Gelation of gellan – A review

Gellan is an anionic extracellular bacterial polysaccharide discovered in 1978. Acyl groups present in the native polymer are removed by alkaline hydrolysis in normal commercial production, giving the charged tetrasaccharide repeating sequence: → 3)-β-d-Glcp-(1 → 4)-β-d-GlcpA-(1 → 4)-β-d-Glcp-(1 → 4)-α-l-Rhap-(1 →. Deacylated gellan converts on cooling from disordered coils to 3-fold double helices. The coil–helix transition temperature (T_m) is raised by salt in the way expected from polyelectrolyte theory: equivalent molar concentrations of different monovalent cations (Group I and Me₄N⁺) cause the same increase in T_m; there is also no selectivity between different divalent (Group II) cations, but divalent cations cause greater elevation of T_m than monovalent. Cations present as counterions to the charged groups of the polymer have the same effect as those introduced by addition of salt. Increasing polymer concentration raises T_m because of the consequent increase in concentration of the counterions, but the concentration of polymer chains themselves does not affect T_m. Gelation occurs by aggregation of double helices. Aggregation stabilises the helices to temperatures higher than those at which they form on cooling, giving thermal hysteresis between gelation and melting. Melting of aggregated and non-aggregated helices can be seen as separate thermal and rheological processes. Reduction in pH promotes aggregation and gelation by decreasing the negative charge on the polymer and thus decreasing electrostatic repulsion between the helices. Group I cations decrease repulsion by binding to the helices in specific coordination sites around the carboxylate groups of the polymer. Strength of binding increases with increasing ionic size (Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺); the extent of aggregation and effectiveness in promoting gel formation increase in the same order. Me₄N⁺ cations, which cannot form coordination complexes, act solely by non-specific screening of electrostatic repulsion, and give gels only at very high concentration (above ∼0.6 M). At low concentrations of monovalent cations, ordered gellan behaves like a normal polymer solution; as salt concentration is increased there is then a region where fluid “weak gels” are formed, before the cation concentration becomes sufficient to give true, self-supporting gels. Aggregation and consequent gelation with Group II cations occurs by direct site-binding of the divalent ions between gellan double helices. High concentrations of salt or acid cause excessive aggregation, with consequent reduction in gel strength. Maximum strength with divalent cations comes at about stoichiometric equivalence to the gellan carboxylate groups. Much higher concentrations of monovalent cations are required to attain maximum gel strength. The content of divalent cations in commercial gellan is normally sufficient to give cohesive gels at polymer concentrations down to ∼0.15 wt %. Gellan gels are very brittle, and have excellent flavour release. The networks are dynamic: gellan gels release polymer chains when immersed in water and show substantial recovery from mechanical disruption or expulsion of water by slow compression. High concentrations of sugar (∼70 wt % and above) inhibit aggregation and give sparingly-crosslinked networks which vitrify on cooling. Gellan forms coupled networks with konjac glucomannan and tamarind xyloglucan, phase-separated networks with kappa carrageenan and calcium alginate, interpenetrating networks with agarose and gelling maltodextrin, and complex coacervates with gelatin under acidic conditions. Native gellan carries acetyl and l-glyceryl groups at, respectively, O(6) and O(2) of the 3-linked glucose residue in the tetrasaccharide repeat unit. The presence of these substituents does not change the overall double helix structure, but has profound effects on gelation. l-Glyceryl groups stabilise the double helix by forming additional hydrogen bonds within and between the two strands, giving higher gelation temperatures, but abolish the binding site for metal ions by changing the orientation of the adjacent glucuronate residue and its carboxyl group. The consequent loss of cation-mediated aggregation reduces gel strength and brittleness, and eliminates thermal hysteresis. Aggregation is further inhibited by acetyl groups located on the periphery of the double helix. Gellan with a high content of residual acyl groups is available commercially as “high acyl gellan”. Mixtures of high acyl and deacylated gellan form interpenetrating networks, with no double helices incorporating strands of both types. Gellan has numerous existing and potential practical applications in food, cosmetics, toiletries, pharmaceuticals and microbiology.     

Gelling Properties of Gellan Solutions Containing Monovalent and Divalent Cations

Gelling temperatures of gellan solutions with the addition of Na⁺ and K⁺ ranging from 15 to 450 mM or Ca⁺⁺ and Mg⁺⁺ from 2 to 40 mM were determined by dynamic rheological testing at four polymer concentrations between 0.4 and 2.0% (w/w). Gelling temperatures were much higher for gellan solutions containing divalent cations than for those containing the same amount of monovalent cations. Solutions containing K⁺ gelled at higher temperatures than those containing Na⁺. Effects of Ca⁺⁺ and Mg⁺⁺ on gelling temperatures were not significantly different. A general model was developed to predict the gelling temperature of gellan solutions as functions of cation and polymer concentrations.     

Thermal stability and transition temperatures of monovalent salts of high‐acyl gellan gels

Thermogravimetry/derivative thermogravimetry (TG/DTG), rheometry and differential scanning calorimetry (DSC) were used to study the thermal stability and determine the transition temperatures of the sodium and potassium salts of high‐acyl gellan (HAG) in the presence of 0–100 mm NaCl and KCl, respectively. TG/DTG revealed the potassium gellan (KHAG) gels to be more stable than those of sodium gellan (NaHAG), regardless of external cation concentration. Rheometry and DSC showed the melting (T_m) and gelling (T_g) temperatures to increase with cation concentration. The DSC peak temperatures showed thermal hysteresis contrary to rheometry. In most cases, DSC revealed KHAG to exhibit higher T_m and T_g than NaHAG. Consequently, thermal characteristics of NaHAG and KHAG gels depend on the size of the external cation and its ability to coordinate water molecules. Cation salts of HAG exhibit significantly lower transition temperatures than the commercial preparation from which they were produced.     

Gel—sol transition in gellan gum solutions. II. DSC studies on the effects of salts

The thermal properties of sodium form gellan gum solutions with and without sodium chloride, potassium chloride, calcium chloride and magnesium chloride were studied by differential scanning calorimetry (DSC). The DSC cooling or heating curves for 1% gellan gum solutions without salt showed a single exothermic or endothermic peak at ~30°C. DSC cooling curves showed a single exothermic peak, with the setting temperature (Ts) shifting to progressively higher temperatures with increasing concentration of the added NaCl or KCl. At low concentrations of NaCl or KCl, DSC heating curves showed a single endothermic peak; however with more addition of salt the endothermic peak gradually developed a bimodal character and eventually split into more than two distinct peaks. The onset of detectable splitting occurred at a high salt concentration which coincided with that at which elastic gels are formed at even a higher temperature as was observed by viscoelastic measurements. With a sufficient addition of monovalent cations the endothermic curve became again a single peak shifting to higher temperatures. In the presence of divalent cations, although Ts shifted to higher temperatures with increasing concentration of added CaCl₂ or MgCl₂, the melting temperature (Tm) in heating DSC curves shifted to higher temperatures (up to a certain temperature) and then shifted to lower temperatures with increasing concentration of salt. With increasing concentration of CaCl₂ or MgCl₂, the exothermic and endothermic enthalpies estimated for a main peak increased up to a certain salt concentration and then decreased; however many other peaks were observed at higher temperatures. The endothermic peaks for gels with excessive divalent cations were too broad to be resolved from the baseline; in contrast the exothermic peaks were much sharper and readily recognized. In comparing thermal properties with rheological properties, gellan gum solutions with excessive divalent cations form firm gels on cooling to below the setting temperature, and then it was difficult to remelt them. This was quite different from the behaviour of thermoreversible gels formed in the presence of monovalent cations. It seems that the mechanism of gel formation in gellan gum with divalent cations is markedly different from that with monovalent cations.     

Segregative interactions and competitive binding of Ca in gelling mixtures of whey protein isolate with Naκ-carrageenan

Gelling mixtures of Na⁺κ-carrageenan with whey protein isolate (WPI) at pH 7.0 have been studied rheologically and by differential scanning calorimetry (DSC), with comparative measurements for the individual constituents of the mixtures. The concentration of WPI was held fixed at 10.0 wt% and carrageenan concentration was varied in the range 0.05–3.0 wt%. Ca²⁺ cations, which have been shown previously to be particularly effective in inducing gelation of κ-carrageenan, were introduced as CaCl₂. The concentration of CaCl₂ used in most of the experiments was 8 mM, but other concentrations were also studied. Mixtures were prepared in the solution state at 45 °C, and showed no evidence of either phase separation or complex formation. Rheological changes were monitored by low-amplitude oscillatory measurements of storage modulus, G′, during (i) cooling (1 °C/min) and holding at 5 °C, to induce gelation of the carrageenan in the presence of non-gelled WPI; (ii) heating and holding at 80 °C to dissociate the carrageenan network and induce gelation of WPI; (iii) cooling and holding again at 5 °C, to give composite networks with both components gelled; and (iv) re-heating to 80 °C to dissociate the carrageenan network. Gel structure was characterised further by creep–recovery measurements at the end of each holding period, and by torsion measurements at 5 °C, before and after thermal gelation of WPI.     

Dissociation of natural actomyosin from kuruma prawn muscle induced by pyrophosphate

Effect of pyrophosphate (PP) on the dissociation and stability of natural actomyosin (NAM) from kuruma prawn muscle was studied in comparison with adenosine 5′-triphosphate (ATP). In the presence of PP up to 5 mM, NAM exhibited lower Mg²⁺-ATPase activity (P < 0.05), while no marked change was observed in NAM treated with ATP at all concentrations tested (0.25–10 mM) (P > 0.05). Ca²⁺-ATPase activity of NAM treated with 5 mM PP decreased markedly when incubated at temperatures greater than 30 °C, suggesting lowered thermal stability of the liberated myosin molecule. Nevertheless, Ca²⁺-ATPase activity of ATP-treated NAM was similar to the control NAM. In the presence of 5–10 mM MgCl₂, NAM treated with 5 mM PP underwent dissociation effectively, as evidenced by a greater decrease in Mg²⁺-ATPase activity as well as an increased band intensity of actin released. Therefore, addition of PP in combination with MgCl₂ was more effective than was ATP in dissociating the actomyosin complex of prawn muscle.     

Thermal and Calcium Pretreatment Affects Texture, Pectinesterase and Pectic Substances of Frozen Sweet Cherries

Freezing causes loss of turgidity and firmness in sweet cherries. Thermal pretreatment at 50°C for 10 min followed by immersion in 100 mM CaCl₂ and thermal pretreatment at 70°C/2 min with or without immersion in 100 mM CaCl₂ prevented freezing-induced loss of firmness. Thermal pretreatments increased the pectin fraction soluble in EDTA, reduced the degree of pectin esterification, and increased both the concentration of divalent cations in the cell wall and the pectinesterase activity bounded to the cell wall. Immersion in CaCl₂ increased the concentration of Ca²⁺ cations in the cell wall and enhanced the effect of thermal pretreatments on pectinesterase activity.     

Characterization of polyphenoloxidase (PPO) and total phenolic contents in medlar (Mespilus germanica L.) fruit during ripening and over ripening

Characterization of polyphenoloxidase (PPO) enzyme and determination of total phenolic concentrations during fruit ripening and over ripening in medlar (Mespilus germanica L.) were determined. During ripening, PPO substrate specificity, optimum pH and temperature, optimum enzyme and substrate concentrations were determined. Among the five mono- and di-phenolic substrates examined ((p-hydroxyphenyl) propionic acid, l-3,4-dihydroxyphenylalanine, catechol, 4-methylcatechol and tyrosine), 4-methylcatechol was selected as the best substrate for all ripening stages. A range of pH 3.0–9.0 was also tested and the highest enzyme activity was at pH 7.0 throughout ripening. The optimum temperature for each ripening stage was determined by measuring the enzyme activity at various temperatures over the range of 10–70 °C with 10 °C increments. The optimum temperatures were found to be 30, 20 and 30 °C, respectively, for each ripening stage. Optimum enzyme and substrate concentrations were found to be 0.1 mg/ml and 40 mM, respectively. The V_max and K_m value of the reaction were determined during ripening and found to be 476 U/mg protein and 26 mM at 193 DAFB (days after full bloom) – stage 1, 256 U/mg protein and 12 mM at 207 DAFB – stage 2, 222 U/mg protein and 8 mM at 214 DAFB – stage 3. For all ripening stages sodium metabisulfite markedly inhibited PPO activity. For stage 1 of ripening, Cu²⁺, Hg²⁺ and Al³⁺, for stage 2, Cu²⁺ and Hg²⁺, and for stage 3, Cu²⁺, Hg²⁺, Al³⁺ and Ca²⁺ strongly inhibited diphenolase activity. Accordingly, it can be concluded that as medlar fruit ripen there is no significant changes in the optimum values of polyphenoloxidases, although their kinetic parametres change. As the fruit ripening progressed through ripe to over-ripe, in contrary to polyphenoloxidase activity, there was an apparent gradual decrease in total fruit phenolic concentrations, as determined by using the aqueous solvents and water extractions.     

Dilute solution properties of wild sage (Salvia macrosiphon) seed gum

In this paper, the effect of salt type (sodium and calcium chlorides), salt concentration (0, 0.5, 20 and 50 mM) and temperature (20, 30 and 40 °C) on the properties of dilute sage seed gum (SSG) solutions were investigated. SSG was evaluated for intrinsic viscosity by various models i.e. Huggins, Kraemer, Higiro and Tanglertpaibul and Rao equations. The results showed that the Tanglertpaibul & Rao and Higiro equations were chosen as the best models for intrinsic viscosity determination of SSG at different temperatures and salts concentrations, respectively. The increase in ionic strength of the NaCl and CaCl₂ from 0 to 0.5 mM caused increase in intrinsic viscosity, but increasing the temperature from 20 to 40 °C and salts concentrations from 0.5 to 50 mM decreased the intrinsic viscosity. Divalent ions from CaCl₂ showed a more pronounced effect on the intrinsic viscosity compared with monovalent ions from NaCl. SSG solutions at all temperatures and salts concentrations were in the dilute domain. The weight-average molecular weight of sage seed gum was obtained as 1.5 × 10⁶ Da.     

The effect of starch isolation method on physical and functional properties of Portuguese nut starches. II. Q. rotundifolia Lam. and Q. suber Lam. acorns starches

A new starch was isolated from fruits of two acorn species, Quercus rotundifolia and Quercus suber by alkaline (A3S) and enzymatic (ENZ) methods and physical and functional properties were studied. The isolation method induced changes in most of those properties in the isolated starches, mainly in resistant starch content, syneresis, pasting, thermal and rheological properties. Isolated acorn starches presented high amylose content (53–59%) and resistant starch content (30.8–41.4%). Acorn starches showed limited and similar solubility values and swelling power values, showing a gradual increase from 60 °C to 90 °C. The pasting temperatures ranged from 67.5 to 72.0 °C and pastes did not present breakdown, which is suggestive of a high paste stability of acorn starches during heating. At ambient temperature the turbidity and syneresis values were low, but when held at freezing temperatures the syneresis significantly increased. Thermal analysis revealed that the acorn starches easily undergo transition phenomena as shown by the low T_o and enthalpy values (4.1–4.3 J/g), these effects were more evident in starches isolated by ENZ method. Pastes are more elastic than viscous and form strong gels after cooling. Q. suber starch was shown to be more sensitive to the effect of isolation method. Generally, starch isolated by enzymatic method presented less interesting functional properties, since this isolation procedure greater affected the raw structure of starches.     

Thermophysical properties and thermal behavior of leafy vegetables packaged in clamshells

Little is known about the thermophysical properties of fresh-cut lettuce other than heat of respiration. Empirical correlations based on food composition remain the only way to estimate the thermophysical properties of fresh-cut lettuce. The objectives of this study were (i) to determine the thermophysical properties of several baby-leaf lettuce and brassica greens and (ii) to verify the measured thermophysical properties by using them in a heat transfer model and comparing the predicted product temperatures with measured product temperatures in a simulated interruption of a cold chain. Density, leaf thickness, thermal conductivity, specific heat and water activity from nine varieties of baby-leaf lettuce and brassica greens were measured. A broken cold chain was simulated in a low temperature incubator set at 10 °C for a length of time before readjustment at 2 °C. Results showed that density (1078–1112 kg m⁻³), leaf thickness (0.18–0.54 mm), thermal conductivity (0.55–0.70 W (m °C)⁻¹) and specific heat (3.1–4.3 kJ (kg °C)⁻¹) varied significantly (P < 0.05) between varieties. However, no significant differences were observed for water activity (0.959 ± 0.006). Using thermophysical properties as input in the heat transfer model, experimental and calculated temperatures were well correlated (R² = 0.98) with a root mean square error of 0.57 °C over the 10–40 mg CO₂ (kg h)⁻¹ range of respiration rate. The measured thermophysical properties adequately predicted the temperature of the baby-leaf greens during simulated broken cold chains. A sensitivity analysis performed with the heat transfer model showed that the thermal conductivity, the specific heat and the density were relatively more important on the thermal behaviour of the baby-leaf greens than the heat of respiration.     

Rheological properties of Salecan as a new source of thickening agent

Salecan is a novel soluble glucan produced by Agrobacterium sp. ZX09, displaying the ability to inhibit pancreatic amylase and reduce postprandial glucose. The research here provides an investigation of the rheological properties of Salecan solution over a wide range of shear rate (0.001–1000 s⁻¹), frequency (0.1–100 rad/s), concentrations (0.3%, 0.5%, 1.0% and 1.5%), temperature (5–95 °C) and pH (1.0–13.0). The power law model well described the rheological behavior of the solutions, in the shear-thinning region, with high determination coefficients, R². Salecan solutions showed a non-Newtonian viscosity behavior at all concentrations and temperatures. The solutions exhibited excellent stability below 55 °C. Viscosities were not affected after being frozen (−20 °C). Over a wide pH range from 6.0 to 12.0, the viscosity almost kept invariant. With increasing frequency, the storage and loss moduli G′ and G″ of Salecan solution increased and complex viscosities decreased continuously, which showed an elastic behaviour. All data indicated that Salecan has excellent rheological properties and could be utilized in food industry as a new source of thickening agent.     

Modelling the effect of water immersion thermal processing on polyacetylene levels and instrumental colour of carrot disks

C₁₇ polyacetylenes are a group of bioactive compounds present in carrots which have recently gained scientific attention due to their cytotoxicity against cancer cells. In common with many bioactive compounds, their levels may be influenced by thermal processes, such as boiling or water immersion. This study investigated the effect of a number of water immersion time/temperature combinations on concentrations of these compounds and attempted to model the changes. Carrot samples were thermally treated by heating in water at temperatures from 50–100 °C and holding times of 2–60 min. Following heating, levels of falcarinol (FaOH), falcarindiol (FaDOH), falcarindiol-3-acetate (FaDOAc) and Hunter colour parameters (L^∗, a^∗, b^∗) were determined. FaOH, FaDOH, FaDOAc levels were significantly reduced at lower temperatures (50–60 °C). In contrast, samples heated at temperatures from 70–100 °C exhibited higher levels of polyacetylenes (p < 0.05) than did raw unprocessed samples. Regression modelling was used to model the effects of temperature and holding time on the levels of the variables measured. Temperature treatment and holding time were found to significantly affect the polyacetylene content of carrot disks. Predicted models were found to be significant (p < 0.05) with high coefficients of determination (R²).     

Food-grade microemulsions,nanoemulsions and emulsions: Fabrication from sucrose monopalmitate & lemon oil

Sucrose monopalmitate (SMP) is a non-toxic, biodegradable, non-ionic surfactant suitable for use in foods and beverages. This study aimed to establish conditions where stable microemulsions, nanoemulsions or emulsions could be fabricated using SMP as a surfactant and lemon oil as an oil phase. Emulsions (r > 100 nm) or nanoemulsions (r < 100 nm) were formed at low surfactant-to-oil ratios (SOR < 1) depending on homogenization conditions, whereas microemulsions (r < 10 nm) were formed at higher ratios (SOR > 1). The impact of simple mixing, thermal treatment, and homogenization on the formation of the different colloidal systems was investigated. Blending/heating was needed to produce microemulsions or emulsions, whereas blending/heating/homogenization was needed to produce nanoemulsions. The impact of environmental stresses (pH, ionic strength, temperature) on the functional performance of nanoemulsions and microemulsions was examined. Relatively stable nanoemulsions could be formed at pH 6 and 7 and stable microemulsions at pH 5 and 6, but extensive particle growth/aggregation occurred at lower and higher pH values. Microemulsions were relatively stable to salt addition (0–200 mM NaCl), but nanoemulsions exhibited droplet aggregation/growth at ≥50 mM NaCl after 1 month storage at pH 7. Microemulsions formed gels at low temperatures (5 °C), were stable at ambient temperatures (23 °C), and exhibited particle growth at elevated temperatures (40 °C). Nanoemulsions were stable at refrigerator (5 °C) and ambient (23 °C) temperatures, but exhibited coalescence at elevated temperatures (40 °C). This study provides important information for optimizing the application of sucrose monoesters to form colloidal dispersions in food and beverage products.     

Formation and stability of emulsions using a natural small molecule surfactant: Quillaja saponin (Q-Naturale)

Q-Naturale^® is a natural food-grade surfactant isolated from the bark of the Quillaja saponaria Molina tree. The major surface active components of Q-Naturale^® are believed to be saponin-based amphiphilic molecules. In this study, we compared the effectiveness of this natural surfactant for forming and stabilizing emulsions with a synthetic surfactant (Tween 80) that is widely used in the food industry. We examined the influence of homogenization pressure, number of passes, and emulsifier concentration on the particle size produced. Q-Naturale^® was capable of forming relatively small droplets (d < 200 nm) at low surfactant-to-oil ratios (SOR < 0.1) using high pressure homogenization (microfluidization), but the droplets were not as small as those produced using Tween 80 under similar conditions (d < 150 nm). Q-Naturale^®-coated droplets were stable to droplet coalescence over a range of pH values (2–8), salt concentrations (0–500 mM NaCl) and temperatures (20–90 °C). However, some droplet flocculation was observed under highly acidic (pH 2) and high ionic strength (≥400 mM NaCl) conditions, which was attributed to screening of electrostatic repulsion. Indeed, Q-Naturale^®-coated droplets had a relatively high negative charged at neutral pH that decreased in magnitude with decreasing pH. These results indicate that Q-Naturale^® is an effective natural surfactant that may be able to replace synthetic surfactants in food and beverage products.     

Acrylamide formation is prevented by divalent cations during the Maillard reaction

The effects of mono- and divalent cations on the formation of acrylamide were studied in a fructose-asparagine model system at 150 and 180 °C. At amounts equivalent to those of asparagine and fructose, added divalent cations, such as Ca²⁺, were found to prevent acrylamide formation completely, whereas monovalent cations, such as Na⁺, almost halved the acrylamide formed in the model system. It was confirmed by mass spectrometric analyses of pyrolyzates that the formation of the Schiff base of asparagines, which is the key intermediate leading to acrylamide, was prevented by the cations. Meanwhile, the reaction proceeded to form brown coloured products. Dipping potatoes into calcium chloride solution inhibited the formation of acrylamide by up to 95% during frying. The sensory quality of fried potato strips, in terms of golden yellow colour and crispy texture, was not adversely affected by this treatment.     

A NaCl-stable serine proteinase from Virgibacillus sp. SK33 isolated from Thai fish sauce

An extracellular proteinase from Virgibacillus sp. SK33, isolated from 1 month-old fish sauce, was purified to electrophoretic homogeneity, using hydrophobic interaction chromatography and hydroxyapatite with purification fold of 2.5 and 7% yield. The anomalous molecular weight (MW) of 19 kDa was obtained from SDS–PAGE, whereas a MW of 33.7 kDa was determined by MALDI-TOF. Optimum conditions for catalytic activity were 55 °C and pH 7.5. The proteinase was strongly inhibited by phenylmethanesulfonyl fluoride (PMSF) and preferentially hydrolysed Suc-Ala-Ala-Pro-Phe-AMC, indicating a serine proteinase with subtilisin-like characteristics. K_m and k_cat of the purified proteinase were 27 μM and 12 s⁻¹, respectively. Proteinase activity, toward both synthetic and anchovy substrates, increased with NaCl up to 25%. The proteinase exhibited high stability in both the absence and presence of NaCl up to 25%. Approximately 2.5-fold increase in activity was observed in the presence of divalent cations, including Ca²⁺, Mg²⁺ and Sr²⁺ at 100 mM. MALDI-TOF MS and LC–ESI-MS/MS analyses, as well as N-terminal sequences, revealed that the purified enzyme did not match microbial proteinases in the database, indicating it to be a novel proteinase.     

Sodium lactate,sodium diacetate and pediocin: Effects and interactions on the thermal inactivation of Listeria monocytogenes on bologna

The effects and interactions of temperature (56.3–60 °C), sodium lactate (SL; 0–4.8%), sodium diacetate (SD; 0–0.25%) and pediocin (0–10,000 AU) on Listeria monocytogenes on bologna were studied and a predictive inactivation model was developed. Bologna was manufactured with different SL/SD concentrations in the formulation, dipped in pediocin solution and treated at different temperatures using combinations of parameters determined by central composite design. D-values were calculated and analyzed using second order response regression. Predicted D-values were also calculated. The observed D-values for L. monocytogenes on bologna ranged from 2.10 to 35.59 min. Temperature alone decreased predicted D-values from 99.02 min at 56.3 °C to 44.71 min at 60.0 °C. Adding SL decreased D-values (85.43–22.71 min) further; however, heat and SD combined was the most effective for reducing L. monocytogenes on bologna. An SD level of 0.25% at 58.2 °C had the overall lowest predicted D-value (15.95 min). Combination treatments increased or decreased D-values, depending on the temperature. Pediocin (2500 and 5000 AU) and heat decreased D-values, but exhibited a protective effect at higher concentrations (≥7500 AU). The results showed that interactions between additives in formulations can vary at different temperatures/concentrations, thereby affecting thermal inactivation of foodborne pathogens in meat products. Hence, food processors should modify food formulations carefully, and verify with adequate testing so that product safety is not compromised.     

Modulation of thermal stability and heat-induced gelation of β-lactoglobulin by high glycerol and sorbitol levels

The influence of glycerol and sorbitol on the thermal stability and heat-induced gelation of β-lactoglobulin (β-lg) in aqueous solutions was investigated. The thermal stability of β-lg was characterized by measuring the thermal denaturation temperature (T_m) using differential scanning calorimetry, while its gelation properties were characterized by measuring the gelation temperature (T_gel) and final gel rigidity (G^∗) using dynamic shear rheology. All experiments were carried out using aqueous solutions containing 10% (w/w) β-lg, glycerol (0–70% w/w) or sorbitol (0–55% w/w), and 5 mM phosphate buffer (pH 7.0). No salt was added to these solutions so that there was a relatively strong electrostatic repulsion between the protein molecules, which usually prevents gelation. When the cosolvent concentration was increased from 0% to 50%, T_m increased from 74 to 86 °C for sorbitol, but only from 74 to 76 °C for glycerol, which indicated that sorbitol was much more effective at stabilizing the native state of the globular protein than glycerol. Protein solutions containing sorbitol (0–55%) did not form a gel after heating, but those containing glycerol formed gels when the cosolvent concentration exceeded about 10%, with G^∗ increasing with increasing glycerol concentration. We attribute these effects to differences in the preferential interactions of polyols and water with the surfaces of native and heat-denatured proteins, and their influence on the protein–protein collision frequency.     

Effects of storage temperature and duration on chemical properties,proximate composition and selected bioactive components of pomegranate (Punica granatum L.) arils

Effects of storage temperature on nutritional composition of pomegranate cultivars (‘Arakta’, ‘Bahgwa’ and ‘Ruby’) were investigated. Pomegranate arils were stored at 1, 4, and 8 °C at 95% relative humidity for 14 days. Arils stored at 1 °C were removed from storage after 14 days and kept at ambient conditions for one day. Anthocyanins, ascorbic acid and proximate composition were measured on day 0, 7, 14 and 15. O₂ consumption and CO₂ production increased at elevated temperatures. Proximate composition of pomegranate arils was similar amongst cvs.: 0.8 g/g moisture, 5.2 g kg⁻¹ ash, 13–15 g kg⁻¹ fat, 27–29 g kg⁻¹ dietary fibre, 11–12 g kg⁻¹ protein, 140–150 g kg⁻¹ carbohydrate and 310–320 kJ 100 g⁻¹ energy. ‘Arakta’ had highest titratable acidity (TA) level (0.3 ± 0.01 g 100 mL⁻¹) and ascorbic acid (26.0 ± 1.3 mg kg⁻¹). Low β-carotene levels (1.7–3.5 mg kg⁻¹) were detected in all samples. Nutritional composition of arils was not significantly affected at 1 and 4 °C for 14 d. Temperature did not affect total soluble solids (TSS), but increased TA and reduced TSS/TA. No mould growth was observed in arils stored at 1 °C after 14 d.