Effect of whey protein enzymatic hydrolysis on the rheological properties of acid-induced gels

A protein dispersion blend of β-lactoglobulin and α-lactalbumin was heat-denatured at pH 7.5, hydrolyzed by α-chymotrypsin and then acidified with glucono-δ-lactone to form gels at room temperature. Heat treatment induced the formation of whey protein polymers with high concentration of reactive thiol groups (37 μmol/g). The reactive thiol group concentration was reduced by half after 40 min enzymatic hydrolysis. It was further reduced after enzyme thermal deactivation. During acidification, the first sign of aggregation for hydrolyzed polymers occurred earlier than for non hydrolyzed polymers. Increasing the hydrolysis duration up to 30 min resulted in more turbid gels characterized by an open microstructure. Elastic and viscous moduli were both reduced, while the relaxation coefficient and the stress decay rate constants were increased by increasing the hydrolysis duration. After one week storage at 5 °C, the hardness of gels made from hydrolyzed polymers increased by more than 50%. The effect of polymer hydrolysis on acid-induced gelation is discussed in relation to the availability and reactivity of thiol groups during gel formation and storage.     

Kinetic study of β-lactoglobulin thermal aggregation at low pH

Kinetics of β-lactoglobulin (β-lg) thermal aggregation at pH 3.35, 85 °C, and 2-8% w/w protein concentration was studied using high performance liquid chromatography (HPLC) coupled with multi-angle laser light scattering (MALS) and rheology. Rate of β-lg aggregation was found to be of first order with respect to the initial protein concentration, and the conversion of native-like β-lg monomers/dimers (<36 kDa) to aggregates increased with initial concentration and heating times. The size of the aggregates formed during heating was dependent on the initial protein concentration. A simple nucleation and growth model was described for the β-lg aggregation at pH 3.35, where nucleation was found to be a rate limiting step below the previously identified critical concentration, C_c ∼ 6.4% protein. Above the C_c, nucleation occurred quickly and was not rate limiting. Critical size of the nucleus varied with protein concentration, with larger critical size needed at lower protein concentrations.     

Impact of cosolvents on formation and properties of biopolymer nanoparticles formed by heat treatment of β-lactoglobulin–Pectin complexes

The purpose of this study was to determine the influence of neutral cosolvents on the formation and properties of biopolymer nanoparticles formed by thermal treatment of protein–polysaccharide electrostatic complexes. Biopolymer particles were formed by heating (85 °C, 20 min) an aqueous solution containing a globular protein (β-lactoglobulin) and an anionic polysaccharide (beet pectin) above the thermal denaturation temperature (T_m) of the protein under pH conditions where the biopolymers formed electrostatic complexes (pH 5). The impact of two neutral cosolvents (glycerol and sorbitol) on the self-association of β-lactoglobulin and on the formation of β-lactoglobulin–pectin complexes was examined as a function of solution pH (3–7) and temperature (30–95 °C). Glycerol had little impact on the pH-induced self-association or aggregation of the biopolymers, but it did increase the thermal aggregation temperature (T_a) of the protein–polysaccharide complexes, which was attributed to its ability to increase aqueous phase viscosity. Sorbitol decreased the pH where insoluble protein–polysaccharide complexes were formed, and greatly increased their T_a, which was attributed to its ability to increase T_m, alter biopolymer–biopolymer interactions, and increase aqueous phase viscosity. This study shows that neutral cosolvents can be used to modulate the properties of biopolymer nanoparticles prepared by thermal treatment of protein–polysaccharide electrostatic complexes.     

Optimisation of mild-acidic protein extraction from defatted sunflower (Helianthus annuus L.) meal

For protein isolation from defatted sunflower meal, mild-acidic extraction was investigated to minimise concomitant oxidation and polymerisation of phenolic compounds and their irreversible binding to proteins. Because of the impaired solubility of sunflower proteins at low pH, the potential of sodium chloride (NaCl) to improve protein extractability was firstly screened for pH 2–11. Increasing NaCl concentrations of the aqueous solvent (c_NaCl) up to 2.8 mol/L enhanced the relative protein yield to almost 80% at ambient temperature and pH 5.6–7.4. As to improved protein recovery at minimal interactions with phenolic acids, the concerted effects of pH (3.2–7.4), c_NaCl (1–3 mol/L), temperature (T, 15–45 °C), and meal-to-solvent ratio (MSR, 0.03 and 0.05 g/mL) on the protein concentration of the extract (c_PE) and the relative protein yield (RPY) were examined, using response surface methodology (RSM). Aside from the prevailing influence of pH value and salt concentration, elevated temperature slightly enhanced protein extraction, whereas MSR mainly influenced c_PE, but hardly RPY. Calculated models proved suitable for the evaluation of extraction processes and the prediction of optimum conditions in terms of high protein yields at the lowest pH possible. Extraction at pH 6.0 was shown to be an appropriate compromise yielding 76–83% of the meal protein, depending on the constraints given. With elevated NaCl concentrations compensating for unfavourable pH conditions, mild-acidic extraction was found to be suitable for the recovery of high-quality sunflower protein in terms of light-coloured protein isolates.     

The influence of phospholipids and food proteins on the size and stability of model sub-micron emulsions

The influence of both the nature of the surfactant and surfactant concentration on the processes of droplet breakup and coalescence in the formation of decane in water sub-micron emulsions in a high-pressure homogenizer were investigated. Emulsions were produced using a Microfluidics inc. M110-S microfluidizer with an impinging jet high-shear chamber. For all the food grade emulsifiers studied, the droplet size decreased with increasing concentration (weight %) reaching a limiting droplet size between 0.5 and 1% for the proteins and 1.5% for the Phospholipids. A hydrophobic fluorescent dye (1-undecylpyrene) was used to establish the extent of competition between droplet breakup and coalescence in the emulsification process. For the food proteins and phosphatidylglycerol, droplet coalescence in the process reduced as the amount of emulsifier increased, becoming zero at concentrations of about 0.5–1% i.e. the same concentration as that required to produce the limiting minimum droplet size. For phosphatidylcholine some coalescence in the process was observed up to the highest concentration studied (2%) which is indicative of the fact that it normally stabilises water in oil emulsions so favouring coalescence in the process. The data collected in this study show that in the emulsification process droplet size is determined by both breakup and coalescence events, and that the final droplet size is probably a consequence of multiple breakup events until at higher emulsifier concentrations the droplet size is reached which is limited by the breakup capabilities of the homogenizer. Emulsion stability over 400 h was investigated by measuring changes in the droplet size using dynamic light scattering. For the proteins the increase in droplet volume was shown to be linear with respect to time, indicating an Ostwald ripening process. Although there was coalescence on storage at the lowest concentrations of phospholipid used, there was no observed ripening at any emulsifier concentration showing that phospholipid interfaces are structured in such a way as to resist ripening even though decane has a solubility in water. The ripening rate for whey and β-lactoglobulin were observed to be approximately 10 times higher than the ripening rate calculated using the Lifshitz–Slesov–Wagner (LSW) theory 10–20 nm³ s⁻¹. Ripening rates are explained in terms of the nature of the interface formed.     

Structural and mechanical characterization of κ/ι-hybrid carrageenan gels in potassium salt using Fourier Transform rheology

The mechanical and structural properties of κ/ι-hybrid carrageenan gels obtained at various concentrations in the presence of 0.1 m KCl were studied with Fourier Transform rheology (FTR) and cryoSEM imaging. FTR data show that gels formed at concentration below 1.25 wt% exhibit a strain hardening behavior. The strain hardening is characterized by a quadratic increase of the scaled third harmonic with the strain and a third harmonic phase angle of zero degree. Both features are weakly depending on the concentration and conform to predictions from a strain hardening model devised for fractal colloidal gels. However, the phase angle of the third harmonic reveals that κ/ι-hybrid carrageenan gels obtained at higher concentrations show shear thinning behavior. Colloidal gel models used to extract structural information from the concentration scaling of gel equilibrium shear modulus G₀ and the strain dependence of FTR parameters suggest that κ/ι-hybrid carrageenan gels are built from aggregating rod-like strands (with fractal dimension x = 1.13) which essentially stretch under increasing strain. The mechanically relevant structural parameters fairly match the gel fractal dimension (d = 1.66) obtained from the cryoSEM analysis.     

Water soluble inner aqueous phase markers as indicators of the encapsulation properties of water-in-oil-in-water emulsions stabilized with sodium caseinate

The aim of this study was to evaluate the suitability of Methylene Blue (MB) and Vitamin B₁₂ (Vit-B₁₂) as water soluble inner aqueous phase (W₁) markers for measuring the encapsulation efficiency and stability of water-in-oil-in-water (W₁/O/W₂) double emulsions stabilized by sodium caseinate (NaCN). The encapsulation efficiency and stability were determined by centrifugation of the double emulsion to separate the cream phase (W₁/O) and the outer aqueous phase (W₂) and measuring the concentration of marker in W₂ by absorbance spectrophotometry. To validate this method the marker concentration measurable and the stability of the marker in W₂ were measured. Both markers could be accurately measured in W₂ and there was no change in the concentration of marker on storage of a W₂ solution for 7 days at 45 °C. The recovery yields of MB and Vit-B₁₂ in the recovered W₂ of an oil-in-water (O/W₂) emulsion, determined using the procedure normally used for measuring encapsulation efficiency and stability, were 78% and 99%, respectively, and 52 and 100%, respectively. Double emulsions had encapsulation efficiency of 61.9 ± 21.4% and 16.6 ± 1.1% and encapsulation stability of 62.0 ± 22.6% and 10.7 ± 0.7% for MB and Vit-B₁₂, respectively. Recovery yield and encapsulation efficiency/stability data for MB indicate that it is not a suitable marker for measuring the encapsulation properties of NaCN stabilized double emulsions while similar data for Vit-B₁₂ indicate that it is a suitable marker for studying the encapsulation properties of double emulsions stabilized with NaCN. Methods used in other studies to measure encapsulation properties of double emulsions are discussed in light of the results obtained in this study.     

Protein–polysaccharide interactions: The determination of the osmotic second virial coefficients in aqueous solutions of β-lactoglobulin and dextran

Solutions containing dextran and solutions containing mixtures of dextran +β-lactoglobulin are studied by membrane osmometry. The low concentration range of these solutions is considered. From the measured osmotic pressures the virial coefficients are obtained. These are analyzed using the osmotic virial coefficient of β-lactoglobulin solutions published earlier by us [Schaink, H.M., & Smit, J.A.M. (2000). Determination of the osmotic second virial coefficient and the dimerization of beta-lactoglobulin in aqueous solutions with added salt at the isoelectric point. PCCP, 2, 1537–1541]. The second cross-virial coefficient A₁₂ is found to be positive indicating a repulsive and probably mainly steric interaction between neutral in nature dextran and and practically uncharged β-lactoglobulin (pH=5.18). The measurements show that the β-lactoglobulin has only a small tendency to form multimers in the presence of dextran. The phase diagram of solutions of dextran+Whey Protein Isolate (appr. 60% β-lactoglobulin) is also presented. The McMillan–Mayer equation of state that considers only the second virial coefficients is found to be unreliable for the extrapolation up to the concentrations at which phase separation is expected.     

Kinetics of phase separation during pressure-induced gelation of a whey protein isolate

The kinetic process of pressure-induced gelation of whey protein isolate (WPI) solutions (20–28%, w/v) was studied using in situ light scattering. The gelation of WPI solutions could be induced by pressurization at 250 MPa, a pressure lower than that reported in other studies. The gelation time decreased with increasing WPI concentration and followed an exponential rule. The relationship of the logarithm of scattered light intensity (I) versus time (t) was linear after the induced time and could be described by the Cahn–Hilliard linear theory. With increasing time, the scattered intensity deviated from the exponential relationship, and the time evolution of the scattered light intensity maximum I_m and the corresponding wavenumber q_m could be described in terms of the power-law relationship as I_m  t^β and q_m  t^−α, respectively. These results indicated that phase separation occurred during the gelation of WPI solutions under high pressure.     

Influence of gliadins on rheology of methylcellulose in 70% (v/v) aqueous ethanol

Rheological properties of methylcellulose in 70% (v/v) aqueous ethanol with and without 100 g L⁻¹ gliadins were studied as a function of methylcellulose concentration C from 0 g L⁻¹ to 20 g L⁻¹. The overlapping concentration of methylcellulose is 7.5 g L⁻¹ in the absence of gliadins while methylcellulose is in the semidilute region at C ≤ 20 g L⁻¹ in the presence of gliadins. The presence of gliadins weakens the shear thinning under strain rate sweep. However, dynamic stress sweep reveals that gliadins may enhance the stress thinning and the solutions at C > 7.5 g L⁻¹ exhibit two Newtonian plateaus which are not observed in the absence of gliadins. The results suggest that the mutual interaction between methylcellulose and gliadin leads to the appearance of two Newtonian plateaus. The methylcellulose solutions in the absence of gliadins do not gel upon heating. On the other hand, gliadins in the solution facilitate the hydrophobic association of methylcellulose molecules upon heating, which leads to the thermal gelation of the solution at C = 20 g L⁻¹.     

Study of xanthan gum/waxy corn starch interaction in solution by viscometry

The objective of the study was to investigate the molecular interactions of xanthan gum and waxy corn starch in a ternary system. Solutions of xanthan gum/(waxy corn starch) blends dissolved in a mixed solvent of 90% DMSO/10% H₂O (v/v) were studied by means of viscometry as a function of total polymer concentration and composition. The classical Huggins equation, η_sp/C=[η]+bC, which expresses the specific viscosity (η_sp) of a polymer as a function of the concentration C, was found to be suitable for all blends in the dilute system. The intrinsic viscosity ([η]) and Huggins parameter b of the blends increased with the increase of xanthan fraction in the blends. Meanwhile, the addition of xanthan gum also dramatically lowered the overlap concentration (C*) of the blends. The results indicate that xanthan gum is a good thickener, and xanthan gum and waxy corn starch are attracted to each other in the solution under study.     

Tuning the properties of β-lactoglobulin nanofibrils with pH,NaCl and CaCl2

We investigated the effects of pH (1.6–2.4), NaCl and CaCl₂ (0–100 mm) on the kinetics of β-lactoglobulin fibril formation during heating at 80 °C. The morphology of fibrils was also examined. At pH 1.8–2.4 fibril formation occurred slightly faster with decreasing pH. At pH 1.6 fibril formation during the growth phase occurred much faster than at any other pH. Fibril morphology was unchanged between pH 1.6 and pH 2.0. Addition of NaCl or CaCl₂ accelerated fibril formation during the growth phase, and CaCl₂ shortened the lag phase as well. Worm-like fibrils were seen at ≥60 mm NaCl or ≥33 mm CaCl₂, and these had a persistence length which was much shorter than the long semi-flexible fibrils formed without salts. The efficiency of fibril formation can be substantially enhanced by varying pH and salt concentration.     

Ascospore inactivation and germination by high pressure processing is affected by ascospore age

The effects of age on high pressure resistance of the ascospores of heat resistant moulds Byssochlamys fulva, B. nivea, Neosartorya fischeri and N. spinosa were determined. Ascospores were harvested from cultures grown for 3–15 weeks at 30 °C on malt extract agar. Following filtration and determination of concentration, the ascospores were subjected to high pressure processing (HPP) at 600 MPa for 10 min in 0.1 M citrate phosphate buffer (pH 4 and 6) and mango puree (pH 5). The results supported our hypothesis that age (maturity) affects high pressure resistance of ascospores of heat resistant moulds. A reduction of log₁₀ 2.5 cfu mL^− 1 was achieved for three week old ascospores ofB. nivea whereas for nine week old ascospores only a half log reduction was achieved. Similar results were observed for B. nivea and N. fischeri. The HPP treatment caused activation of ascospores of N. spinosa, with older ascospores showing increased activation.

Industrial relevance

The observation of activation of some ascospores by HPP, indicates that HPP alone is insufficient for elimination of these problematic spoilage microorganisms. HPP would need to be combined with other hurdles in order to produce high quality pressure-treated shelf-stable fruit products.     

Chitosan-coated pectin beads: Characterization and in vitro release of mangiferin

Microspheres of low degree of esterification (DE) pectin with calcium and the same sphere coated with chitosan (PCaC) were prepared. The spheres have diameters in the range 650–680 μm. The layer of chitosan is about of 5–15 μm thick. To obtain firm and stable PCaC beads, chitosan was reacetylated. Two different degrees of acetylation, giving PCaC50 and PCaC80 were adopted. The beads were characterized by FTIR, SEM and swelling measurements. Mangiferin was loaded in PCaC reacetylated in two different ways: by addition in pectin solution (M_p) and by addition in CaCl₂ solution (M_c). The yield in producing the beads, the efficiency in encapsulation and the content of mangiferin in beads were determined. A swelling kinetics study was done in simulated gastric fluid (SGF, pH 1.2) and in simulated intestine fluid (SIF, pH 7.4). The release of mangiferin from the beads was performed in SGF followed by the release in SIF. Based on the yield and efficiency in encapsulation the best bead was found to be PCaC50-M_p. The highest release (7.8 mg of mangiferin/g of bead) was achieved by the PCaC50-M_c. For all beads more bioactive was released in SGF than in SIF.     

Microstructural characterization of β-lactoglobulin–konjac glucomannan systems: Effect of NaCl concentration and heating conditions

In the initial part of this study, the high temperature (85 °C) microscopic phase behaviour of β-lactoglobulin (0.4–6%, w/w)–konjac (0.05–0.75%, w/w) mixtures containing 50 mM NaCl was established using confocal laser scanning microscopy (CLSM). Also, the effects of heating time (heating temperature: 78 °C) and NaCl concentration (0–75 mM) on protein denaturation kinetics and the phase behaviour in 2%, w/w, β-lactoglobulin–0.4%, w/w, konjac mixtures were investigated using turbidimetry, protein denaturation measurement, CLSM and image analysis techniques. Segregative phase separation occurred in heat-treated β-lactoglobulin–konjac mixtures containing biopolymer and NaCl concentrations exceeding certain critical levels, due to heat and NaCl induced β-lactoglobulin denaturation/aggregation. The microstructural properties of selected heated (to 85 °C for 30 min) and cooled (to 25 °C) β-lactoglobulin–konjac mixtures containing different NaCl levels were studied using CLSM and rheological measurements and the results showed that the microstructure can be distinguished as miscible, phase separated or phase separated containing stable protein inclusions dependent on NaCl concentration. Response surface methodology was used to determine the minimum NaCl concentrations required for phase separation and for formation of phase separated systems containing stable inclusions in a wide concentration range of heated and cooled β-lactoglobulin (0.8–2%, w/w)–konjac (0.2–0.75%, w/w) mixtures. The results show that the microstructural and rheological properties of β-lactoglobulin–konjac mixtures can be controlled by selecting appropriate mixture biopolymer and NaCl concentrations and heating conditions.     

Lipid production in Porphyridium cruentum grown under different culture conditions

Autotrophic growth of Porphyridium cruentum under 18:12 h and 12:12 h light:dark cycles showed the maximum cell concentration of 2.1 g-dry wt./L, whereas the specific growth rate, 0.042 (1/h), at 18:6 h is faster than that of 12:12 h, 0.031 (1/h), respectively. The highest lipid accumulation level, 19.3 (%, w/w), was achieved at 12:12 h cycle. Under dark cultivation condition with 10 g/L of glucose, the lipid accumulation in the cell was 10.9 (%, w/w), whereas the heterotrophic growth with glycerol as the carbon resource showed low level of cell concentration and lipid production, compared to that of glucose. The glucose was decided to be a suitable carbon resource for the heterotrophic growth of P. cruentum. The lipids from P. cruentum seemed be feasible for biodiesel production, because over 30% of the lipid was C₁₆–C_18:1. The cultivation time and temperature were important factors to increase the maximum cell concentration. Extending the cultivation time helps maintain the maximum cell concentration, and higher lipid accumulation was achieved at 25 °C, compared to 35 °C. The fed-batch cultures showed that, under the light condition, the specific production rate was slightly decreased to 0.4% lipid/g-dry wt./day at the later stage, whereas, under the dark condition, the specific production rate was maintained to be a maximum value of 1.1% lipid/g-dry wt./day, even in the later stage of cultivation. The results indicate that the heterotrophic or 12:12 h cyclic mixotrophic growth of P. cruentum could be used for the production of biodiesel in long-term fed-batch cultivation of P. cruentum.     

Kinetic studies of the thermal inactivation of plasmin in acid or sweet whey

The thermal behaviour of the milk alkaline proteinase, plasmin, was studied in acid and sweet (rennet) whey; indigenous (bovine) plasmin was studied in the former system, but endogenous porcine plasmin was added in the latter, due to the very low levels of residual plasmin. The inactivation of plasmin in both systems followed first-order inactivation kinetics, which was consistent with previous observations of plasmin inactivation in milk and model milk systems. The thermal inactivation of plasmin in acid whey (D_{90 °C}=108±29 min, z=24.5±1.2 °C) was much slower than in the sweet whey system (D_{90 °C}=0.021±0.006 min, z=7.3±0.3 °C). Similarly, denaturation of β-lactoglobulin (β-lg) followed a first-order inactivation profile and this protein was also more heat stable in acid whey (D_{90 °C}=86±76 min, z=13.7±1.5 °C) than sweet whey (D_{90 °C}=0.81±0.29 min, z=9.1±0.5 °C). While it is possible that the increased heat stability of plasmin in acid whey is due to reduced sulphydryl/disulphide interchange reactions between plasmin and β-lg, it also appears that structural changes in the plasmin molecule were a significant contributory effect on the thermal stability of plasmin in this system. Increasing the pH of acid whey decreased the heat stability of plasmin. However, adjusting the pH of sweet whey had little effect on the heat stability of plasmin. Overall, severe heat treatments may be required to ensure inactivation of the enzyme in acid whey, but a balance is required between reducing the activity of plasmin and maintaining the functionality of whey proteins as food ingredients.     

A novel global hydrodynamic analysis of the molecular flexibility of the dietary fibre polysaccharide konjac glucomannan

Konjac glucomannans have been widely considered in health food products although their hydrodynamic properties have been poorly understood. The weight-average molecular weight (M_w); sedimentation coefficient (s⁰_20,w) and intrinsic viscosities ([η]) have been estimated for five different preparations. The decrease in both intrinsic viscosity and sedimentation coefficient with molecular weight enables the estimation of molecular flexibility in terms of persistence length (L_p) using the traditional Bohdanecky–Bushin and Yamakawa–Fujii analyses for intrinsic viscosity and sedimentation data respectively. However, this requires an assumption of the mass per unit length M_L. Advantage can now be taken of a recent development in data interpretation which allows the estimation of L_p from combined intrinsic viscosity and sedimentation coefficient data and also an estimate for M_L. Using this “global” procedure an estimate of (13 ± 1) nm is found for L_p and a value of (330 ± 10) g mol⁻¹ nm⁻¹ for M_L.The value for L_p suggests a molecule of considerable flexibility, comparable to galactomannans (L_p  8–10 nm) but not as flexible as pullulan (L_p  1–2 nm).     

Combined effect of an O2 absorber and oregano essential oil on shelf-life extension of Greek cod roe paste (tarama salad) stored at 4 °C

In the present study the combined effect of an O₂ absorber and oregano essential oil (0.1% v/w) on shelf life extension of Greek cod roe paste (tarama salad) stored under refrigeration (4 °C) was investigated. The study was based on microbiological [Total viable count (TVC), Lactic Acid Bacteria (LAB), Enterobacteriaceae, H₂S-producing, yeast and molds and Clostridium spp.), physicochemical (pH, fatty acid composition, thiobarbituric acid (TBA), and color) and sensory (color, odor, and taste) changes occurring in the product as a function of treatment and storage time. Aerobically packaged tarama salad stored at 4 °C was taken as the control sample. Results showed that TVC exceeded 7 log cfu/g on day 12–13 of storage for control samples and day 31–32 for samples containing oregano oil. Samples containing either the O₂ absorber or the O₂ absorber plus oregano oil never reached 7 log cfu/g during the 60 day storage period. LAB were only partially inhibited by the oregano oil and/or the O₂ absorber. Yeasts and molds were totally inhibited by the O₂ absorber. Enterobacteriaceae populations were below the method detection limit (2 log cfu/g) H₂S-producing bacteria were the dominant spoilage microorganisms. Clostridum spp. was absent in 25 g sample. pH decreased from an initial value of 4.36 to 3.03 depending on specific treatment. Color parameters L and b increased and a decreased in control samples as well as in samples containing oregano oil. Color parameters remained unaffected in samples containing the O₂ absorber. TBA expressed as malondialdehyde (MDA) values increased from 1.5 mg/kg to 3.4, 3.2 and 2.9 mg/kg for samples containing oregano oil, the O₂ absorber and O₂ absorber plus oregano oil at the point of sensory rejection, respectively. Saturated fatty acids (SFA) increased during storage with a respective decrease in monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA) both in control samples and samples containing oregano oil. Fatty acid composition remained unaffected in all samples containing the O₂ absorber. Sensory shelf life was 24 days for the control samples, 32 days for samples containing oregano oil, 60 days for samples containing the O₂ absorber and at least 60 days for samples containing the O₂ absorber plus oregano oil.

Industrial relevance

Oxygen absorbers as well as plant essential oils are considered natural means of preservation and may substantially extend the shelf life of foodstuffs while maintaining desirable sensory attributes (taste, odor and color).     

Physical and thermo-mechanical properties of whey protein isolate films containing antimicrobials, and their effect against spoilage flora of fresh beef

The effectiveness of antimicrobial films against beef's spoilage flora during storage at 5 °C and the impact of the antimicrobial agents on the mechanical and physical properties of the films were examined. Antimicrobial films were prepared by incorporating different levels of sodium lactate (NaL) and -polylysine (-PL) into sorbitol-plasticized whey protein isolate (WPI) films. The moisture uptake behavior and the water vapor permeability (WVP) were affected only by the addition of NaL at all concentrations used since an increased water uptake and permeability were observed with the addition of NaL into the protein matrix. An increase of the glass transition temperature (5–15 °C) of the sorbitol region, as determined by Dynamic Mechanical Thermal Analysis (DMTA), was caused by the addition of -PL into the WPI specimens. Instead, incorporation of NaL into the protein matrix did not alter its thermo-mechanical behavior. The addition of NaL at concentrations of 1.0% and 1.5% w/w in the film-forming solution resulted in a decline of maximum tensile strength (σ_max) and Young modulus (E). A decrease of E and σ_max, accompanied with an increase in elongation at break (%EB), was also observed with increasing -PL concentration, at moisture contents higher that 10% (w/w). The antimicrobial activity of the composite WPI films was tested on fresh beef cut portions. The maximum specific growth rate (μ_max) of total flora (total viable count, TVC) was significantly reduced with the use of antimicrobial films made from 0.75% w/w -PL in film-forming solutions (p < 0.05), while the growth of Lactic Acid Bacteria was completely inhibited. Significant inhibition of growth of the total flora and pseudomonads was also observed with the use of films made with protein solutions containing 2.0% w/w NaL. These results pointed to the effectiveness of the antimicrobial whey protein films to extend the shelf life of fresh beef.