Effect of surface-active stabilizers on the surface properties of coconut milk emulsions

Previously we have demonstrated improved stability of coconut milk emulsions homogenized with various surface-active stabilizers, i.e., 1 wt% sodium caseinate, whey protein isolate (WPI), sodium dodecyl sulfate (SDS), or polyoxyethylene sorbitan monolaurate (Tween 20) [Tangsuphoom, N., & Coupland, J. N. (2008). Effect of surface-active stabilizers on the microstructure and stability of coconut milk emulsions. Food Hydrocolloids, 22(7), 1233–1242]. This study examines the changes in bulk and microstructural properties of those emulsions following thermal treatments normally used to preserve coconut milk products (i.e., −20 °C, −10 °C, 5 °C, 70 °C, 90 °C, and 120 °C). Calorimetric methods were used to determine the destabilization of emulsions and the denaturation of coconut and surface-active proteins. Homogenized coconut milk prepared without additives was destabilized by freeze–thaw, (−20 °C and −10 °C) but not by chilling (5 °C). Samples homogenized with proteins were not affected by low temperature treatments while those prepared with surfactants were stable to chilling but partially or fully coalesced following freeze–thaw. Homogenized coconut milk prepared without additives coalesced and flocculated after being heated at 90 °C or 120 °C for 1 h in due to the denaturation and subsequent aggregation of coconut proteins. Samples emulsified with caseinate were not affected by heat treatments while those prepared with WPI showed extensive coalescence and phase separation after being treated at 90 °C or 120 °C. Samples prepared with SDS were stable to heating but those prepared with Tween 20 completely destabilized by heating at 120 °C.     

Associative and segregative interactions between gelatin and low-methoxy pectin: Part 2—co-gelation in the presence of Ca

The effect of segregative interactions with gelatin (type B; pI=4.9; 0–10 wt%) on the networks formed by low-methoxy pectin on cooling in the presence of stoichiometric Ca²⁺ at pH 3.9 has been investigated by rheological measurements under low-amplitude oscillatory shear. Samples were prepared and loaded at 85 °C, cooled (1 °C/min) to 5 °C, held for 100 min, and re-heated (1 °C/min) to 85 °C, with measurement of storage and loss moduli (G′ and G″) at 10 rad s⁻¹ and 2% strain. The final values of G′ at 5 °C for mixtures prepared at the same pH without Ca²⁺ were virtually identical to those observed for the same concentrations (0.5–10.0 wt%) of gelatin alone, consistent with the conclusion from the preceding paper that electrostatic (associative) interactions between the two polymers become significant only at pH values below 3.9. Increases in moduli on cooling in the presence of Ca²⁺ occurred in two discrete steps, the first coincident with gelation of calcium pectinate alone and the second with gelation of gelatin. Both processes were fully reversible on heating, but displaced to higher temperature (by 10 °C), as was also observed for the individual components. The magnitude of the changes occurring over the temperature range of the gelatin sol–gel and gel–sol transitions demonstrates that the gelatin component forms a continuous network; survival of gel structure after completion of gelatin melting shows that the calcium pectinate network is also continuous (i.e. that the co-gel is bicontinuous). On progressive incorporation of NaCl (to induce phase separation before, or during, pectin gelation) the second melting process, coincident with loss of calcium pectinate gel structure, was progressively abolished, indicating conversion to a gelatin-continuous network with dispersed particles of calcium pectinate. These qualitative conclusions are supported by quantitative analyses reported in the following paper.     

Encapsulation of emulsified tuna oil in two-layered interfacial membranes prepared using electrostatic layer-by-layer deposition

Tuna oil-in-water emulsions (5 wt% tuna oil, 100 mM acetate buffer, pH 3.0) containing droplets stabilized either by lecithin membranes (primary emulsions) or by lecithin–chitosan membranes (secondary emulsions) were produced. The secondary emulsions were prepared using a layer-by-layer electrostatic deposition method that involved adsorbing cationic chitosan onto the surface of anionic lecithin-stabilized droplets. Primary and secondary emulsions were prepared in the absence and presence of corn syrup solids (a carbohydrate widely used in the micro-encapsulation of oils) and then their stability to environmental stresses was monitored. The secondary emulsions had better stability to droplet aggregation than primary emulsions exposed to thermal processing (30–90 °C for 30 min), freeze-thaw cycling (−18 °C for 22 h/30 °C for 2 h), high sodium chloride contents (200 mM NaCl) and freeze-drying. The addition of corn syrup solids decreased the stability of primary emulsions, but increased the stability of secondary emulsions. The interfacial engineering technology used in this study could lead to the creation of food emulsions with novel properties or improved stability to environmental stresses.     

Influence of emulsifier type on freeze-thaw stability of hydrogenated palm oil-in-water emulsions

The influence of emulsifier type (Tween 20, whey protein isolate, casein) on the physical properties of 20 wt% hydrogenated palm oil-in-water emulsions after crystallization of (i) the oil phase only or (ii) both the oil and water phases has been examined. Emulsion stability was assessed by differential scanning calorimetry measurements of fat destabilization after cool–heat cycles, and by measurements of mean particle size, oiling off, and gravitational separation after isothermal storage (−20 to 37 °C). Tween 20-stabilized emulsions showed appreciable fat destabilization at temperatures where the oil phase was partially crystalline, which was attributed to partial coalescence. Protein-stabilized emulsions were stable under these conditions, which was mainly attributed to the relatively thick interfacial membranes surrounding the droplets. When both oil and water phases crystallized, there was complete destabilization of Tween 20- and casein-stabilized emulsions, and extensive destabilization of whey protein-stabilized emulsions, which was attributed to ice crystallization. The results of this study could facilitate the development of frozen food products with improved properties.     

Competitive adsorption of α-lactalbumin in the molten globule state

The molten globule state of α-lactalbumin is a partially denatured form with native-like secondary structure and disordered tertiary structure. Using circular dichroism measurements, it was demonstrated that the molten globule state was produced by decreasing the pH to 2.0 at 25°C or by removing bound Ca²⁺ by treatment with ethylenediamine—tetraacetic acid (EDTA) at pH 7.5 and 40°C. Tension measurements showed that α-lactalbumin in the molten globule state is more easily unfolded at liquid interfaces than is the native protein. Results of competitive adsorption experiments involving α-lactalbumin and β-lactoglobulin at the oil droplet surface in emulsions are consistent with preferential adsorption of α-lactalbumin during emulsification when it is in the molten globule state. In contrast to the difficulty of exchange between α-lactalbumin and β-lactoglobulin at the oil-water interface in emulsions at 25°C, it has been found that the two whey proteins are able partially to displace one another from the oil—water interface at 40°C. While native α-lactalbumin was found to be readily displaced from the oil—water interface by β-lactoglobulin at 40°C, it was found that α-lactalbumin in the molten globule state in the presence of EDTA at 40°C had itself the capacity for displacing β-lactoglobulin from the interface.     

Survival kinetics of Ephestia kuehniella eggs during 46–75 °C heat treatment

The effect of heat treatment on the survival of Ephestia kuehniella eggs was examined. Samples of 60 eggs were immersed in hot water at constant temperature in the 46–75 °C range for 5–1200 s. Following heat treatment and cooling, the eggs were stored at 24 ± 1 °C in a growth chamber for 7 days before survival evaluation. Statistical analysis of the data demonstrated that the thermal survival kinetics were best represented by a first-order reaction. The rate constant had an Arrhenius-type dependence over the 54–75 °C temperature range. Kinetic parameters were estimated by non-linear regression. The activation energy (E_a) and rate constant (k_ref) at the reference temperature (T_ref = 64.8 °C), were determined as 102.2 ± 6.2 kJ mol⁻¹ and 0.061 ± 0.003 s⁻¹, respectively, over the 54–75 °C temperature range. A 0.01% survival rate was obtained after 50 s at 75 °C. The data at temperatures below 50 °C were not in accordance with those at higher temperatures. Above this temperature, mortality was likely due to physiological disorders, as noted on a DSC thermogram.     

Characterization of gelation time and texture of gelatin and gelatin–polysaccharide mixed gels

The effects of cooling rate, holding temperature, pH and polysaccharide concentration on gelation characteristics of gelatin and gelatin–polysaccharide mixtures were investigated using a mechanical rheometer which monitored the evolution of G′ and G″. At low holding temperatures of 0 and 4 °C, elastic gelatin gels were formed whereas a higher holding temperature of 10 °C produced less elastic gels. At slow cooling rates of 1 and 2 °C/min, gelling was observed during the cooling phase in which the temperature was decreased from room temperature to the holding temperature. On the other hand, at higher cooling rates of 4 and 8 °C/min, no gelation was observed during the cooling phase. Good gelling behavior similar to that of commercial Strawberry Jell-O^® Gelatin Dessert was observed for mixtures of 1.5 and 15 g sucrose in 100 ml 0.01 M citrate buffer containing 0.0029–0.0066 g low-acyl gellan. Also, these mixed gels were stronger than Strawberry Jell-O^® Gelatin Desserts as evidenced by higher G′ and gel strength values. At a very low gellan content of 0.0029 g, increasing pH from 4.2 to 4.4 led to a decrease in the temperature at the onset of gelation, G′ at the end of cooling, holding and melting as well as an increase in gel strength. The gelation time was found to decrease to about 40 min for gelatin/sucrose dispersions in the presence of 0.0029 g gellan at pH 4.2 whereas the corresponding time at pH 4.4 was higher (79 min). In general, the gelation time of gelatin/sucrose dispersions decreased by a factor of 2 to 3 in the presence of low-acyl gellan. The addition of low-acyl gellan resulted in an increase in the gelation rate constant from 157.4 to 291 Pa. There was an optimum low-acyl gellan content for minimum gelation time, this optimum being pH dependent. Addition of guar gum also led to a decrease in gelation time to 73 min with a corresponding increase in the gelation rate constant to 211 Pa/min though these values were not sensitive to guar gum content in the range of 0.008–0.05 g. The melting temperature of gelatin/sucrose/gellan as well as gelatin/sucrose/guar mixtures did not differ significantly from that of pure gelatin or Strawberry Jell-O^® Gelatin Desserts. At pH 4.2, the melting rate constant was highest at a low-acyl gellan content of 0.0029 g whereas the rate constant was insensitive to low-acyl gellan content at pH 4.4. Addition of guar did not seem to affect the melting temperature or the melting rate constant.     

Migration of α-tocopherol from an active multilayer film into whole milk powder

Antioxidant active packaging is a promising technology for whole milk powder (WMP) protection. In this study, the migration of α-tocopherol from a multilayer active packaging (made of high density polyethylene, ethylene vinyl alcohol and a layer of low density polyethylene containing the antioxidant) to WMP was studied. A model based on the Fick’s diffusion equation was used to calculate the diffusion coefficients (D) of α-tocopherol as 2.34 × 10⁻¹¹, 3.06 × 10⁻¹¹, and 3.14 × 10⁻¹¹ cm² s⁻¹ at 20, 30 and 40 °C, respectively. The D at 20 °C was different from those at 30 and 40 °C (P < 0.05); but it was similar at 30 and 40 °C. This low influence of temperature on the migration of α-tocopherol from 20 to 40 °C assures the release at real storage and commercialization conditions in regions with warm/hot climate. The antioxidant delivering system delayed the lipid oxidation of WMP and it was more effective at 30 and 40 °C since the rate of oxidative reactions was higher at these temperatures than at 20 °C.     

Inhibition of Clostridium sporogenes growth in mascarpone cheese by co-inoculation with Streptococcus thermophilus under conditions of temperature abuse

The ability of a biological control system to inhibit the outgrowth of Clostridium sporogenes spores during storage of mascarpone cheese under temperature-abuse conditions was investigated. Challenge studies were carried out on mascarpone cheese artificially contaminated with spores of C. sporogenes (10 cfu g⁻¹), and with or without the coinoculum of a Streptococcus thermophilus strain (10⁵cfu g⁻¹). During storage at 4, 12, and 25°C, the outgrowth of clostridia spores, the growth of S. thermophilus, and the pH changes were evaluated at 10, 20, 30, and 40 days. In mascarpone cheese stored at 4° and 12°C, S. thermophilus and C. sporogenes did not show any growth. The initial pH (6·14) of the product also remained unchanged. During storage at 25°C S. thermophilus grew up to about 10⁷cfu g⁻¹after 10 days, resulting in a pH decrease of mascarpone cheese to values close to 4·5. The cell number decreased progressively during storage reaching values near to 10¹cfu g⁻¹after 40 days, whereas product acidity remained constant. C. sporogenes, when inoculated alone, also grew at 25°C. The cell number increased to levels of about 10⁷cfu g⁻¹after 20–40 days of storage according to the different mascarpone cheese lots used. No growth was found when C. sporogenes was co-inoculated in mascarpone cheese with S. thermophilus and stored at 25°C. The study on the behaviour of C. sporogenes, known as a non-toxigenic variant of Clostridium botulinum, allowed us to obtain useful information for setting up an effective biological control system to inhibit growth of the toxigenic species as well. The use of an additional barrier, besides refrigerated storage, may help to maintain the safety of mascarpone cheese in the event it was exposed to elevated temperatures.     

Repair and growth of heat- and freeze-injured Escherichia coli O157:H7 in selective enrichment broths

Two Escherichia coli O157:H7 strains, ATCC 35150 and 43894, were heat injured in a beef infusion at 53°C for 40 and 50 min, respectively (1· 5–2·0 log₁₀cfu ml⁻¹of injury) and freeze injured at −25°C for 30 days (1 log₁₀cfu ml⁻¹of injury) as determined by plating on MacConkey agar with 0·60% bile salts #3 (Mac-BS) as the selective medium and on Brain Heart Infusion agar (BHIA) as the non-selective medium. Repair of injury was measured in five selective enrichment broths [buffered peptone water supplemented with vancomycin, cefsulodin, and cefixime (BPW-VCC), modified EC broth with novobiocin (mEC+n), enterohaemorrhagic E. coli enrichment broth (EEB), double modified TSB (dmTSB), and BCM^®E. coli enrichment broth (BCM^®-EB)] versus TSB as the non-selective control broth over 3 h incubation at 37°C and 42°C. Repair was measured as the increase in cfu ml⁻¹enumerated on Mac-BS with time vs the total cfu ml⁻¹(injured and uninjured cells) enumerated on BHIA. In mEC+n, EEB, and dmTSB some death of both heat- and freeze-injured cells occurred immediately during the 3 h incubation (decrease on BHIA plates), and there was either minimal or no repair of the injured cells at both temperatures. Efficient repair of heat injury was obtained with both BPW-VCC and BCM^®-EB, but the latter produced a growth rate and final cell concentration closer to TSB. In freeze-injury repair however, BPW-VCC gave poor results while repair in BCM^®-EB was equal to TSB. Both BCM^®-EB and BPW-VCC inhibited the growth of all Gram-positive and a select number of Gram-negative bacteria tested. The ability of the selective enrichment broth BCM^®-EB to resuscitate heat- and freeze-injured E. coli O157:H7 efficiently within 3 h, warrants further testing with other types of stress in both artificially and naturally contaminated foods.     

Evaluation of buoyant density centrifugation as a sample preparation method for NASBA-ELGA detection ofCampylobacter jejuni in foods

Buoyant densities of four Campylobacter jejuni strains were in the range of 1·084–1·087 g ml⁻¹. Milk (3% fat) and chicken skin homogenates had buoyant densities beneath 1·033 g ml⁻¹. Discontinuous buoyant density centrifugation (BDC) in 40% Standard Isotonic BactXTractor medium respectively succeeded in recovering C. jejuni (5×10³–5×10⁴cfu ml⁻¹) from spiked milk (3% fat) and chicken skin. NASBA–ELGA detection of C. jejuni (5×10²–5×10³cfu ml⁻¹) in 12 different food samples using four different sample preparation methods was performed: RNA extraction by heating (filter and non-filter stomacher bag), RNA extraction by BDC (non-filter stomacher bag), RNA extraction by GuSCN–Triton-X-100 lysis and silica-purification (non-filter stomacher bag). BDC succeeded in overcoming inhibition of the amplification reaction except for one of the soft cheese samples. It was noticed that for chicken skin, chicken meat, pork, chicken sausage, turkey meat, milk (3% fat) and skimmed milk a simple heat treatment at 96°C without any additional precautions to prevent inhibition accomplished NASBA–ELGA detection of the pathogen. The use of a filter stomacher bag improved the quality of the NASBA–ELGA detection signal for beef but did not prevent inhibition of the amplification reaction in the cases of ground beef, prepared minced meat, soft cheese and hard cheese. The silica purification of RNA (which was used as a control treatment) accomplished NASBA–ELGA detection of C. jejuni for all of the latter food homogenates.     

Inactivation of Escherichia coli O157:H7 on surface-uninjured and -injured green pepper (Capsicum annuum L.) by chlorine dioxide gas as demonstrated by confocal laser scanning microscopy

Cells of Escherichia coli O157:H7 on uninjured and injured surfaces of green pepper were inactivated by 0·15–1·2 mg l⁻¹ClO₂gas treatments. A membrane-surface-plating method was used for resuscitation and enumeration of E. coli O157:H7 treated with ClO₂. The location and viability ofE. coli O157:H7 on uninjured and injured green pepper surfaces after ClO₂gas treatments were visualized using confocal laser scanning microscopy (CLSM). Live and dead cells of E. coli O157:H7 on pepper surfaces were labeled with a fluorescein isothiocyanate-labeled antibody and propidium iodide, respectively. A 7·27 log reduction of E. coli O157:H7 on uninjured green pepper surfaces was obtained with a 0·60 mg l⁻¹ClO₂gas treatment for 30 min at 20°C under 90–95% relative humidity. For injured surfaces, a 6·45 log reduction was achieved with a 1·2 mg l⁻¹ClO₂gas treatment. Each ClO₂gas treatment (0·15–1·2 mg l⁻¹ClO₂) for inoculated bacteria on uninjured surfaces showed significantly more reductions (1·23–4·24 log) than for those on injured surfaces (P<0·05). The microphotographs of CLSM showed that bacteria preferentially attached to injured surfaces and those bacteria could be protected from bacterial reduction by the injuries. This study indicates that ClO₂gas treatment can be a potential effective method of pathogen reduction for fresh fruits and vegetables.     

Effect of controlled gelatinization in excess water on digestibility of waxy maize starch

An aqueous dispersion of waxy maize starch (5%, w/w) was controlled gelatinized by heating at various temperatures for 5 min. The treated samples were analysed using in vitro Englyst assay, light microscopy, differential scanning calorimetry, X-ray diffraction, and Fourier-transform infrared spectroscopy. When heated, SDS and RS levels were decreased inversely with RDS. A high SDS content (>40%) was kept prior to the visible morphological and structural changes (before 60 °C). Swelling factor began to increase slightly at 50–60 °C and continued to maximum value at 80 °C. A large decrease in ΔH, crystallinity, and ratio of 1047/1022 cm⁻¹ attributed to partially dissociation of crystalline clusters and double helices occurred at 65–80 °C. These changes showed that controlled gelatinized starch with slow digestion property occurred in the molecular rearrangement process before granule breakdown and SDS mainly consists of amorphous regions and a small portion of less perfect crystallites.     

Destruction of acid- and non-adapted Listeria monocytogenes during drying and storage of beef jerky

Presence of Listeria monocytogenes in ready-to-eat meat products is not desired and strictly regulated in the US. Inactivation of acid- and non-adapted L. monocytogenes inoculated on beef slices was studied during drying and storage of jerky formulated with modified marinades. The inoculated (five-strain composite, c. 6·2 log cfu cm⁻²) slices were subjected to marinades (4°C, 24 h) prior to drying (60°C for 10 h) and aerobic storage (25°C for 60 days). The predrying marinade treatments tested were, first, no treatment, control (C); second, traditional marinade (TM); third, double amount of TM modified with 1·2% sodium lactate, 9% acetic acid, and 68% soy sauce containing 5% ethanol (MM); fourth, dipping into 5% acetic acid for 10 min and then applying the TM (AATM); and fifth dipping into 1% Tween 20 for 15 min and then into 5% acetic acid for 10 min followed by the TM (TWTM). Bacterial survivors on beef slices were determined during drying and storage using tryptic soy agar with 0·1% pyruvate (TSAP), and PALCAM agar. Results indicated that drying reduced bacterial populations in the order of pre-drying treatments TWTM (5·9–6·3 log cfucm⁻² in 10 h)≥AATM≥MM>TM≥C (3·8−4·6 log cfucm⁻² in 10 h). No significant (P≥0·05) difference was found in inactivation of acid-adapted and non-adapted inocula within individual treatments. Bacterial populations dropped below the detection limit (−0·4 log cfucm⁻²) as early as 4 h during drying or remained detectable even after 60 days of storage depending on acid-adaptation, predrying treatment, and agar media. These results indicated that acid-adaptation may not increase resistance to microbial hurdles involved in jerky processing and that use of modified marinades may improve the effectiveness of drying in inactivating L. monocytogenes.     

Effect of lactic acid on Listeria monocytogenes and Edwardsiella tarda attached to catfish skin

This study evaluated the use of lactic acid to decontaminate Listeria monocytogenes andEdwardsiella tarda attached to catfish skin with or without mucus. At the highest inoculum levels (10⁴–10⁵cfu skin⁻¹), lactic acid (0·5–2·0%) exposure for 10 min reduced counts of L. monocytogenes firmly attached to catfish skin by 0·9–>1·9 log₁₀cfu skin⁻¹and cells loosely attached by 2·7–>3·7 logs. Counts of E. tarda firmly attached to catfish skin were reduced by 0·9–>3·0 logs and cells loosely attached by 1·5–>3·5 logs. Overall bacterial numbers of lactic acid-treated cells that were firmly attached to skin with mucus were higher than on skin without mucus. Firmly attached L. monocytogenes was more resistant to lactic acid than was firmly attached E. tarda. Catfish skin mucus decreased the antimicrobial effect of lactic acid against attached L. monocytogenes and E. tarda.     

Aflatoxin M1 determination and stability study in milk samples using a screen-printed 96-well electrochemical microplate

A highly sensitive method for the determination of aflatoxin M₁ (AFM₁) is described that involves the use of a disposable multichannel microplate coupled to intermittent pulse amperometry (IPA) for immunosensor development based on a direct competitive assay. Immunoassay parameters were evaluated and optimized to establish an electrochemical enzyme-linked immunosorbent assay (ELISA) procedure for AFM₁ in milk samples. The suitability of the immunosensors for the direct analysis of the toxin in milk was assessed. AFM₁ was measured with a working range of 5–250 pg mL⁻¹ and a detection limit of 1 pg mL⁻¹. Good recovery values (90–105%) were obtained. The method was used to study the recovery of this toxin in frozen (−30 °C) or lyophilized (4 °C) milk samples stored for up to 3 months. A significant, but variable, decrease (>50%) of the measured AFM₁ concentration with respect to the initial toxin contamination levels was found.     

Mechanical, water vapor barrier and thermal properties of gelatin based edible films

Edible films are thin materials based on a biopolymer. The objectives of this work were to determine the water vapor permeability and the mechanical and thermal properties of edible films based on bovine hide and pigskin gelatins. These films were prepared with 1 g of gelatin/100 ml of water; 15–65 g sorbitol/100 g gelatin; and at natural pH. The samples were conditioned at 58% relative humidity and 22°C for 4 days before testing. The mechanical properties were determined by the puncture test and the water vapor permeability by gravimetric method at 22°C. For DSC analysis, samples were conditioned over silica gel for 3 weeks. Samples (10 mg) were heated at 5°C/min, between −150 and 150°C in a DSC TA 2010. A second scan was run after cell cooling with liquid nitrogen. As expected, the puncture force decreased and the puncture deformation and water vapor permeability increased with the sorbitol content. The origin of gelatin was important only above 25 g sorbitol/100 g gelatin. The DSC traces obtained in the first scan of samples with 15–35 g sorbitol/100 g gelatin, showed a well visible glass transition followed by a sol–gel transition. However, with the increase of sorbitol concentration, the glass transition became broader, typical of the system presenting a phase separation. The model of Couchman and Karazs for ternary system, was used to predict the Tg values as a function of sorbitol concentration.     

Rheological behavior of aqueous dispersions of cashew gum and gum arabic: effect of concentration and blending

Rheological properties of cashew gum (CG) and gum arabic (AR), the exudate polysaccharides from Anacardium occidentale L. and Acacia, at different solutions (0.4–50% w/v) were studied. The intrinsic viscosity, [η], of CG in water at 20°C was ≈0.1 dl g⁻¹, while that of AR was ≈0.6 dl g⁻¹. The apparent viscosity of the unheated and the heated (at 80°C for 30 min) CG and AR solutions showed a progressive increase with increasing concentration. The flow curves of blends with equal viscosity solutions of AR/CG: 25/75, 50/50 and 75/25, showed no major interaction. The apparent viscosity (η_a) vs. shear rate data for both the AR and CG dispersions (4–50% w/v) exhibited shear-thinning characteristics at low shear rates (< about 10 s⁻¹) and Newtonian plateaus at shear rates >100 s⁻¹, and the Sisko model described well the η_a vs. data of all the dispersions.     

Green tea extract as a natural antioxidant to extend the shelf-life of fresh-cut lettuce

Green tea extract (GT) was evaluated as a preservative treatment for fresh-cut lettuce. Different quality markers, e.g. respiration, browning, ascorbic acid and carotenoid content were evaluated. GT concentration (0.25, 0.5 and 1 g 100 mL^− 1) and temperature (20 °C and 50 °C) were tested. Optimal GT treatment (0.25 g 100 mL^− 1 at 20 °C) was compared with chlorine (120 ppm at 20 °C). High GT concentrations (0.5 g 100 mL^− 1 and 1.0 g 100 mL^− 1) maintained better prevent ascorbic acid and carotenoid loss than 0.25 g 100 mL^− 1 GT and chlorine. GT increased browning of samples, probably due to the content of polyphenols of the treatment; the use of heat-shock reduced this negative effect. GT and heat-shock combined also showed negative effects, reducing the antioxidant content (ascorbic acid and carotenoids). No significant differences were observed between chlorine and optimal GT (0.25 g 100 mL^− 1 at 20 °C) in browning appearance and sensory properties. GT better kept the antioxidant activity of the samples than chlorine.

Industrial relevance

An alternative treatment for minimally processed Iceberg lettuce is tested, based on its antioxidant capacity. Minimally processed industry is constantly looking for new treatments to avoid the use of chlorine which is a standard at the moment.     

Contamination of infant powdered milk in use with enterotoxigenic Staphylococcus aureus

One-hundred and ten of the 114 samples of infant milk formulae collected from nursing mothers contained viable staphylococci, with the highest mean count of 1·0 × 10² cfu g⁻¹, from samples collected the day the tin was opened. The highest mean total aerobic plate count was 2·6 × 10³ cfu g⁻¹. Titratable acidity and pH of the reconstituted milk ranged from 0·06 to 0·8 and 6·40 to 6·52 respectively. 52·0% of the 123 isolates were S. aureus, 49·6% produced β-hemolysin and 17·1% produced α-hemolysin. Enterotoxin A was produced by 6·5%, B by 1·6%, C by 2·4%, D by 1·6% and E by 0·8% of the isolates. The total staphylococcal and aerobic plate counts were not affected by either the period elapsed from the opening of tin to sampling or the brand of milk.