells increased with increasing pH values. The increased efficacy of protamine at alkaline pH may be explained on the basis of an increase in electrostatic affinity for the cell surface of target cells. E. coli produced a protamine‐degrading enzyme, however, was still susceptible to protamine.

Miniaturizing the fermentation assay: Effects of fermentor size and fermentation kinetics on detection of premature yeast flocculation

This article reports on fermentation assays used to test the fermentability of malt, especially malt implicated in premature yeast flocculation (PYF). No standard small-scale method currently exists. The current study involved 12°C fermentations with three vessel configurations: a 4.4-cm high spectrophotometer cuvette, a 12.5-cm high test tube, and a 122-cm high “tall” tube fermentor. Worts tested were made from control and PYF malts. We included in our experiments the method of Jibiki and associates, using wort supplemented with 4% glucose and fermented at 21°C. The decline in Plato values (or real extract) was fit with a sigmoidal model. The model parameters and fermentor height were used to estimate CO2 evolution and average shear rates. It was confirmed that both fermentor height and fermentation speed are the major independent variables determining CO2 evolution and agitation within the fermenting wort. In examining the fermentation data, it is evident that the yeast did not stay equally suspended in all three fermentation vessels. When fermented at 12°C, yeast fell out of suspension rapidly in both the cuvette and test tube fermentors. In the tall tubes, the yeast remained in suspension as expected. The 21°C test tube fermentations supplemented with glucose had fermentation profiles similar to the tall tube fermentations but were complete in less than 72 hr. It is notable that these profiles could be used to distinguish between PYF and control malts. The minimum average shear rate required to keep yeast suspended was determined to be between 4 and 7.5 sec−1. Thus, when downscaling a fermentation assay by reducing the fermentor height, the rate of fermentation must be increased to maintain adequate shear rates. We confirmed that the PYF fermentation assay could be reduced to 15 mL, resulting in reduced labor, time, and material costs.

Storage stability of paralytic shellfish poisoning toxins

Abstract Variations in C toxins (C1- 2), GTX (gonyautoxin) 1-4, STX (saxitoxin) and NEO (neosaxitoxin) in scallop digestive glands and a mixture of purified paralytic shellfish poisoning (PSP) toxins were studied during storage at −35, 5 and 25°C and at different pH levels. Heated and unheated samples of homogenates and purified toxin mixtures were stored for 1 year and 4 months, respectively, and analyzed at different times by HPLC. C toxin levels decreased rapidly during storage at 25°C with a significant decrease after the first 2 months. GTX 2/3 in the unheated samples did not change initially at pH 3–4, but decreased at pH 6–7 with the fastest rate at 25°C followed by 5°C. There was no significant change in any toxin type stored at −35°C regardless of pH. GTX 1/4 decreased significantly after 4 months at 25°C. NEO and STX remained unchanged at all temperatures at low pH (pH 3), whereas NEO levels continued to decrease at higher pH (pH 6–7) at 25°C. All toxins were stable at low pH (pH 3–4) and −35°C.