Sanguansri Charoenrein

Effects of freezing and thawing on texture, microstructure and cell wall composition changes in papaya tissues

BACKGROUND During storage, frozen fruit may be thawed and refrozen many times before consumption, which may be extremely damaging to the texture of the frozen fruit and reverse the advantage of fast freezing. The effects of freezing and thawing on texture, microstructure and cell wall composition changes in papaya tissues were investigated. RESULTS The frozen-thawed papayas had an increase in drip loss and a decrease in firmness with increasing number of freeze-thaw cycles. Light microscopy showed irregular shapes and cell damage in parenchyma cells of frozen-thawed papayas, whereas transmission electron microscopy showed loss of cell wall materials in middle lamella. Moreover, destruction of cell wall was observed after being subjected to five freeze-thaw cycles. These changes related with a significant decrease in alcohol-insoluble solids, Na₂CO₃- and 24% KOH-soluble fractions and an increase in the water-, EDTA- and 4% KOH-soluble fractions. This was due to a decrease in the molecular mass of pectic and hemicellulosic polymers in frozen-thawed papayas using high-performance size-exclusion chromatography. CONCLUSION The freezing and thawing processes caused fine structural damage and cell wall composition changes which contributed to a loss of drip volume and firmness of papaya tissues.

Effect of waxy rice flour and cassava starch on freeze–thaw stability of rice starch gels

Repeatedly frozen and thawed rice starch gel affects quality. This study investigated how incorporating waxy rice flour (WF) and cassava starch (CS) in rice starch gel affects factors used to measure quality. When rice starch gels containing 0-2% WF and CS were subjected to 5 freeze-thaw cycles, both WF and CS reduced the syneresis in first few cycles. However CS was more effective in reducing syneresis than WF. The different composite arrangement of rice starch with WF or CS caused different mechanisms associated with the rice starch gel retardation of retrogradation, reduced the spongy structure and lowered syneresis. Both swollen granules of rice starch and CS caused an increase in the hardness of the unfrozen and freeze-thawed starch gel while highly swollen WF granules caused softer gels. These results suggested that WF and CS were effective in preserving quality in frozen rice starch based products.

Texture improvement of fresh and frozen mangoes with pectin methylesterase and calcium infusion

BACKGROUND A major problem of mango products is texture loss. The effect of commercial pectin methylesterase (PME) and calcium infusion on improvement of the texture of both fresh and frozen-thawed mango cubes was investigated in the present study. RESULTS The weight gain and moisture content of mango samples were greater at relatively high vacuum level (10 kPa). The PME activity of samples infused with PME and calcium at 10 kPa increased fourfold in comparison with that of control and water-infused samples. The combined effect of PME and calcium was found to improve the texture and microstructure of both fresh and frozen-thawed mangoes. Fresh mangoes infused with PME and calcium at 10 kPa showed significantly higher firmness than control fresh samples. Frozen-thawed mangoes infused with PME and calcium at 50 kPa and atmospheric pressure had superior texture and microstructure in comparison with control frozen-thawed samples. CONCLUSION The results of the present analysis allow for a better appreciation of the role of PME, calcium and appropriate infusion conditions in improving the texture of both fresh and frozen-thawed mangoes.

Influence of waxy rice flour substitution for wheat flour on characteristics of batter and freeze-thawed cake

This study aimed to improve the freeze-thawed cake properties by10-20% waxy rice flour (WRF) substitution for wheat flour (WF). Viscosity of WRF-substituted batters was lower; consequently, trapped air was less uniformly distributed than WF batter. After five freeze-thaw cycles, firmness and enthalpy of melting retrograded amylopectin of WF- and WRF-substituted cakes increased and the matrix surrounding the air pores from SEM images was denser than in fresh-baked cakes. Sensory evaluation showed an increase in firmness and a decrease in firmness acceptability of freeze-thawed cakes. However, freeze-thawed cake with WRF substitution had significantly less firmness, less dense matrix and more acceptability than WF cake. This could have been due to a low amylose content of WRF and the spread of ruptured waxy rice starch granules around swollen wheat starch granules as observed by CLSM. Thus, WRF could be used for WF substitution to improve the firmness in freeze-thawed cake.

The effect of moisture content on physicochemical properties of extruded waxy and non-waxy rice flour.

The properties of waxy rice flour (WRF) and non-waxy rice flour (RF) were modified using an extrusion process with different feeding material moisture contents. WRF was more affected by the thermomechanical stress from extrusion; consequently, it had a lower glass transition temperature but higher water solubility index (WSI) indicating higher molecular degradation than extruded RF. The lower moisture content of the feeding flour caused more severe flour damage (coarser surface of the extruded flour) and lowered relative crystallinity compared to higher moisture content processing. Moreover, low moisture content processing led to complete gelatinization, whereas, partial gelatinization occurred in the higher moisture content extrusion. Consequently, the extruded flours had a lower peak viscosity and gelatinization enthalpy but a higher water absorption index and WSI than native flour. In conclusion, the rice flour type and the moisture content of the extrusion feeding flour affected the physicochemical properties of the extruded flour.

Color and gelling properties of dried egg white: Effect of drying methods and storage conditions

ABSTRACT Dried egg white is extensively used as a food ingredient due to its unique functional properties and extensive shelf life. This study investigated the effect of drying methods and storage conditions on the color and gelling properties of dried egg white. Egg white was dried with two drying methods; freeze drying and hot-air drying, then stored at 25 and 40°C for 4 months. The result showed that the color of hot-air-dried egg white, especially after storage at 40°C, was darker yellow than for freeze-dried egg whites. The gelling properties of both samples were altered during storage, however, substantial changes in the gelling properties were found in the gel made from stored hot-air-dried egg white. A decrease in the enthalpy of protein denaturation that indicated a partially unfolded protein conformation, an increase in exposed sulfhydryl and a decrease in the total sulfhydryl contents were found, especially in stored samples of hot-air-dried egg whites. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis pattern of proteins in the stored samples showed protein aggregation, and this was related to the increased glass transition temperature. This study revealed that drying egg white using hot-air drying and storage especially at high temperature induced changes in their color and protein conformation. These have contributed to protein aggregation which affected dried egg white’s gel properties.

Changes in physical and gelling properties of freeze-dried egg white as a result of temperature and relative humidity.

BACKGROUND The quality of dried egg white with respect to functional properties can be affected by storage conditions. The effect of temperature and relative humidity (RH) on changes in colour and gelling properties in freeze-dried egg white (FDEW) during storage was investigated. RESULTS The glass transition temperature (Tg ) of FDEW decreased with increasing % RH. The colour of FDEW stored at 60 °C was darker yellow than those at 40 and 25 °C, particularly at high % RH. RH had no effect on hardness and water-holding capacity (WHC) of gels made from FDEW stored at 25 °C for 1 week. However, hardness and WHC of gels from FDEW stored at higher temperatures; 40 °C, 48% RH and 60 °C, 11% RH dramatically increased. These results related to the differential scanning calorimeter thermograms which showed a broadening peak with lower enthalpy of protein denaturation. Moreover, the proteins SDS-PAGE pattern in the samples stored at high temperatures or RH levels showed protein aggregation. CONCLUSION Storage of FDEW at high temperature and RH levels induced protein conformation changes. These have contributed to protein aggregation which affected the gelling properties of FDEW.

Porosity and Water Activity Effects on Stability of Crystalline β-Carotene in Freeze-Dried Solids

UNLABELLED Stability of entrapped crystalline β-carotene as affected by water activity, solids microstructure, and composition of freeze-dried systems was investigated. Aliquots (1000 mm(3) , 20% w/w solids) of solutions of maltodextrins of various dextrose equivalents (M040:DE6, M100:DE11, and M250:DE25.5), M100-sugars (1:1 glucose, fructose and sucrose), and agar for gelation with dispersed β-carotene were frozen at -20, -40, or -80 °C and freeze-dried. Glass transition and α-relaxation temperatures were determined with differential scanning calorimetry and dynamic mechanical analysis, respectively. β-Carotene contents were monitored spectrophotometrically. In the glassy solids, pore microstructure had a major effect on β-carotene stability. Small pores with thin walls and large surface area allowed β-carotene exposure to oxygen which led to a higher loss, whereas structural collapse enhanced stability of β-carotene by decreasing exposure to oxygen. As water plasticized matrices, an increase in molecular mobility in the matrix enhanced β-carotene degradation. Stability of dispersed β-carotene was highest at around 0.2 a(w) , but decreasing structural relaxation times above the glass transition correlated well with the rate of β-carotene degradation at higher a(w) . Microstructure, a(w) , and component mobility are important factors in the control of stability of β-carotene in freeze-dried solids. PRACTICAL APPLICATION β-Carotene expresses various nutritional benefits; however, it is sensitive to oxygen and the degradation contributes to loss of nutritional values as well as product color. To increase stability of β-carotene in freeze-dried foods, the amount of oxygen penetration need to be limited. The modification of freeze-dried food structures, for example, porosity and structural collapse, components, and humidity effectively enhance the stability of dispersed β-carotene in freeze-dried solids.

Influence of Annealing Temperature on Tg′ of Cooked Rice Stick Noodles

Tg′, a glass transition temperature under conditions of maximal freeze concentration, is important to the stability of frozen food. The procedures for using differential scanning calorimetry (DSC) to measure Tg′ have been reported under different experimental conditions. The aims of this study were to investigate the Tg′ of cooked rice stick noodles, and to determine the effect of the annealing temperature on its Tg′ value using DSC. Cooked rice stick noodles in aluminum DSC pans were scanned from −60°C to 25°C at 5°C/min. to locate the apparent Tg′, which was −5.3°C in the non-annealed state. When subjected to 4 different annealing temperatures of −2, −6, −8 and −10°C for 15 minutes, the Tg′ of cooked rice stick noodles was −5.3, −4.0, −4.2, and −4.9°C, respectively. The value of Tg′ was clearly observed in the annealed sample at −6°C. The annealing process allowed time for the maximum formation of ice. This study showed that annealing at a temperature slightly below Tg′ gave a higher and more accurate value.

Effect of Cultivar and Ripening Stage on Quality and Microstructure of Frozen Mangoes (Mangifera indica Linn.)

The aim of this work was to investigate the quality and microstructure of different frozen mango cultivars. Since freezing can cause severe textural damage to fruit, the proper selection of a ripening stage and cultivar can reduce such damage. Two mango cultivars (Nam Dok Mai and Chok Anan) were allowed to ripen for four ripening stages. During ripening, titratable acidity, alcohol-insoluble residues, and firmness significantly decreased while total soluble solids, total pectin, and water-soluble pectin levels increased. Mangoes from the three final ripening stages were frozen until the temperature of the samples reached –25°C. After thawing, the partially ripe mango samples exhibited the highest firmness, lowest drip loss, lowest water-soluble pectin, and the highest sensory firmness scores. The microstructure found using light microscopy and transmission electron microscopy showed that all frozen-thawed mangoes exhibited a swelling and folding of the cell wall due to freezing damage. The cell walls of fully ripe mangoes were dramatically damaged, which explained the low firmness values. Tissue and cell wall damage in Nam Dok Mai were the greatest, which contributed to the large drip loss. The cheaper Chok Anan cultivar was more resistant to freezing damage than Nam Dok Mai.