Tomato pulp as source for the production of lycopene powder containing high proportion of cis-isomers

Cis-isomers of lycopene has been demonstrated to possess higher biological activity than its all-trans form. The objectives of this study were to compare the extraction efficiency and degree of isomerization of lycopene by employing supercritical carbon dioxide (SCD) and solvents, as well as process the lycopene extract into powder containing high proportion of cis-isomers from tomato pulp as source. Results showed that a high yield of lycopene was achieved by SCD at 350 bar and 70 °C, with cis-isomers making up 41.4% of total lycopene. However, with temperature at 80 and 90 °C, the maximum isomerization was accomplished, and cis-lycopene constituted 51.0 and 53.8% of total lycopene, respectively. For solvent extraction, the highest yield of all-trans-lycopene was attained by ethanol–hexane (4/3 v/v) at 25 and 50 °C, whereas the maximum isomerization (47.0% cis-lycopene) occurred at 75 °C. A powder product containing 34.5% cis-isomers of lycopene was obtained by spray-drying, and the total amount of lycopene in spray-dried powder was much greater than that in freeze-dried powder. The maximum yield of lycopene in the powder product could be obtained through processing by adding sodium alginate to the eluate directly after SCD extraction and open-column chromatography with formation of 41.4% cis-lycopene. The method developed in this study may be used for possible commercial production of highly active lycopene powder.     

Carotenoid transfer to oil during thermal processing of low fat carrot and tomato particle based suspensions

Carotenoid solubilization in the oil phase is a prerequisite for carotenoid bioaccessibility during digestion. However, the level of bioencapsulation and the hydrophobicity of carotenoids were proven to strongly affect their transfer to oil during in vitro digestion. Therefore, thermal processing (95–110 °C) was exploited to favor carotenoid transfer from tomato- and carrot-based fractions to the oil before digestion. Initially, the total (all-trans + cis) carotenoid content in the oil increased quickly, thereafter, depending on the temperature applied, either a drop or a plateau was reached at longer treatment times. Treatment conditions of > 100 °C for 10 min significantly favoured carotenoid transfer to oil (≥ 75%). The rates of transfer to oil were as follows: β-carotene ≈ α-carotene > lycopene. The results revealed that the cell wall hinders carotenoid transfer to oil during thermal processing. Overall, the results indicate that typical high temperature short time thermal processing can be sufficient to achieve maximal carotenoid transfer to oil with minimal degradation in real food systems/food emulsions and this can be crucial to improve the nutritional quality of carrot and tomato based products.     

Lycopene degradation and isomerization in tomato dehydration

Lycopene is an important nutrient, since it appears to provide protection against a broad range of epithelial cancers. Tomatoes and tomato products are the major source of lycopene, and are considered to be an important source of carotenoids in the human diet. Biodegradation of lycopene not only affects the attractive color of the final products, but also their nutritive value. The main cause of lycopene degradation in tomato dehydration is isomerization and oxidation. The objectives of this study were to determine the retention of total lycopene and isomerization in different dehydration methods, and to optimize processing technology for the retention of lycopene biological potency in the tomato products. Experiments were carried out to compare the effect of osmotic treatment, vacuum-drying, air-drying and their combination on the retention of lycopene bioactivity. Firstly a skin treatment was applied to the tomatoes, following an osmotic treatment at 25°C in 65°Brix sucrose solution for 4 h, then vacuum-drying at 55°C for 4–8 h, or air-drying at 95°C for 6–10 h. In the fresh tomato samples, lycopene content is 75.5 μg/100 g on dry weight basis. Lycopene occurs in nature primarily in the more stable all-trans form. A significant increase in the cis-isomers with simultaneous decrease in the all-trans isomers can be observed in the dehydrated tomato samples in the different dehydration methods. The cis-isomers increased with temperature and processing time. In the osmotic treatment, the predominating mechanism is isomerization of lycopene. Since the total lycopene content remained essentially constant, but the distribution of trans- and cis-isomers changed. In the air-drying processing, isomerization and oxidation (autoxidation) as two strong factors affected simultaneously the decrease of total lycopene content, distribution of trans-and cis-isomers, and biological potency. A possible explanation of this result is that sugar enters the tomato matrix and strengthen the binding force on lycopene in the tomato matrix. Osmotic solution (sugar) remaining on the surface layer of the tomato prevents oxygen from penetrating and oxidizing lycopene. The osmotic treatment could reduce lycopene losses in comparison with other dehydration methods.     

Comparison of lycopene stability in water- and oil-based food model systems under thermal- and light-irradiation treatments

Lycopene can undergo degradation via isomerization and oxidation during processing and storage. These degradative reactions affect its bioactivity and health benefit functionality. Degradation kinetics and isomerization of lycopene in water- and oil-based tomato model systems were investigated as a function of thermal treatments and light irradiation. Results showed that 80 and 100 °C heating favoured the stability of lycopene in oil-based tomato products. The high heating temperatures (120 and 140 °C) increased isomerization of lycopene and resulting in degradation of total lycopene and cis-isomers in both water- and oil-based tomato products. However, the levels of degradation of total lycopene contents and cis-isomers were greater in water-based samples than in oil-based model systems under different treatments. Heat and light both promoted lycopene isomerization of the all-trans form to the cis-isomers and further oxidation of cis-isomers. The major effect of thermal degradation and photosensitized oxidation was a significant decrease in the total lycopene content, especially the content of cis-isomers. These research results could be useful in assisting industry to improve processing technology and to improve the nutritional value and health-benefits of tomato-based foods.     

The ripening influence of two papaya cultivars on carotenoid biosynthesis and radical scavenging capacity

Carotenoid biosynthesis in papaya fruits from the cultivars (cv.) ‘Golden’ and ‘Sunrise Solo’ was studied throughout three different ripening stages. The content of these secondary metabolites was assessed using HPLC–PDA–MSⁿ. Carotenoid levels increased during ripening, with all-trans-lycopene varying from 0.73 to 1.58 μg/g in the cv. ‘Golden’ and from 0.68 to 1.67 μg/g in the cv. ‘Sunrise Solo’. The all-trans-β-cryptoxanthin content varied from 1.29 to 3.0 μg/g in the cv. ‘Golden’ and from 0.28 to 5.13 μg/g in the cv. ‘Sunrise Solo’. The Zds gene showed a different pattern of expression during the ripening and between cultivars, while the Lcyβ gene expression was up-regulated in the two cultivars. The capacity to scavenge peroxyl radicals did not show a significant difference among the ripening stages and between the different cultivars. This study describes, for the first time, a tentative correlation between carotenoid biosynthesis in papaya pulp and the gene expression of the enzymes related to this pathway.     

Role of microbial strain and storage temperatures in the degradation of isoflavone phytoestrogens in fermented soymilk with selected β-glucosidase producing Lactobacillus casei strains

Soymilk fermented with 2 Lactobacillus casei strains were stored at various temperatures (80 °C, 4 °C, 24.8 °C and 37 °C) for 8 weeks and isoflavone concentration analysed at weekly intervals using RP-HPLC. The degradation of each isoflavone compound at each storage temperature was found to fit first order kinetic model. Aglycone as well as glucosides generally appeared to be stable during storage (P < 0.01) at the 4 storage temperatures. Aglycone forms had smaller degradation constants compared to glucosides at all storage temperature and in the presence of both microorganisms. Specifically, aglycones showed a unique trend of smaller degradation at lower storage temperatures (−80 °C and 4 °C) than at higher temperatures (24.8 °C and 37 °C). Glucoside genistin was least stable at all storage temperatures compared to other isoflavones in the fermented soymilk with each strain while aglycone daidzein was the most stable. L. casei 2607 in fermented soymilk stored at 4 °C after 8 weeks gave the least degradation for daidzein of a mere 3.78% loss from 9.53 to 9.17 ng/μL. L. casei 2607 showed greater hydrolytic potential than L. casei ASCC 290 as denoted by higher degradation of isoflavone glucosides in fermented soymilk at lower storage temperatures.     

Heat transfer coefficients during deep fat frying of sweetpotato: Effects of product size and oil temperature

Heat transfer coefficient, h during deep fat frying of sweetpotato was determined from designed experiments. Samples of three different disc sizes were fried at four oil temperatures between 150 and 180 °C. Heat transfer coefficient estimation was based on heat energy balance between the oil and sample during frying. Maximum h was reached at the earlier stage of frying (80–120 s). The range of maximum h reached was 710–850 W/m² °C. h at latter period of frying (200–300 s) was 450–550 W/m² °C. h was found to vary directly with oil temperature but inversely with sample size (p <0.05).     

Enrichment of the conjugated linoleic acid content of bovine milk fat by dry fractionation

This study investigated the effect of fat fractionation on the conjugated linoleic acid (cis-9, trans-11-C_18 : 2) content of bovine milk fat. Anhydrous milk fat was fractionated into hard and soft fractions using controlled cooling and agitation. Fractionation of milk fat pre-melted at 60°C using a temperature programme of 33–10°C and a cooling rate of 0.58°C h⁻¹ yielded a soft fraction containing 63.2% more conjugated linoleic acid (2.22 g 100 g⁻¹ FAME), which was also enriched in polyunsaturated fatty acids and vaccenic acid (trans-11-C_18 : 1) compared with the parent fat. Agitation following fractionation was found to have a negative effect on the conjugated linoleic acid content of the soft fraction. Refractionation of the soft fraction did not increase the yield of conjugated linoleic acid. The conjugated linoleic acid and trans fatty acid content of 26 selected food products ranging in milk fat content from 0 to 100% is reported. Conjugated linoleic acid concentrations ranged from 0 to 16.2 mg g⁻¹ fat and were generally lower than the trans fatty acid content which ranged from 0 to 155.7 mg g⁻¹ fat. Spreads containing vegetable oils contained higher trans fatty acid and lower conjugated linoleic acid contents than milk fat-containing products. This study highlights that a milk fat fraction enriched in conjugated linoleic acid may be achieved by dry fractionation.     

Rapid method for resveratrol determination by HPLC with electrochemical and UV detections in wines

The study describes an improved version of the reverse-phase-HPLC method. To determine free resveratrol isomers, the method uses isocratic elution and electrochemical detection with the use of a glassy working electrode at a potential of 0.75 V. The effects of important factors – such as the isomerization of standard resveratrol solution in diffused daylight and its dependence on temperature or resveratrol isomerization in wine samples – were investigated. The equilibrium between trans- and cis-isomer was achieved after a 5-h exposure to the daily diffused light and it was not significantly influenced by the temperature (30–60 °C). Linearity was obtained in the concentration range from 0.01 to 10 mg l⁻¹. The detection limit (S/N=3) was 3 μg l⁻¹ for trans-resveratrol and 15 μg l⁻¹ for cis-resveratrol. The trans- and cis-resveratrol were determined in selected Czech red and white wines and in the extract from Vitis vinifera plant. The concentration of trans-resveratrol ranged from 0.7 to 11 mg l⁻¹, that of cis-resveratrol from 0.6 to 5.1 mg l⁻¹.     

Effects of a hydrodynamic process on extraction of carotenoids from tomato

We evaluated the results of using a proprietary hydrodynamic method, which was introduced with the hope of increasing accessibility of beneficial nutrition-enhancing fruit and vegetable products. Tomato, a major dietary source of carotenoids, notably lycopene, was tested because of its many health benefits to consumers. Samples before and after treatment were compared for lycopene, phytoene, and phytofluene contents. Extractable lycopene and other carotenoids increased significantly. In nature, lycopene exists almost exclusively as the all-trans stereoisomer. Cis-lycopene isomers form during cooking and digestion, resulting in higher percentages in plasma and tissues than ingested. Cis-lycopene isomers are more bioavailable than all-trans lycopene. Extraction using this proprietary method increased extracted cis-lycopene to as high as 43% of the total lycopene, indicating increased isomerisation. This method could therefore contribute significantly to the delivery of health benefits of biologically available lycopene from tomato products for metabolic functions.     

Assessment of conventional and microwave heating induced degradation of carotenoids in olive oil by VIS Raman spectroscopy and classical methods

Raman spectroscopy, as a fast non-destructive method, has shown its potential in revealing heat-induced degradation of extra virgin olive oil carotenoids during microwave versus conventional heating processes. A progressive degradation in carotenoids was observed, starting at 180 °C and 140 °C in microwave and conventional heating processes respectively; this was followed by a rapid degradation at 180 °C only with conventional heating. As the main difference, the Raman bands due to carotenoids completely disappeared at 203 °C with conventional heating, while these bands could still be observed even up to 225 °C with microwave heating. Furthermore a loss of cis double bonds and slightly changes in free fatty acid was also observed in both heating processes. A precise calibration model was established using partial least squares regression, which can be used for monitoring carotenoids content degradation during heating. The accuracy of the model was estimated using the root mean square errors and the correlation coefficient.     

Bioavailability of β-carotene isomers from raw and cooked carrots using an in vitro digestion model coupled with a human intestinal Caco-2 cell model

Not only is there limited information in the literature regarding the β-carotene (BC) isomer profile of micelles from digested foods; few studies have looked at their subsequent uptake and transport by human intestinal Caco-2 cells. Therefore, the aims of the present study were, first, to assess the profile of BC isomers in micelles from digested raw and cooked carrots; and, second, to determine their cellular uptake and transport. Greater amounts of all-trans-, 13-cis- and 15-cis-BC isomers were present in the micelles of cooked carrots compared with raw carrots. Furthermore, micelle fractions obtained from the most highly processed (pureed) carrots had greater (P < 0.05) amounts of all-trans-, 13-cis- and 15-cis-BC compared with those derived from raw and boiled carrots. A similar trend was seen with BC isomer uptake and transport. Our data suggest that the food matrix and degree of processing play important roles on carotenoid isomerization and bioavailability.     

Effect of pH and level of concentrate in the diet on the production of biohydrogenation intermediates in a dual-flow continuous culture

Milk fat depression in cows fed high-grain diets has been related to an increase in the concentration of trans-10 C_18:1 and trans-10,cis-12 conjugated linoleic acid (CLA) in milk. These fatty acids (FA) are produced as a result of the alteration in rumen biohydrogenation of dietary unsaturated FA. Because a reduction in ruminal pH is usually observed when high-concentrate diets are fed, the main cause that determines the alteration in the biohydrogenation pathways is not clear. The effect of pH (6.4 vs. 5.6) and dietary forage to concentrate ratios (F:C; 70:30 F:C vs. 30:70 F:C) on rumen microbial fermentation, effluent FA profile, and DNA concentration of bacteria involved in lipolysis and biohydrogenation processes were investigated in a continuous culture trial. The dual-flow continuous culture consisted of 2 periods of 8 d (5 d for adaptation and 3 d for sampling), with a 2 × 2 factorial arrangement of treatments. Samples from solid and liquid mixed effluents were taken for determination of total N, ammonia-N, and volatile fatty acid concentrations, and the remainder of the sample was lyophilized. Dry samples were analyzed for dry matter, ash, neutral and acid detergent fiber, FA, and purine contents. The pH 5.6 reduced organic matter and fiber digestibility, ammonia-N concentration and flow, and crude protein degradation, and increased nonammonia and dietary N flows. The pH 5.6 decreased the flow of C_18:0, trans-11 C_18:1 and cis-9, trans-11 CLA, and increased the flow of trans-10 C_18:1, C_18:2n-6, C_18:3n-3, trans-11,cis-15 C_18:2 and trans-10,cis-12 CLA in the 1 h after feeding effluent. The pH 5.6 reduced Anaerovibrio lipolytica (32.7 vs. 72.1 pg/10 ng of total DNA) and Butyrivibrio fibrisolvens vaccenic acid subgroup (588 vs. 1,394 pg/10 ng of total DNA) DNA concentrations. The high-concentrate diet increased organic matter and fiber digestibility, nonammonia and bacterial N flows, and reduced ammonia-N concentration and flow. The high-concentrate diet reduced trans-11 C_18:1 and trans-10 C_18:1, and increased C_18:2n-6, C_18:3n-3 and trans-10,cis-12 CLA proportions in the 1 h after feeding effluent. The increase observed in trans-10,cis-12 CLA proportion in the 1 h after feeding effluent due to the high-concentrate diet was smaller that that observed at pH 5.6. Results indicate that the pH is the main cause of the accumulation of trans-10 C_18:1 and trans-10, cis-12 CLA in the effluent, but the trans-10,cis-12 CLA proportion can be also affected by high levels of concentrate in the diet.     

Thermal inactivation of Bacillus anthracis Sterne in irradiated ground beef heated in a water bath or cooked on commercial grills

The thermal stability of heat-shocked and non-heat-shocked spores of the virulence-attenuated Sterne strain of Bacillus anthracis was evaluated at select temperatures in irradiated, raw ground beef (25% fat) heated in a water bath or cooked using two different commercial grills. For the former, 3-g portions of inoculated ground beef were packaged in bags that were completely immersed in a temperature-controlled circulating water bath held at 65 °C (149 °F), 70 °C (158 °F), 75°(167 °F), and 80 °C (176 °F) for a predetermined length of time. For the latter, formed ground beef patties (95-g each) were inoculated with spore stock A or B of the Sterne strain and then cooked on a commercial open-flame gas grill or on a commercial clamshell electric grill to achieve target internal temperatures of either 71.1 °C (160 °F), 82.2 °C (180 °F), or 93.3 °C (200 °F). Cooking ground beef patties on commercial grills, resulted in reductions of ca. 0.8 to 3.5 log₁₀ CFU/g for spore stocks A and B of B. anthracis Sterne after heating to 71.1 °C (160 °F), 82.2 °C (180 °F), or 93.3 °C (200 °F) on either the open-flame gas grill which required ca. 9.6 min to reach the target internal temperatures or on the clamshell electric grill which required ca. 4.0 min to reach the target internal temperatures. In comparison, our data using a water bath system and heating at 65° to 80 °C predict nearly 4 log reductions in spore levels for short times, ~½ min, depending possibly on the temperature. Thus, our data suggest that models based on heating ground beef in a water bath is not a good predictor of reductions of levels of spores of B. anthracis Sterne strain that would be obtained when cooking ground beef patties on commercial grills under conditions that may be typically used by consumers and/or retail establishments. Nevertheless, our data validated that cooking ground beef patties on a commercial grill at a temperature considered to be “well-done” and a temperature (71.1 °C;160 °F) recommended by the USDA/FSIS, is effective at killing spores of B. anthracis Sterne.Industrial relevanceHeating ground beef in a water bath or cooking ground beef patties on commercial grills under conditions simulating those that are used by consumers and/or that occur in retail food service establishments is effective at killing spores of B. anthracis Sterne.     

Cis-trans isomerisation of lycopene and colour stability of foam—mat dried tomato powder during storage

Tomato powder with ～3% moisture was obtained by foam-mat drying tomato paste (29% solids). Samples of powder were stored at various temperatures (?10° +2° +20° and +37°c), atmospheres (air, nitrogen) and humidities (with and without in-package desiccation), and colour changes were observed for periods of up to 1 year. The main cause of colour fading was the oxidation of lycopene. However, another phenomenon, trans—cis isomerisation of all-trans lycopene, can occur, with apparent loss of coloration, which is later restored by re-isomerisation. The existence of this mechanism was demonstrated by simultaneous measurements of total liposoluble colour at 420 nm and separations of carotenoid pigments by column chromatography on Al₂O₃. Neo-lycopene A (6-mono-cis-lycopene) and at least one other cis-lycopene isomer were found in fresh powder and in samples stored up to 5 months. cis-Isomers were more susceptible to oxidation. Colour changes depend upon the interplay of isomerisation and oxidation; in some cases, tomato powder was more intensively coloured after storage than when fresh. Storage at +20°c was more favourable for colour retention than storage at +2°c, or — 10°c. Excessive desiccation strongly promoted oxidation losses. Storage in an inert atmosphere improved colour retention. At +37°c non-enzymic browning, accompanied by formation of water, caused rapid darkening although carotenoid pigments were not lost to a great extent.     

Migration of α-tocopherol from LDPE films to corn oil and its effect on the oxidative stability

The migration of α-tocopherol (α-T) from low density polyethylene (LDPE) films, added with 20 (film A) and 40 mg g^?1 (film B) to corn oil for 12 weeks at 5, 20 and 30 °C was determined. A LDPE film added with no α-T was used as control (film C). Diffusion coefficient (D) values for the film A system were 1.4 × 10^?11, 7.1 × 10^?11 and 30.3 × 10^?11 cm² s^?1 at 5, 20 and 30 °C, respectively. Meanwhile, D values for the film B system were 1.3 × 10^?11, 9.6 × 10^?11 and 51.1 × 10^?11 cm² s^?1 at the same temperatures. The activation energy (E_a) for the diffusion of α-T was 126.5 (film A) and 105.9 kJ mol^?1 (film B). The effect of the migration of α-T on the oxidative stability of corn oil was evaluated by monitoring hexanal content by solid phase micro-extraction (SPME) and gas chromatography. The hexanal content in the oil showed that both films added with α-T resulted suitable to maintain the oxidative stability of the oil for about 16 weeks at 30 °C, compared to 12 weeks for the oil in contact with the film C.     

Effects of cooked temperature on pork tenderness and relationships among muscle physiology and pork quality traits in loins from Landrace and Berkshire swine

The effect of, and associations between, loin muscle morphology and pork quality indicator traits were assessed at three cooked temperatures in loin chops from 38 purebred Berkshire and 52 purebred Landrace swine. Three loin chops from each pig were randomly assigned to cooked temperature treatments of 62, 71, or 79 °C and loin tenderness was assessed as Warner–Bratzler shear force (WBSF). Cooked temperature (P < 0.001), breed (P < 0.001) and breed × cooked temperature (P < 0.001) effects influenced loin chop WBSF, whereby WBSF increased as cooked temperature increased. Chops from Landrace pigs had greater WBSF at each cooked temperature compared with chops from Berkshire pigs. Chops from Landrace pigs became less tender with increasing cooked temperature, whereas chops from Berkshire pigs became less tender only when cooked to 79 °C. In loins from Landrace pigs, Minolta a1 at 62 °C (R² = 0.07), and average muscle fiber diameter at 71 °C and 79 °C (R² = 0.07 and 0.24, respectively), contributed to WBSF variation. In contrast, for loins from Berkshire pigs, loin ultimate pH and intramuscular fat percentage accounted for 27% and 30% of the variation in WBSF at 62 °C and 71 °C, respectively, and loin ultimate pH accounted for 7% of variation in WBSF at 79 °C. Results suggest that loins from Berkshire pigs have properties that resist toughening at greater cooked temperatures and that associations between quality measures and loin tenderness differ between Landrace and Berkshire pigs.     

Effect of a 10-day ageing at 30 °C on the texture of dry-cured hams processed at temperatures up to 18 °C in relation to raw meat pH and salting time

The aim of this study was to investigate the effect of a 10-day ageing at 30 ± 2 °C on the texture of dry-cured hams processed at temperatures up to 18 ± 2 °C for 12 months in relation with raw ham pH and salting time. Three pH groups (semimembranosus muscle at 24 h post-mortem: Low pH < 5.7, Medium pH = 5.7 ⩽ pH ⩽ 5.9, and High pH > 5.9), three salting times (6 d, 10 d and 14 d) and two ageing temperatures (18 °C and 30 °C) were investigated. Physicochemical characteristics, instrumental and sensory texture and product sliceability were evaluated on biceps femoris and semimembranosus muscles. Hams with pH_SM24 < 5.7 should be avoided in order to reduce the incidence of texture problems in dry-cured ham elaboration. Texture problems are especially important in hams with a reduced salt content that are mechanically sliced (not frozen). A 10-day ageing at 30 °C could be useful for reducing the soft texture problems in dry-cured hams processed at temperatures up to 18 °C for 12 months without affecting the product flavour.     

Behaviour of soyasapogenol B under optimised hydrolysis and ESI mass spec conditions

Soyasaponins are oleanane triterpenoids found in soy (Glycine max Merr). The analysis of the main aglycone (soyasapogenol B) is hampered by inconsistent ion fragmentation patterns produced during MS analysis. The greatest (p < 0.05) increase in soyasapogenol B during acid hydrolysis HCl (5% (v/v)) was at 100 °C (84.59 ± 4.73 μg/ml), followed by 120 °C (69.82 ± 6.20 μg/ml) and 80 °C (46.54 ± 6.41 μg/ml). The capillary temperature (LC–ESI-MS) of 220 °C produced the greatest (p < 0.05) intensity of soyasapogenol B ion. Temperatures between 200 and 250 °C consisted the optimum temperature range for analysing soyasapogenol B. At low capillary temperatures (<200 °C), the soyasapogenol B ion ([M?H]^? = 457.4) was converted to an ion m/z at 441, while, at temperatures ?250 °C the soyasapogenol B ion produced ions m/z at 440 and 437. We report an analysis procedure for detecting intact soyasapogenol B molecular ion without the typical fragmentation reported in the literature.     

Effect of transglutaminase on rheological properties and microstructure of chemically acidified sodium caseinate gels

The mechanical properties and microstructure of 2.7% and 4.5% sodium caseinate gels chemically acidified by glucono-δ-lactone (GDL) and cross-linked by microbial transglutaminase (TG) were studied. The acidification was performed at different temperatures. According to SDS–PAGE TG clearly caused polymerisation of caseinate irrespective of the treatment temperature (4–50 °C), The cross-linking of the proteins was more extensive at temperatures 22–50 °C. Low amplitude viscoelastic measurements showed that 4.5% caseinate gels acidified at 50 °C were formed much faster than gels acidified at 22 °C. TG only slightly increased the time of gelling. Control gels prepared without TG at temperatures of 4, 22, 37 and 50 °C were mechanically weak. Examination of the control gels with a confocal laser scanning microscope showed that gels formed at 37 and 50 °C were coarse and porous with large cavities between particle aggregates, whereas those formed at 22 °C were much more homogeneous. The TG-treated and acidified sodium caseinate dispersions formed firm gels, indicating cross-linking of casein proteins. Interestingly, the strongest gels were formed at 22 and 37 °C. TG treatment improved the homogeneity of the gel structure at temperatures of 37 and 50 °C. The hardness of TG-treated gels acidified at 4 °C increased during 1 week of storage.