食品挤压过程中的反应动力学

食品挤压过程中发生的有利或不利反应决定产品的最终品质。物料在挤压机内同时受热和剪切,经历一个非等温过程,且停留时间出现分布,这些给挤压导致的反应动力学研究带来很大困难。本文首先介绍了挤压变化过程的动力学描述及其简化处理,随后综述了挤压过程中的淀粉转化/ 糊化和降解、蛋白质聚合、营养素及毒素损失、理化特性变化、酶抑制剂失活动力学的研究结果,最后对今后的研究方向作了展望。     

Thiamine destruction during extrusion cooking as an indicator of the intensity of thermal processing

Assessment of the intensity of high temperature-short time (HTST) extrusion cooking was studied by measuring the extent X_A of thiamine destruction. Small quantities of thiamine hydrochloride (0.1–0.7 g per 100 g) were mixed with wheat flour (containing ∼ 14% water) prior to extrusion experiments. Results indicate that thiamine destruction follows an apparent first-order reaction and that the rate constant K of destruction, calculated from near isothermal extrusion experiments at 133, 142 or 152°C (product temperature), follows an Arrhenius relationship with an activation energy of 49.9 kJ/mol.
K values were calculated from experimental determination of X_A and of the residence time distribution (RTD) in the effective heating zone of the extruder.
The influence of product temperature (just before the die), water content during extrusion and speed of screw rotation on the overall K (for non-isothermal extrusion experiments) was determined and analysed by multiple regression. Results show that thiamine destruction X_A can be predicted by means of a linear mathematical model.
Each experimental X_A value can be used, in combination with Arrhenius equation constants and RTD, to express a given extrusion process in terms of equivalent isothermal temperature and equivalent plug flow residence time.     

Kinetics of Thermomechanical Destruction of Thiamin During Extrusion Cooking

Maize grits were extrusion cooked in a CM45-F conical, counter-rotating twin-screw extruder, at different barrel temperatures (140–200°C), feed moistures (11.8–14.2% w.b.), feed rates (37–51 kg/h), screw speeds (65–81 rpm), and initial thiamin concentrations (9–93 mg/kg). Residence time distribution was measured by a dye tracer technique. A first-order rate equation was used to model the reaction kinetics, which allowed the calculation of the destruction rate constant using residence time distribution curves. The k-values were most dependent on barrel temperature, feed moisture, and screw speed. The destruction rate constant of thiamin during extrusion cooking was a function of product temperature and shear stress.     

A Model for Mechanical Degradation of Wheat Starch in a Single-Screw Extruder

The effect of extrusion cooking in a single screw extruder on the molecular weight distribution of wheat starch was correlated to the starch melt rheology. A simple first-order model has been developed which defines the extent of mechanical degradation of the amylopectin component as a function of nominal shear stress and residence time in the extruder. A limited set of experimental observations were used to test the model. Iso-shear stress curves were calculated for the experimental range of conditions. The curves can be used, in conjunction with estimates of the residence time, to calculate equivalent processing conditions, and to predict the extent of amylopectin degradation. The model may allow the use of small extruders to determine fundamental process parameters for full scale extrusion.     

Kinetics of quality changes during food frying

This paper deals with kinetics of quality changes during food frying. The quality parameters of interests include color, texture, viscoelastic properties, volume/density, and nutraceuticals (Omega 3 fatty acids). The kinetic theory and determination methodology under isothermal/non-isothermal conditions are also reviewed. This paper presents the reported reaction rate constant, order of reaction, and activation energy for specific quality changes including phenomenological observations and important speculations. The changes of color, textural, and viscoelastic properties generally followed a first order reaction. In some studies, activation energy analysis could have been improved by applying corresponding system temperatures for the quality change reactions of interest. The kinetic information on volume/density changes and thermal degradation of nutraceutical is still limited to experimental observations of the changes.     

APPLICABILITY of A SHEAR INDUCED RATE CONSTANT MODEL to CONVERSION of CORN MEAL IN A TWIN-SCREW EXTRUDER

Data obtained from extruding corn meal in a corotating twin-screw extruder was used to test an Arrhenius type shear induced rate constant model, k_s= k_s,o exp (—E_s/), where k_s was shear induced rate constant, E_s was shear activation energy, was shear stress, and was molar volume. A rheological zone was created by placing a reverse screw 300 mm from the die where the temperature was at 60C, sufficiently low that thermal effect on conversion of the material into gel or melt could be neglected. Throughput was used as an independent variable to generate various levels of shear input, while screw speed and feed moisture content were kept constant. Conversion of the material in the rheological zone was determined using differential scanning calorimetry, and the rate contant k_s was calculated assuming zero order kinetics. A plot of k_s vs. 1/ followed a straight line, confirming the applicability of the model to twin-screw extrusion under the experimental conditions. the shear activation energy E_s was 40 cal/mole, comparable to the literature values obtained from single screw extrusion and capillary rheometry.     

Application of an In‐line Viscometer to Determine the Shear Stress of Plasticised Wheat Starch

An in‐line viscometer was designed and constructed to enable determination of the shear stress of plasticised wheat starch during extrusion cooking. The viscometer was installed between the end of the barrel section and the extruder die plate so that the shear stress could be determined for the plasticised material, irrespective of the geometrical shape into which it was subsequently moulded by the extruder die. The extrusion conditions were described in terms of the process parameters, i. e. water content, barrel temperature, screw speed and screw configuration; and of the system parameters, which were the specific mechanical energy input (SME), product temperature (PT) and mean residence time (MRT). The parameters were measured and the results evaluated using statistical methods. Regression equations were used to describe functional relationships between the shear stress and the extrusion conditions on the one hand, and between the shear stress and the product characteristics of the extrudates on the other. The shear stress of plasticised wheat starch determined in‐line can be used to predict the morphological structure (volumetric expansion) and the functional properties (cold paste viscosity and cold water solubility) of extruded, directly expanded starch with a high degree of accuracy. The measurement technique used and the results of the extrusion tests undertaken for this project will therefore enable the shear stress of plasticised material in an extruder to be used as the principle parameter for controlling extruders on‐line.     

Model for Gelatinization of Wheat Starch in a Twin-Screw Extruder

Wheat starch was processed in a co-rotating twin-screw extruder, at moisture contents 25% and 30%, screw speeds 200 and 300 rpm, feed rate 30 kg/hr, and barrel temperature settings 100, 120, 140, and 160°C. Degree of starch gelatinization at each point along the extruder channel was determined by sampling and analyzing material inside the extruder. Kinetics of the gelatinization during extrusion was investigated. A first-order rate equation was developed to predict degree of starch gelatinization during extrusion. The rate constant was a function of both temperature and shear stress.     

Residence Time Distribution for Evaluating Flow Patterns and Mixing Actions of Rice Extruded with Thermostable <Emphasis Type="Italic">α</Emphasis>-Amylase

The effect of enzyme concentration, feed rate, screw speed, moisture content, and barrel temperature on residence time distributions (RTDs) of rice flour in a co-rotating twin-screw extruder had been broadly investigated. Feed rate and screw speed as the most important operating variables affecting the mean residence time (MRT) were further emphasized by using a second-order central composite design for achieving high-efficiency enzymatic extrusion with other variables constant. Increasing enzyme concentration to 1‰ (db of starch), the MRT increased probably due to the largely reduced viscosity of extrudate caused by rapid gelatinization and degradation of starch. Limited range of moisture content and barrel temperature had only slight impact on MRT and axial mixing considering the required high enzymatic activity relying on their levels sensitively. Flow pattern of extruded rice with thermostable α-amylase was close to perfect mixing flow. Seven flow models were compared to fit the RTD data, and Yeh-Jaw simplified model showed the best fitting results for enzymatic extrusion. Overall, the mechanism of introduced enzyme affecting MRT and mixing actions of extrusion process is of industrial implication for main ingredients reactions and nutrients to retain during enzymatic extrusion.     

Modeling Shearing Resistance of Powdery Starch for Simulation Studies of Extrusion Cooking Processes

Modeling extrusion cooking of starch requires the use of appropriate intrinsic kinetic equations. Knowledge of shear stress profile in a particulate system such as that in the transition zone of an extruder is crucial for calculation of degrees of conversion (gelatinization and melting) of starch. Shearing resistance of moistened powder starch as affected by temperature, normal pressure, effective shear rate, moisture content, and degree of starch conversion was studied in a model system. A modified vane shear testing device was used for experimental measurement of each individual effect. Within the experimental ranges studied (temperature 30 to 80°C, moisture content 16 to 35%, effective shear rate 0 to 550 s⁻¹, starch conversion 0 to 0.36 (36%), and normal pressure 0 to 83 kPa) parameter values in each functionality were evaluated. A single mathematical model useful for calculating shear stress was obtained by combining these effects in a single relationship. A good correlation was shown when the calculated shear stress was plotted against the experimentally measured. The methodologies developed here are useful for the studies of powdery food materials. The potential application of this correlation to the simulation of extrusion processes was discussed.     

Kinetics of Lysine and Other Amino Acids Loss During Extrusion Cooking of Maize Grits

ABSTRACT: Maize grits were extrusion-cooked in a conical, counter-rotating twin-screw extruder at different barrel temperatures, feed moistures, and screw speeds. Residence time distribution was measured by a dye tracer technique. Experiments with lysine-fortified maize grits showed a 1st order reaction for lysine loss. A detailed kinetic study has been performed for the losses during extrusion cooking of lysine, cystine, and arginine. The 1st-order rate constants were dependent mainly on product temperature and feed moisture, whereas screw speed had no influence. Activation energy of lysine, arginine, and cystine loss was 127, 68, and 76 kJ/mol, respectively. Shear stress significantly affected the rate constants of amino acids loss in extrusion cooking.     

植物蛋白螺杆挤压组织化技术的研究进展

螺杆挤压组织化技术是植物蛋白肉生产加工的主要方式,具有规模化、产业化、连续化、高效节能等优点。旨在对植物蛋白螺杆挤压组织化技术进行梳理,从螺杆挤压组织化技术及其设备方面综述了植物蛋白螺杆挤压组织化技术的研究进展,阐述了螺杆挤压组织化过程的影响因素。螺杆挤出机是植物蛋白挤压组织化加工的主要设备,螺杆为螺杆挤出机的核心部件,螺杆元件及螺杆构型对挤压压力、剪切力、扭矩和平均停留时间具有较大影响。温度、剪切力、压力、原料等都是影响植物蛋白螺杆挤压组织化过程的重要因素。     

Shear Induced Starch Conversion During Extrusion

Using a capillary rheometer and a singlerscrew extruder, waxy corn starch was extruded at a relatively low temperature range. Notable conversions induced by shear energy were obtained in very short residence times (from several sec to 1 min). Conversions up to > 70% were also achieved. Kinetic studies showed that conversion caused by shear energy was more efficient than that by thermal energy. The shear rate constant, k, was an exponential function of shear stress and a linear function of shear rate. The minimum shear stress required to cause starch conversion was from 10⁴ to 10⁶ N/m² for 100°C to 21°C. In such “shear” experiments, the maximum temperature rise due to viscous thermal dissipation, measured experimentally and calculated theoretically, was too low to cause any significant thermal cooking of starch. The shear converted starch extrudates were unpuffed and had shapes of traditional pasta.     

Analysis of Shear and Thermal History During Co-Rotating Twin-Screw Extrusion

A computation procedure was developed for calculating contributions of frictional and viscous heat generation during co-rotating twin-screw extrusion. Application of energy balance to experimental data (mechanical energy input, feed rate, feed moisture, screw speed, die pressure, barrel wall and dough temperature profiles) indicated how conditions affected fill, filled length, shear and thermal energy inputs to the foods. Kneading blocks increased degree of fill in partially filled sections, and dissipated 4–6 times more viscous heat than conveying elements in completely filled sections. Energy input profiles along extruder screw axis clearly demonstrated how heat generation from shear and heat transfer between extruder and foods influenced total energy input.     

The Effect of Density on Melt Viscosity Calculations

Previous studies of starch viscosity during extrusion have been made without detailed data on the density of starch/water systems, at the temperatures and pressures common to extrusion. Density data are needed to convert the extruder mass output into volumetric output for calculations of shear rates. Earlier studies have assumed constant density, despite changes in temperature, pressure, and moisture content. Using recently published data on starch density, it is shown that density corrections have minimal impact on results compared to the constant density assumption. The power law index remains unchanged, and changes in the consistency are within experimental error. The choice of a reference density is the determining factor in shear rate calculations. Similar results are obtained for low density polyethylene, and, by extension, other polymer melts.     

Immobilized α-amylase from Bacillus licheniformis: a potential enzymic time—temperature integrator for thermal processing

Biphasic and nth-order models were tested as to their usefulness to fit experimental inactivation data of Bacillus licheniformis α-amylase, immobilized on glass beads, and were discussed with respect to their suitability to characterize the considered enzymic system as a time—temperature integrator (TTI) to evaluate heat processes. Both isothermal and non-isothermal inactivation experiments were carried out. Model (kinetic) parameters (rate constant k, activation energy E_A and reaction order n) were estimated using a non-linear regression procedure. The results obtained, especially the activation energy of about 293 kJ mole^–1, indicated a potential use of this system as a TTI for heating processes in the temperature range of 96–108°C.     

Polymerization and Mechanical Degradation Kinetics of Gluten and Glutenin at Extruder Melt-Section Temperatures and Shear Rates

Thermal polymerization and mechanical degradation rates were measured for wheat gluten and glutenin at 25–30% moisture. Changes in soluble protein and disulfide bonds were measured at 85°C to 180°C, residence times of 2–60 set and shear rates of 10–275 sec^-l. Polyrnerization rates without shear were evaluated using isothermal analysis. An extrusion rheometer was used to simulate extruder conditions to determine the combined polymerization-degradation rates. Molecular weight distributions of soluble reaction products were determined using liquid size exclusion chromatography. Activation energies were 0.60–2.1 kcal/ mol, 2.0–8.2 kcal/mol, and 0.50–0.90 kcaVmo1 and reaction orders were 0.5–2.0, 0.8–1.5, and 0.6–0.7, respectively for polymerization without shear, polymerization with shear, and degradation.     

Rheological behaviour of wheat flour dough in twin-screw extrusion cooking

A capillary-type viscometer was designed to measure the apparent viscosity of the cooked wheat flour dough on -line in a twin-screw extruder. The effect of mean residence time and specific energy were studied in the ranges 20–45s and 300–800 kJ kkg⁻¹, respectively. A viscosity model was developed which accounts for effects of wall shear rate, moisture content and specific energy. It was shown that the extent of the cooking reaction is largely determined by the specific energy applied.     

System parameters and product properties response of soybean protein extruded at wide moisture range

In order to explore the effect of water during extrusion process, soybean protein isolate (SPI) was extruded using a pilot-scale twin-screw extruder at 28%, 36%, 44%, 52% and 60% moisture content and 140, 150 and 160 °C cooking temperature. The extrusion system parameters like in-line viscosity at die, mean residence time and specific mechanical energy (SME), product textural properties including tensile strength, hardness, chewiness and degree of texturization, and the molecular weight distribution characterized by SDS–PAGE were investigated. And the interrelationship between system parameters and product properties were analyzed. The results showed that moisture content was a more important factor on system parameters and product properties than cooking temperature. Higher moisture content resulted in lower viscosity of dough in the extruder, shorter residence time and lower conversion ratio of extruder mechanical energy into heat energy, finally reducing significantly the tensile strength, hardness, chewiness and the degree of aggregation. The data from extrusion system parameters and product properties correlate well and could be used to explain and control the characteristics of extrudate.