冻干圣女果粉的水分吸附性质及玻璃化转变温度

根据吸附理论,在水分活度为0.11～0.90范围内,环境温度为25 ℃条件下,采用静态称质量法研究冻干圣女果粉的吸附和解吸等温线;通过差示扫描量热仪（differential scanning calorimeter,DSC）测量不同水分含量下的玻璃态转变温度（glass transition temperature,Tg）,并采用Gordon-Taylor方程对其进行了非线性拟合。结果表明：冻干圣女果粉在25 ℃条件下的吸附等温线和解吸等温线的类型都为J型,属于Ⅲ型等温线;在水分活度为0.23～0.76范围内存在明显的解吸-吸附滞后现象,属于H3型等温线。GAB和Peleg模型都描述冻干圣女果粉的吸附特性。随着水分含量的增加,冻干圣女果粉的Tg显著降低;Gordon-Taylor方程能够较好地拟合其玻璃化转变曲线。对比水分活度贮藏理论和玻璃化转变理论,发现二者在预测冻干圣女果粉贮藏稳定性上存在一定的差异。     

INFLUENCE OF PLATE TEMPERATURE AND MODE OF REHYDRATION ON TEXTURAL PARAMETERS OF PRECOOKED FREEZE-DRIED BEEF

The effects of various plate temperatures and rehydration procedures in texture of precooked freeze-dried beef, were evaluated. Frozen precooked commercial beef was freeze-dried at 20, 40, 60 and 80C during 24 h. The rehydration procedure involved two main variables: distilled water at room-temperature and at 80C, for 30 min. The texture profile analysis was performed with a TA-XT2 Texture Analyzer (SMS) for freeze-dried rehydrated samples and compared to frozen precooked beef. The results showed that with a same drying period, the 60C plate temperature and rehydrated in distilled water at room-temperature resulted in a better product texture, in regards to texture parameters hardness and chewiness.     

Equilibrium Water Content and the State of Water in Dehydrated White Cabbage

The water sorption isotherms for freeze-dried cabbage were determined at 15°C and 23°C, and the state of the water was studied using differential scanning calorimetry (DSC). The water sorption capacity of the freeze-dried cabbage depended on the temperature. The effects of free sugars were clear in both adsorption and desorption. The thermal behavior of freeze-dried cabbage was highly moisture dependent. In the capillary condensation range (>0.9 a_w) solution of low molecular components of the freeze-dried cabbage affected its freezing behavior.     

MEASUREMENT OF TEXTURE PARAMETERS OF FREEZE-DRIED BEEF*

A modified texture profile analysis with the Instron Universal Testing Apparatus was used to measure hardness and chewiness of precooked freeze-dried beef cubes in order to determine the influence of processing and reconditioning. Two different plate temperatures were used during freeze-drying, and the resulting product was reconditioned at four different water activities in addition to the dry and fully rehydrated product. Reconditioning temperatures of 52, 70, and 100°F were investigated. The results revealed that the hardness and chewiness values were the highest at water activities of 0.4 to 0.6 and with the product freeze-dried at a plate temperature of 105°F. The influence of reconditioning temperature was inconsistent, apparently due to an interaction between the water activity and the plate temperature.     

Technical note: Sorption isotherms and monolayer moisture content of raw peas, and peas dehydrated after pressure cooking

The moisture sorption isotherms of raw and pre-cooked Century, MP 888 and MP 889 peas at 10, 25 and 40°C were determined by the standard salt solution technique. Pea cultivars, although different in composition, did not differ in equilibrium moisture content. At 40°C and in the water activity range 0.50 a _w 0.90, the sorption capacity of raw peas was higher than that of pre-cooked peas. All adsorption-desorption isotherms were of sigmoid shape and the quantity of sorbed water, at a given relative humidity, increased as the temperature was decreased. The monolayer moisture content of pre-cooked dehydrated peas stored at 40°C was 6.2%, with corresponding equilibrium relative humidity value of about 27%.     

Effect of sodium chloride, sodium nitrite and temperature on desorption isotherms of previously frozen beef

Moisture desorption isotherms of beef were determined in the relative humidity range of 23 to 90% at 5, 15 and 25 °C and at 2.5% NaCl and 2.5% NaCl + 150 ppm NaNO₂ content. Desorption isotherms were found to be typical type II sigmoid. The water content at equilibrium was higher in beef with NaCl and NaCl + NaNO₂ than control samples. Experimental data were fitted to various mathematical models and it was found that the Peleg model was best in describing the equilibrium moisture content relationship for beef samples over the entire range of temperatures. The net isosteric heat of sorption was estimated from equilibrium desorption data, using the Clausius-Clapeyron equation. Isosteric heats of desorption were found to increase with decreasing moisture content.     

Mathematical modelling of moisture sorption isotherms and determination of isosteric heat of blueberry variety O'Neil

The sorption isotherms of blueberry variety O'Neil were determined at 20, 40 and 60 °C, for a range of water activity of 0.10–0.95. The isotherms showed that the equilibrium moisture content increased when temperature decreased at constant water activity. The BET, GAB, Halsey, Henderson, Caurie, Smith, Oswin and Iglesias-Chirife equations were tested for modelling the sorption isotherms. The results showed that GAB, BET and Halsey models gave the best fit quality for the experimental desorption data, and BET, Oswin and Henderson for adsorption data as suggested by the statistical tests employed. The net sorption heat was calculated using the Clausius–Clapeyron equation giving 38.62 kJ mol⁻¹ (desorption) and 30.88 kJ mol⁻¹ (adsorption) at a moisture content of 0.01 g water (g d.m.⁻¹). Tsami equation was applied to estimate the net isosteric heat of sorption as function of equilibrium moisture content with satisfactory results.     

Effect of Cooking on Moisture Sorption Isotherms of Sheanut (Vitellaria paradoxa Gaertn.) Kernels: Evidence from Light and Scanning Electron Microscopy

The standard static gravimetric method was used to determine moisture sorption isotherms of both raw and cooked sheanut kernels at 40, 50 and 60 °C in the water activity range 0.11–0.96 in order to evaluate the influence of cooking on the moisture sorption capacity of the kernels. Preliminary analysis showed that the kernels lost a significant quantity of its proteins and carbohydrate during the cooking process. Results of analysis of the moisture sorption isotherms revealed that cooked kernels generally had significantly (p?<?0.05) lower equilibrium moisture contents (EMC) than the raw ones for both desorption and adsorption processes. The effect of temperature on the sorption processes as portrayed by the desorption isotherms revealed that EMC decreased steadily with increase in temperature within the water activity range 0.10–0.8 a_w but increased rapidly with increase in temperature above 0.8 a_w resulting in the overlap of isotherms for all sorption processes. This crossing over of isotherms was attributed to the dissolution of sugars at higher water activities. The protein matrix in the kernel was observed using light microscopy and was found to have been disorientated after cooking. Studies using light and scanning electron microscopy revealed that the reduced ability of cooked kernels to sorb water could be linked to changes in the structure of the kernels brought about by the cooking of the kernels. It is concluded that cooking had a very significant effect on the amount of water sorbed by sheanut kernels.     

Sorption properties of raw hake muscle

Adsorption and desorption isotherms of raw hake ( Merlucius hubbsi ) muscle were determined at 5, 20 and 30°C. From the temperature dependence of the isotherms, isosteric heats of sorption were evaluated by means of the Clausius-Clapeyron equation. Monolayer moisture contents are calculated by means of the B.E.T. analysis. The magnitude of the hysteresis between desorption and adsorption isotherms was quantified. Results are compared with those reported in the literature for other fish species.     

THE INFLUENCE OF COMPOSITIONAL CHANGES IN BEEFBURGERS ON THEIR TEMPERATURES AND THEIR THERMAL AND DIELECTRIC PROPERTIES DURING MICROWAVE HEATING

This study focused on a microwavable bunburger consisting of a precooked burger in a bread bun. The objective was to assess whether burger formulation changes would affect selected thermal and dielectric properties of the precooked burgers in addition to affecting bunburger temperatures during microwaving. Control burgers and those with elevated fat, water, rusk and salt levels were prepared. Proximal analysis of the raw and precooked burgers revealed a reduction in the fat and moisture content differences following deepfrying. However, burger formulation influenced (P<0.05) the measured thermal and dielectric properties of the precooked burgers in addition to affecting temperatures (P<0.05) from the burger edge and the top and bottom buns. Temperatures from the burger center were not affected (P< 0.05). Adjusting composition does offer the manufacturer a method to alter temperature during microwave heating through the magnitude of the temperature changes observed in this work were small.     

Desorption isotherms for fresh beef: An experimental and modeling approach

Desorption isotherms for fresh beef were determined at 30, 40 and 50 °C by the static gravimetric method. The resulting isotherms exhibited a type II sigmoid shape. The BET, GAB and Halsey models were used to fit these experimental data. The GAB model was most accurate for all temperatures and all levels of water activity, followed by the BET and Halsey models. The temperature dependence of GAB constants was estimated. The isosteric heat of desorption and its evolution in relation to moisture content were calculated using Clausius–Clapeyron equations. The monolayer moisture content was determined using the GAB model: it decreased as the temperature increased. The density of bound water, the number of adsorption sites, the sorption surface area and the percentage of bound water were calculated using the Caurie equation: all these quantities decreased as the temperature increased. The Kelvin and Halsey equations were used for calculation of pore size, which increases with an increase in moisture levels and sorption temperature.     

Water desorption isotherms of two varieties of sweet potato

Water desorption isotherms were determined for New Zealand sweet potato at 25, 40 and 55°C, and for Philippines sweet potato at 28°C. The isotherms were sigmoid in shape and of type II according to the BET classification. The data were fitted to eight two-parameter equations reported in the literature. The effect of water activity, a_w, and temperature, T °C, on the equilibrium moisture content, M _e g water/100g dry solids, was best described for New Zealand sweet potato by: Me = 20.51 T-^0.204 (a_w/(1-a_w)0.39 for a_w= 0.06–0.81     

Physical properties of cassava mash

Sorption and thermophysical properties (desorption curves, density and specific heat) of native and gelatinized cassava mash and of cassava starch were determined, in order to gain a better understanding of the heat and mass-transfer phenomena involved in gari cooking and drying.
Desorption isotherms of native cassava mash are given at four temperatures from 40 to 90°C; the experimental data were described using the GAB model with a maximum root mean square error (RMS) of 5%.
The net desorption enthalpy, calculated from the desorption data, decreases from 902.0 to 80.2 kJ/kg when the moisture content increases from 0.04 to 0.24 kg of water/kg dry matter. Desorption curves are not significantly affected by the degree of gelatinization of the starch, but at constant water activity, native cassava mash has a higher equilibrium moisture content than the other samples.
The density of cassava mash increases from 1239 to 1509 kg/m³ when it is dehydrated from 51.8 to 6% (wet basis), and the specific heats of dry cassava starch and dry cassava mash, determined by differential scanning calorimetry, vary linearly over the temperature range -20°C to 96°C from 1.128 to 1.760 and from 0.979 to 1.591 kJ/kgxK.     

Evaluation of Prerigor Freeze-Dried Beef in the Manufacture of Dry Fermented Sausage

Experiments were designed to determine the acceptability of prerigor freeze-dried beef in the manufacture of dry fermented sausage. Several chemical and physical variables were measured on dry fermented sausages prepared using (1) prerigor salted, freeze-dried, (2) prerigor unsalted, freeze-dried, (3) postrigor salted, freeze-dried, and (4) postrigor frozen beef as ingredients. Results showed that fat and moisture content, total shrinkage, Instron firmness and panel scores for juiciness, texture, and flavor were not affected to a large degree by treatment or rigor state of the beef ingredient. Generally, freezedried beef was found to be equal to conventional beef for dry sausage production regardless of when the freeze-dried beef was salted.     

Moisture isotherms of a processed meat product—Bologna

A study was carried out to obtain moisture isotherms of an intermediate-moisture processed meat product at a refrigerated and three ambient temperatures. Results showed that the isotherms were sigmoidal in shape. The Henderson equation was applied and analysed. It was found that the storage stability of the product is maintained at 5°C. Heats of desorption were presented.     

Moisture sorption isotherms of potato slices

Water vapour sorption isotherms of Norchip, Russet Burbank and Norland potatoes were determined by the salt solution technique at 10, 25 and 40°C. The adsorption isotherms of potatoes were of sigmoid shape and were affected by drying method, temperature and sugars addition. The freeze dried products adsorbed more water vapour than the vacuum dried materials. Addition of glucose, sucrose or lactose to the potatoes caused the equilibrium moisture content to decrease in the low and intermediate water activity ranges. The sorption was temperature independent in the a _w range 0.11–0.35 and showed a hysteresis loop at intermediate moisture levels.     

PHYSICAL, CHEMICAL, AND PALATABILITY TRAITS OF RAW AND PRECOOKED GROUND BEEF PATTIES

Boneless beef (U.S. Utility chucks or triangles; U.S. Choice plates) was ground (0.95 cm plate), standardized to 26% fat, reground (0.38 cm plate) and divided into two lots. One lot remained in Plant A and the other lot was shipped via air-freight to Plant B. On the same day and at the same time, meat at both Plants was formed into patties. Half of the patties at Plant A were precooked in a moisture-controlled oven broiler, half of the patties at Plant B were precooked in an open-hearth broiler; the remaining patties at each Plant were not precooked and were frozen in the raw state. Precooked and raw patties at both Plants were frozen (°30°C) in spiral air blast freezers, boxed, air-freighted to College Station, Tex. and Beltsville, Md. and stored at °20°C. Frozen patties were cooked to 63°C (11 mm at 190°C) and palatability was evaluated by an 84-member consumer panel and by an 8-member trained panel. Pre-cooking of ground beef patties: (a) reduces cube space-weight dimensions, thus economies would accrue in packaging-transportmg-distributing such product in comparison to requirements for raw patties; (b) increases total weight loss by approximately 1.8–9.4 percentage points; (c) resulted in increased moisture retention but increased loss of fat during final heating of product; (d) increases incidence of off-flavors by 7–16 percentage points and decreases flavor desirability if performed by use of a moisture-controlled oven broiler; (e) increases tenderness of heated patties; and (f) has no apparent effect on amount of organoleptically detectable connective tissue, flavor intensity or ready-to-serve appearance. It is very doubtful that the advantages in size reduction and tenderness are sufficient to offset the disadvantages in off-flavor incidence, in cooking-freezing-heating losses, in energy required to cool trie product after precooking, and in the necessity to use heat twice (once for precooking, once for heating prior to being served) to prepare the product for ultimate consumption.     

Effect of temperature and initial moisture content on sorption isotherms of banana dried by tunnel drier

Sorption isotherms of intermediate moisture content (IMC) products are essential to predict shelf-life of packaged moisture-sensitive product by modelling moisture uptake during storage and distribution. The effect of temperature and initial moisture content (MC) of IMC banana on the relationship between MC and water activity were investigated. Raw bananas were dried in a tunnel dryer at 2% relative humidity (RH), 70 °C, and a 3.2 ± 0.2 m s^–1 air velocity. Drying procedure was carried out a number of times until various IMC levels were obtained (5%, 14%, 22% and 33% db for banana). Sorption isotherms of bananas were determined at 10, 20, 30 and 40 °C. The initial MC of IMC banana had no significant effect on the relationship between MC and water activity according to statistical analysis. All the sorption curves were found to be Type II. BET, GAB, modified GAB, Oswin, Halsey and modified Freundlich models were fitted to the data and it was found that the best results were obtained with a modified Freundlich equation. A secondary modified Freundlich model was built accounting for the effect of a _w and temperature on the sample MC.     

Influence of precooking on the firmness and pectic substances of three stem vegetables

Edible portions of the stems of sprouting broccoli, asparagus lettuce, and large-stem mustard were compared for total pectin contents, amounts of different pectin fractions, pectinesterase activities and changes during cooking to investigate effects on the textural changes during cooking. Slices precooked for 30min at temperatures below 60°C (broccoli) or 70°C (lettuce, mustard) were firmer after 15 min recooking in boiling water than those directly cooked without precooking. Optimum temperatures for this firming effect of precooking were 50, 60 and 60°C, respectively, and coincided with the optimum temperatures of activity of pectinesterases extracted from the fresh tissues. Analysis of pectin fractions revealed that the firming effect of precooking is related to the shift from the cold water-soluble fraction to sodium hexametaphosphate-soluble and hot water-soluble fractions of pectins.     

Effect of Water Activity on Lipid Oxidation and Protein Solubility in Freeze-dried Beef during Storage

ABSTRACT: Ground beef patties (15% fat) were freeze-dried to different a_w endpoints and stored at 25°C or 49°C. Water activity, total moisture (%), lipid oxidation (TBARS@450nm), and protein solubility were evaluated. TBARS @450nm was favored at 49°C, but not at 25°C except at high a_w range (p < 0.05), whereas protein solubility was greater at 25°C than 49°C (p < 0.05). TBARS@450nm values at 49°C were generally greatest at low a_w range (p > 0.05). Protein solubility was lowest at medium a_w at 49°C (p < 0.05), while at 25°C low a_w protected protein solubility better than other low or high a_w values.