Lightness and gloss of peach gels: effect of fruit and sugar content

 Lightness and gloss were studied in peach gels prepared with different quantities of sugar and peach pulp, using a mixture of xanthan and locust bean gum as gelling agents. Colour was described in terms of the colour coordinates established by the Commission Internationale de l'Eclairage and gloss was determined as a percentage. The values of lightness determined optically (L^*), when ranked, corresponded well with those determined by eye, but there was less similarity between the ranked values of gloss determined optically and by eye. There was a clear relationship between the sensory evaluation of lightness and gloss and the fruit and sugar content of these gels. Mathematical equations were developed to relate both L^* and gloss values determined optically with fruit and sugar concentrations. It was found that fruit had a greater effect than sugar on the L^* values and that gloss depended only on the sugar content. Finally, the gloss of gels prepared with different gelling agents were compared and it was found that lightness can affect the sensory evaluation of gloss. Received: 15 July 1997 / Revised version: 3 February 1998     

Lightness and gloss of peach gels: effect of fruit and sugar content

 Lightness and gloss were studied in peach gels prepared with different quantities of sugar and peach pulp, using a mixture of xanthan and locust bean gum as gelling agents. Colour was described in terms of the colour coordinates established by the Commission Internationale de l'Eclairage and gloss was determined as a percentage. The values of lightness determined optically (L^*), when ranked, corresponded well with those determined by eye, but there was less similarity between the ranked values of gloss determined optically and by eye. There was a clear relationship between the sensory evaluation of lightness and gloss and the fruit and sugar content of these gels. Mathematical equations were developed to relate both L^* and gloss values determined optically with fruit and sugar concentrations. It was found that fruit had a greater effect than sugar on the L^* values and that gloss depended only on the sugar content. Finally, the gloss of gels prepared with different gelling agents were compared and it was found that lightness can affect the sensory evaluation of gloss. Received: 15 July 1997 / Revised version: 3 February 1998     

Effect of chitin and chitosan on gelling properties of surimi from barred garfish (Hemiramphus far)

The addition of chitin/chitosan significantly increased the breaking force and deformation of gels prepared from barred garfish surimi (P < 0.05). Addition of 7B chitosan with 65.6% degree of deacetylation (% DD) at the level of 15 mg g⁻¹ resulted in the maximum increases in both breaking force and deformation of suwari and kamaboko gels compared to the control and gels containing chitin or chitosan with other % DD (P < 0.05). A chitosan concentration of 10 mg g⁻¹ was found to render the highest breaking force of kamaboko gel compared to other concentrations tested (P < 0.05). Kamaboko gel containing chitosan had an increased breaking force as the calcium chloride concentration increased (P < 0.05), indicating the role of endogenous transglutaminase in cross‐linking of protein–protein and protein–chitosan conjugates. Therefore the incorporation of chitosan and calcium chloride greatly improved the gelling properties of surimi from barred garfish without changes in colour. © 2000 Society of Chemical Industry     

Measurement of the colour and transparency of gels: application to fruit gels

Colour measurement was studied in peach gels prepared with different amounts of peach pulp, using carrageenan or a mixture of carrageenan and locust bean gum as gelling agent. The colour was measured conventionally, in CIELAB terms, applying Kubelka—Munk criteria. The K/S parameter values in relation to lightness are a good representation of the sensory lightness of the samples, whereas the L^* values are not. In the case of colour intensity, both the Kubelka—Munk and CIELAB parameters are a good representation of the sensory colour intensity. The use of carrageenan on its own or in a mixture of carrageenan and locust bean gum does not produce differences in the visual appearance of these two types of gels. Finally, mathematical equations were developed that relate K/S to the fruit and sugar concentrations. It was found that fruit concentration has a greater effect than sugar concentration on the K/S values of these equations, which agrees with the sensory valuation of the appearance of these gels.     

Studies on “sugar-reactivity” of agars extracted from some Indian agarophytes

Sugar reactivity was observed within the sugar–agar complexes in presence of sucrose and glucose with agars of Indian agarophytes viz. Gelidiella acerosa, Gracilaria edulis, Gracilaria crassa and Gelidium pusillum. The sugar reactivity was more pronounced in presence of sucrose than glucose. Oxoid agar was used as the reference material. Control agar gel contained 1.12% agar (w/w) in water. Sucrose–agar and glucose–agar gels in water consisted of 50% (w/w) sucrose and 50% (w/w) glucose, respectively along with 1.12% (w/w) agars of the four seaweeds mentioned above. Addition of sucrose resulted in increase (ca. 25–45%) in gel strength; increase (2–3 °C) in gelling and melting temperatures was observed in the gels prepared with agars from all the agarophytes and Oxoid agar. On the other hand, addition of glucose resulted in increase (19–34%) in the gel strength and gelling and melting temperatures of the agar gels of Oxoid as well as of all other agars decrease (2–3 °C). Maximum sugar reactivity was observed with the 50% level of sucrose and glucose in agar gels. Rheological and thermogravimetric characteristics of these gel samples were studied. The latter showed two patterns e.g. control agar gel of Oxoid agar was thermally less stable than the four control agar samples studied; in sugar–agar gel samples it followed a reverse pattern. To our knowledge, this is the first report of “sugar reactivity” of agar of Indian agarophytes. Sugar reactivity of agar in presence of glucose is also reported for the first time. The results of this study will be useful in bioprospecting as well as in exploring new applications.     

Sweetening power of aspartame in hydrocolloids gels: Influence of texture

Equivalent sweetness of aspartame relative to two sucrose concentrations (10% and 20% w/w) were determined in water and in hydrocolloids gels. The influence of the texture of three hydrocolloids gelled systems—gellan gum, κ-carrageenan, and κ-carrageenan/locust bean gum (LBG)—at two gums concentrations (0.3% and 1.2% w/w) on the equivalent sweetness of aspartame were then studied. For the three gelled systems, the increase in hydrocolloid concentration produced a significant increase in the true rupture stress and in the deformability modulus values. For both κ-carrageenan and mixed gels the true rupture strain values increased when increasing hydrocolloid concentration while for gellan gels, decreased. For the same hydrocolloid concentrations the κ-carrageenan/LBG gels showed the largest strain at rupture and gellan gels the smallest (most brittle). For both soft (0.3% gum) and hard (1.2% gum) gellan gels and κ-carrageenan gels, the concentrations of aspartame needed to deliver a sweetness intensity equivalent to that of gels with 10% sucrose (0.079–0.087% w/w) were similar to those obtained for aqueous solutions (0.084% w/v). For hard κ-carrageenan/LBG gels the corresponding concentration of aspartame was slightly lower. For all gelled systems the concentrations of aspartame needed to deliver a sweetness intensity equivalent to that of gels with 20% sucrose were higher for soft gels than for hard gels.     

Purification and characterization of dibenzothiophene (DBT) sulfone monooxygenase, an enzyme involved in DBT desulfurization, from Rhodococcus erythropolis D-1

Dibenzothiophene (DBT), a model of organic sulfur compound in petroleum, is microbially desulfurized to 2-hydroxybiphenyl by Rhodococcus erythropolis D-1. Three desulfurization (Dsz) enzymes—DszC, A, and B—and flavin reductase are involved in sulfur-specific DBT desulfurization. In this study, DszA was purified, characterized, and crystallized from R. erythropolis D-1. DszA, DBT sulfone monooxygenase, is the second enzyme in microbial DBT desulfurization metabolism and catalyzes the conversion of DBT sulfone to 2′-hydroxybiphenyl 2-sulfinic acid in the presence of flavin reductase with cleavage of the carbon-sulfur bond in the DBT skeleton. Using anion-exchange column chromatography, the four enzyme fractions responsible for DBT desulfurization were separated, and DszA was then purified to homogeneity. Polygonal crystals of DszA were observed within a week. DszA was found to have a molecular mass of 97 kDa and to consist of two subunits with identical masses of 50 kDa. The N-terminal amino acid sequence of the purified DszA completely coincided with the deduced amino acid sequence for dszA of R. erythropolis IGTS8 except for a methionine residue at the latter N-terminal. The optimal temperature and pH for DszA activity were 35°C and about 7.5. The activity of the enzyme was inhibited by Mn²⁺, Ni²⁺, 2,2′-bipyridine, and 8-quinolinol, suggesting that a metal might be involved in its activity. DszA acted on not only DBT sulfone but also on dibenz[c,e][1,2]oxathiin 6-oxide and dibenz[c,e][1,2]oxathiin 6,6-dioxide. Dihydroxybiphenyl was formed from the latter two substrates.     

Effects of thiol reagents and ethanol on strength of whey protein gels

Chemical reagents which interfere with secondary interactions affected gel network formation and the properties of gels made from 20% whey protein isolates. Thus, p-hydroxymercuribenzoate and N-ethylmaleimide 0–16 mmol/dm³ and dithiothreitol 0–32 mmol/dm³ reduced the hardness and cohesiveness of whey protein gels apparently by interfering with formation of disulfide bonds. The addition of ethanol (0–15%) increased the hardness of gels presumably by enhancing electrostatic interactions and hydrogen bonding.     

Non-destructive and objective methods for the evaluation of the maturation level of olive fruit

The maturation level of two olive varieties (Olea europaea, cvs. Arbequina and Picual) was objectively evaluated using two non-destructive methods: checking the fruit firmness using a hand densimeter, and the skin colour with a portable colourimeter. These methods were compared with the Ripening Index (RI), habitually used by olive oil industry, and based on the subjective determination of skin and flesh colour. The values of skin colour [L*(b*–a*)/100] only monitor the olive maturation in both varieties, when the olive is losing its initial green colour, while fruit firmness is decreasing throughout the maturation process. Fruit firmness is directly related to RI and could be useful in objectively estimating the maturation level of the olives.     

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.     

Effects of Cation Properties on Sol-gel Transition and Gel Properties of κ-carrageenan

The phase transition temperatures, rheological properties and gel‐network characteristics for gelation of κ‐carrageenan‐salt (NaCl, KCl and CaCl₂) solutions and their aged gels were investigated. The effectiveness of increasing gelling and gel‐melting temperatures at the salt concentrations examined followed the sequence of K⁺ > Ca2⁺ > Na⁺. This sequence was also true for the gel strength and the melting enthalpy (DH) of the most crosslinked junction zones of aged gels at low salt concentrations. Nonetheless, a different order (Ca⁺⁺ > K⁺ and Na⁺) was found for increasing storage modulus and gelation rate during early‐stage gelation, thermal hysteresis and the DH of aged gels in some salt‐carrageenan systems.     

The isolation and properties of a concentrated red‐purple betacyanin food colourant from Opuntia stricta fruits

A red‐purple food colourant from Opuntia stricta fruits was obtained and studied. Four steps were involved in its isolation: washing, extraction, centrifugation and concentration. Ethanol:water 60:40 (v/v) was selected as the optimum extraction solvent to reduce the viscosity caused by the presence of mucilage and pectins. The resulting 40‐fold concentrated extract had a high colour strength (3.9, OD 535 nm, 1% v/v sol), a high betanin concentration (4.7 g L⁻¹) and low viscosity (59.0 cP). It also showed high stability (t_1/2 = 236.6 d, 4 °C) mainly due to its low pH (3.4) and low water content (571 g kg⁻¹). These characteristics were in the same range as shown by three commercially available liquid concentrated colourants studied (red beet, red carrot and red grape skin). The colour parameters of this concentrated extract of Opuntia stricta were compared with those of commercial red colourants (red beet, red carrot, red grape skin, cochineal, elderberry, hibiscus and red cabbage). CIELAB values of Opuntia stricta (1.5 mL L⁻¹) were L* = 69.8, a* = 59.7 and b* = −23.5. Opuntia stricta presented a vivid red‐purple colour which was distinguishable from the colours shown by the other natural red food colourants. Copyright © 2005 Society of Chemical Industry     

Gel extracted from Khruea‐ma‐noi (Cyclea barbata Miers) leaves: chemical composition and gelation properties

Gel extracted from Khruea‐ma‐noi leaves, which are widely consumed by villagers in Northeast Thailand, has a unique characteristic that could be used for food and nutritional applications, such as in therapeutic diets and in the food industry. Since little is known scientifically about this plant, this study identified the plant's scientific name and determined the chemical composition of the Khruea‐ma‐noi polymer and its gelling mechanism. Results reveal that Khruea‐ma‐noi is classified as Cyclea barbata Miers. Its main polymer is polygalacturonic acid of average molecular weight 741 kDa with 66.3% methylated. Dried powders of extract and purified gels contained 415 and 724 g kg^?1 of soluble dietary fiber and 7.1 and 13.9 g kg^?1 of divalent cations, respectively. Factors affecting gelling ability and characteristics include phenolic compounds, oxidizing and reducing agents, pH, divalent cations and temperature. Most gels formed were thermoreversible except at pH 7. Copyright © 2005 Society of Chemical Industry     

Factors that determine the fracture properties and microstructure of globular protein gels

Protein gel matrices are responsible for the texture of many foods. Therefore an understanding of the chemical reactions and physical processes associated with fracture properties of gels provides a fundamental understanding of select mechanical properties associated with texture. Globular proteins form thermally induced gels that are classified as fine-stranded, mixed or particulate, based on the protein network appearance. The fundamental properties of true shear stress and true shear strain at fracture, used to describe the physical properties of gels, depend on the gel network. Type and amount of mineral salt in whey protein and β-lactoglobulin protein dispersions determines the type of thermally induced gel matrix that forms, and thus its fracture properties. A fine-stranded matrix is formed when protein suspensions contain monovalent cation (Li⁺, K⁺, Rb⁺, Cs⁺) chlorides, sodium sulfate or sodium phosphate at ionic strengths ≤0.1 mol/dm³. This matrix has a well-defined network structure, and varies in stress and strain at fracture at different salt concentrations. At ionic strengths >0.1 mol/dm³ the matrix becomes mixed. This network appears as a combination of fine strands and spherical aggregates, and has high stress values and minimum strain values at fracture. Higher concentrations of monovalent cation salts cause the formation of particulate gels, which are high in stress and strain at fracture. The salt concentration required to change microstructure depends on the salt's position in the Hofmeister series. The formation of a particulate matrix also occurs when protein suspensions contain low concentrations (10–20 mol/dm³) of divalent cation (Ca²⁺, Mg²⁺, Ba²⁺) chloride salts or di-cationic 1,6-hexanediamine at pH 7.0. The divalent cation effect on β-lactoglobulin gelation is associated with minor changes in tertiary structure involving amide—amide interproton connectivities (determined by ¹H NMR) at 40–45°C, increasing hydrophobicity and intermolecular aggregation. The type of matrix formed appears to be related to the dispersed or aggregated state of proteins prior to denaturation. Mixed and particulate matrices result from conditions which favor aggregation at temperatures (25–45°C) which are much lower than the denaturation temperature (~65°C). Therefore, general (e.g. Hofmeister series) and protein-specific factors can affect the dispersibility of proteins and thereby determine the microstructure and fracture properties of globular protein gels.     

Studies of ovalbumin gelation in the presence of carrageenans and after manothermosonication treatments

Ovalbumin is a key ingredient in many food products due to its ability to form a three-dimensional network after heat denaturation, that is, to form gels. We have explored two ways of modifying gels based on ovalbumin: (1) the addition of carrageenans, and (2) manothermosonication (MTS) treatments of ovalbumin suspensions prior to gelation. Ovalbumin gels showed elastic modulus (G′) values of 1216 Pa after a heating and a cooling period. The addition of kappa, lambda, and iota carrageenan enhanced G′ values by 6, 1.7, and 1.7 fold, respectively. Kappa carrageenan was not only the most effective carrageenan in the improvement of ovalbumin gelation but changed also the gelation process at the end of the cooling phase. MTS treatments reduced G′ values of ovalbumin gels by about 50% measured immediately after MTS treatments but only by ca. 25% after 24 h, which reveals that ovalbumin modifications introduced by ultrasound, whatever they are, are reversible. This reduction does not preclude the use of manothermosonicated liquid egg as gelling agent.     

Protein changes after irradiation and ice storage of horse mackerel (Trachurus trachurus)

Anthocyanin-rich concentrates from different fruits can be used as natural food colourants. The pigments' stability is comparatively low and dependent on the composition of food matrices. Food ingredients relevant for soft drinks, jelly fruits and salad dressings were tested in model systems regarding their influence on the colour stability of elderberry and black currant concentrate determined by colour measurement (CIE L ^* a ^* b ^*). In aqueous solutions food-grade organic acids and salt were found to influence anthocyanin stability: colour stability increased with increasing pK _a of acids and decreased with increasing salt concentrations. This may be attributed to altered solvation characteristics of aqueous solutions. A stabilizing influence was found for sugars presumably by reducing water activity. However, when heat treatment was applied, e.g. in the production of hydrocolloid gels, fructose was shown to accelerate anthocyanin decay due to the formation of sugar degradation products. Comparing hydrocolloids, alginate was shown to increase colour stability in aqueous solution and pectin displayed overall highest colour stability in a gel model system, suggesting that polyuronic acids may improve anthocyanin stability by intermolecular association.     

Effect of high pressure processing on heat-induced gelling capacity of blue crab (Callinectes sapidus) meat

There has been increasing use of High pressure processing (HPP) in the fishery industry since this technology facilitates shellfish shucking. Nevertheless, there is limited information about the effect of HPP on protein functional properties of some shellfish. The aim of this study was to evaluate the effect of 100, 300 and 600 MPa/5 min on the gelling capacity of heat-induced (40 °C/30 min + 90 °C/20 min) blue crab (Callinectes sapidus) meat. HPP treatment resulted in crab meat gels with a lighter and reddish colour as compared to the control. HPP at 600 MPa induced the formation of high molecular aggregates from the denaturation-aggregation of myosin heavy chain. Pressurization at 100 MPa promoted the shift of α-helix structures to β-sheet and β-turn as compared with the other pressure levels. TPA values were higher in gels made at 100 MPa than at 300 or 600 MPa. Low pressure levels, then, increased the heat-induced gelling capacity of crab meat, improving the texture through modification of its protein structure.Industrial relevanceHigh pressure processing (HPP) technology has been successfully applied to several seafood products, both for processing and storage. However, in the case of blue crab meat it is important to study the effect of HPP on protein functional properties such as gelling capacity in order to optimize processing parameters for the preparation of high-quality restructured products. This paper reports the development of a HPP process (100, 300 and 600 MPa/5 min 40 °C/30 min + 90 °C/20 min) prior to thermal gelling for the preparation of crab meat gels. The application of 600 MPa produced considerable protein aggregation of gels, whereas with pressures below 300 MPa protein functionality can be modified to produce crab meat gels with adequate brightness, TPA values and a fresh, high-quality appearance. These results could provide a basis for further pressurization applications in the crab industry to create new seafood product analogues based on this kind of crab meat.     

Oxidative changes in pork scratchings,peanuts, oatmeal and muesli viewed by fluorescence,near-infrared and infrared spectroscopy

The possibility of using rapid spectroscopic methods to detect lipid oxidation of four standard food products—peanuts, pork scratchings, oatmeal and muesli—were investigated in a large screening experiment. Fluorescence, near-infrared [including visual (Vis/NIR)] and infrared (IR) spectra have been recorded during and beyond the normal shelf lives of the products and related by means of multivariate data analysis to reference oxidation measures of free radicals and hexanal, as well as to sensory evaluation. For pork scratchings, the three spectroscopic techniques were able to monitor the progressing level of oxidation: IR with good correlation to hexanal (r²=0.86), Vis/NIR with good correlations to free radicals (r²=0.82) and fluorescence with good correlations to both free radicals (r²=0.76) and hexanal (r²=0.83). In the case of IR, the best result was obtained using the region including the out-of-plane C–H of trans-fatty acids. For the other three products, the results were generally inferior except for good correlations between fluorescence spectra and free radicals in peanuts (r²=0.73) and between Vis/NIR spectra and hexanal for muesli products (r²=0.8).     

SWEETNESS PERCEPTION IN RELATION TO SOME TEXTURAL CHARACTERISTICS OF HYDROCOLLOID GELS

The apparent sweetness of five different hydrocolloid gels prepared with equal amounts of sodium Sucaryl varied significantly (P < 0.01) when tested in a multiple paired arrangement by seven trained judges. Results from four replications consistently showed sweetness to be the greatest in carrageenan gels and the least in cornstarch gels. It was intermediate in low methoxyl pectin, agar and gelatin gels. Concentrations of gelling agents were selected to produce gels comparable in hardness. Characterization of mechanical textural properties with the GF-Zenken Texturometer showed that gels of cornstarch and gelatin were more cohesive, springy, chewy and gummy than those of agar, low methoxyl pectin and carrageenan (P < 0.01). These findings suggested that gels which take more effort to disintegrate limit taste perception. Multiple regression analyses showed that the measured mechanical textural parameters accounted for 52.3% of the variability in sweetness scores suggesting that other characteristics may also play a role. Inspection of correlation coefficients of sweetness with single textural parameters showed no one characteristic as a dominating influence over sweetness.     

Effects of drying on the gel strength and cation mobility of furcellaran

Thermal stability, by means of air drying a furcellaran powder, and its impact on gel strength and cation mobility were studied. Halogen heating in the temperature range 90–115°C for 15 min resulted in loss on drying (L_D, %). These results can be described by polynom L_D=−9.583+2.989τ−0.249τ²+0.00729τ³+0.1034t (R²=0.9976), indicating a gradual decomposition of carbohydrates. Air-drying induced a decrease in gel strength and the partial removal of potassium, calcium and sodium ions from the matrix. Air drying above 115°C yielded a remarkable destruction of polysaccharides with a total collapse in gelling power.