Process dynamics of starch-based microcellular foams produced by supercritical fluid extrusion. I: model development

Supercritical fluid extrusion (SCFX) has been used successfully to produce biopolymeric foams with bubble size in the range of 50–200 microns, and bubble density to the order of 10⁶ per cm³. Final bubble size and expansion ratio of extrudates depend on process and material parameters like CO₂ injection rate, nozzle temperature, oven temperature, melt viscosity, melt yield stress, etc. The objective of this study was to describe SCFX process dynamics and post-extrusion drying mechanism by using a mathematical model for bubble growth at the microscopic level in conjunction with a macroscopic model for flow of starch melt through the extruder nozzle, bulk diffusion of CO₂ and water to the atmosphere and heat transfer in the extrudate. The model was written in Visual Basic. The model provided the basis for a good understanding of the mechanisms of bubble growth and collapse, post-extrusion drying and open cell formation during SCFX processing, and is the only such one developed so far for extrusion puffed products. Simulation results and comparison with experimental data are presented in Part II of this paper.     

Physical and chemical properties of tomato, green tea, and ginseng-supplemented corn extrudates produced by conventional extrusion and CO2 injection process

In this study, extrudates were produced from corn flour supplemented with tomato, green tea, and ginseng powder by conventional extrusion and CO₂ injection methods. In conventional extrusion process, the die exit temperatures were adjusted to 80, 110, 130 °C and CO₂ injection method was carried out at 80 °C. In both extrusion methods, the feed moisture, screw speed, and die diameter were kept constant at 20 %, 250 rpm, and 3 mm, respectively. Corn flour was supplemented with tomato powder at three different levels (4, 8, and 12 %), but ginseng and green tea were added only at 8 % level. The bulk density results generally had an inverse relationship with the expansion index values. Higher tomato powder supplementation levels resulted in higher total phenolic compound levels and antioxidant activities. These increases were around 11-fold as compared to control samples in 12 % tomato powder supplementation level at 130 °C extrusion temperature. The results indicated that antioxidant activity increased with increasing extrusion temperature. This increase might be due to higher amount of Maillard reaction products, with antioxidant activity, formed at higher extrusion temperatures. The color (L*, a*, b*) and hydroxymethylfurfural results proved that the rate of Maillard reaction increased at higher extrusion temperatures during the extrusion process. Green tea supplementation resulted in higher total phenolic compounds and antioxidant activity values as compared to tomato and ginseng supplementation under the same extrusion conditions.     

Physicochemical and sensory characteristics of a low-fat tofu produced using supercritical CO2 extracted soy flour

The potential of supercritical CO₂ (SC-CO₂) extracted soy flour was evaluated as a raw material for functional foods. The fat content in SC-CO₂ soy flour was reduced from 19.5% to 7.1%, which influenced some textural and sensory properties of soymilk and tofu. The viscosity of low-fat soymilk decreased from 50 to 40 cp. Low-fat tofu had a higher yield of 69.7%, compared to a yield of 60.8% for full-fat soymilk. The two types of tofu had similar flavor and texture intensity sensory scores, although low-fat tofu had a reduced score for roasted, nutty flavor than full-fat tofu. The pH of a low-fat tofu immersion solution during 30 days of storage was lower than for a full-fat tofu solution, particularly during the first 6 days. A low-fat tofu that is similar to full-fat tofu can be prepared using SC-CO₂ extracted soy flour.     

Hull-less barley flour supplemented corn extrudates produced by conventional extrusion and CO2 injection process

The effects of feed moisture content (20, 25%), exit–die temperature (80, 130 °C), and extrusion-cooking method (with/without CO₂ injection) on β-glucan (BG) levels and physicochemical properties of hull-less barley flour (HBF) supplemented (15, 30, 45%) corn extrudates were investigated. The highest HBF supplementation level (45%) resulted in higher BG levels (2.87–3.28%) in all extrusion conditions. In general, increasing feed moisture content from 20 to 25% and exit–die temperature from 80 to 130 °C resulted in small increases in enzyme resistant starch type-3 (RS₃) levels. However, the investigated extrusion conditions did not form substantial amount of RS₃ and the highest RS₃ content was 540 mg/100 g. Lower feed moisture content and higher exit–die temperature resulted in higher water solubility (WS) and lower water-binding capacity (WBC) values. Cold-paste viscosity (CV) was observed in all extrudates. High WS and WBC values of extrudates and the existence of CV values in RVA curves indicated complete starch gelatinization.Industrial relevanceCO₂ injection has been proved as a reliable alternative method to the conventional extrusion process used in the breakfast cereals and snack food industry. The overall quality of the product processed by CO₂ injection was comparable to that of the current process. The extrudates produced by CO₂ injection method had more uniform expansion and smoother surface. CO₂ injection did not have a reducing effect on β-glucan levels; however, it is expected to result in a better retention of heat labile micronutrients and hence more healthy food products.     

超临界CO2流体萃取的姜油树脂组成与物性研究

超临界CO2流体萃取姜油树脂,采用气相色谱-质谱联用方法分析萃取物的成分,并按其结构和功能分类;对萃取物的折光率、旋光性、在不同植物油的溶解性能、感官特性进行了研究.对常规的生姜溶剂萃取物也进行了以上试验.试验结果表明超临界流体萃取姜油树脂挥发油组分占51.372%,姜辣素类组分占41.936%;而溶剂萃取姜油树脂挥发油组分占11.326%,姜辣速类组分占59.94%;超临界CO2萃取姜油树脂不仅能萃取生姜中的姜辣素组分,还能更大程度的保留生姜中的挥发油组分;超临界流体萃取姜油树脂在植物油脂中具有良好的溶解性能.超临界CO2萃取姜油树脂能很好的保持鲜姜原有的风味,适于进一步加工成食品添加剂,保健品等.     

Chemical composition and antioxidant properties of candlenut oil extracted by supercritical CO2

Candlenut oil was extracted using supercritical CO(2) (SC-CO(2)) with an optimization of parameters, by the response surface methodology. The ground candlenut samples were treated in 2 different ways, that is, dried in either a heat oven (sample moisture content of 2.91%) or dried in a vacuum oven (sample moisture content of 1.98%), before extraction. An untreated sample (moisture content of 4.87%) was used as a control. The maximum percentage of oil was extracted from the heat-oven-dried sample (77.27%), followed by the vacuum-oven-dried sample (74.32%), and the untreated sample (70.12%). At an SC-CO(2) pressure of 48.26 Mpa and 60 min of extraction time, the optimal temperatures for extraction were found to be 76.4 °C, 73.9 °C, and 70.6 °C for the untreated, heat-oven-dried, and vacuum-oven-dried samples, respectively. The heat-oven-dried sample contains the highest percentage of linoleic acid, followed by the untreated and vacuum-oven-dried samples. The antiradical activity of candlenut oil corresponded to an IC(50) value of 30.37 mg/mL.     

Properties of yeast free bread produced by supercritical fluid extrusion (SCFX) and vacuum baking

A novel technology, supercritical fluid extrusion (SCFX), allows for continuous production of yeast-free dough leavened via incorporation of supercritical carbon dioxide (SC-CO₂). In this study, an optimum dough formulation, SCFX leavened dough production and baking procedures were developed. A premixed dough was leavened by 1% (feed basis) SC-CO₂ injection in a twin screw extruder at 37 °C. Specific mechanical energy input was 18 kW h/ton. SCFX leavened dough and bread density, moisture content, bake loss, texture profile analysis (TPA) and stress relaxation were evaluated and compared to conventional yeast leavened breads throughout 5-day storage. A combination of vacuum and conventional baking yielded the lowest SCFX leavened bread density of 0.19 g/cm³ and crumb hardness comparable to conventional yeast leavened and commercial products. This approach could be beneficial for a continuous production of consistent ready-to-bake dough and breads having equivalent quality to commercial products but produced in shorter time and without ethanol emission issues.

Industrial relevance

The dough leavening process in conventional bread production is an industrial hurdle for a number of reasons. First, it is very time consuming. Dough leavening through yeast fermentation can take up to eight hours, which is not conducive to high production rates. This long waiting period also means a great deal of storage space is required to produce a large amount of dough. This storage environment must be carefully controlled in terms of temperature, humidity, and air conditions in order to maintain optimal yeast fermentation, which can also be very costly. Finally, the emission of ethanol, a significant byproduct of yeast fermentation, must be controlled according to the EPA clean air act. Expensive catalytic converters must be installed and maintained in commercial bread production facilities.Bread produced by super critical fluid extrusion (SCFX) overcomes these problems. Total dough production time is less than an hour, and with the proper baking equipment, the entire process can be made continuous. This means constant output, less downtime, and none of the costly storage space used for conventional dough proofing. Furthermore, since there is no yeast being used, there is no ethanol being produced. This means no harmful volatile organic emissions and no expensive catalytic converter. Coupled with vacuum baking, this process results in bread with qualities similar to commercially produced bread in significantly less time.     

超临界CO2处理对PLA纤维微结构和性能的影响

采用超临界CO2流体处理聚乳酸纤维(PLA),探讨了处理后纤维的熔点、纤维对染料的吸附性和纤维取向度的变化.结果表明,经超临界CO2处理后,聚乳酸纤维的熔点提高,取向度降低,纤维对染料的吸附量减小,且上述变化均随着处理温度、压力和时间的增加而加剧.     

C.I.分散棕19在超临界CO2及水中溶解性的分子动力学模拟

为探究分散染料在超临界CO₂和水中各自染色条件下的溶解性,基于分子动力学模拟,采用热力学积分方法分别计算了C.I.分散棕19在2种溶剂中的溶解自由能和结合自由能,并采用平均非键相互作用方法分析了C.I.分散棕19染料分子与2种溶剂分子间的弱相互作用类型及稳定性。模拟结果表明:C.I.分散棕19染料分子在超临界CO₂(24 MPa,130 ℃)和水(0.25 MPa,130 ℃)中的自由能绝对值均较小且相差不大,但其在超临界CO₂中的溶解自由能绝对值稍小,结合自由能绝对值稍大;C.I.分散棕19染料分子与2种溶剂分子间均只存在较弱且不稳定的范德华色散作用,但其与超临界CO₂分子间的弱相互作用相对于水更不稳定。     

Physicochemical properties of extruded germinated wheat and barley as modified by CO2 injection and difference extrusion conditions

Germinated wheat and barley were extruded with a pilot‐scale corotating twin‐screw extruder under different barrel temperatures (100 and 120 °C), feed moisture content (30 and 35%) and CO₂ injection. The effect of germination, barrel temperature, feed moisture content and CO₂ injection on physical properties, proximate composition, γ‐aminobutyric acid, β‐glucan, phytic acid and protease activity was investigated. Results indicated that the barrel temperature, moisture content and CO₂ injection significantly affected water absorption index (WAI), water‐soluble solid index (WSI), specific length, expansion ratio, bulk density, mechanical properties and colour of all extruded products (P < 0.05). Extrusion process significantly decreased γ‐aminobutyric acid and protease activity in extruded germinated wheat (EGW) and barley (EGB) products (with and without CO₂ injection). However, extrusion process (with and without CO₂ injection) increased β‐glucan in EGB and decreased phytic acid in EGW. The content of proximate composition, γ‐aminobutyric acid, β‐glucan, phytic acid and protease activity was slightly affected by CO₂ injection.     

超临界CO2和微波萃取核桃油的比较研究

通过超临界CO2萃取核桃油的正交实验和微波萃取核桃油的均匀设计实验,考察影响核桃油萃取的主要因素,寻求最佳萃取工艺条件.研究结果表明,超临界CO2萃取的最佳工艺条件为:萃取压力35 MPa,萃取温度45℃,萃取时间2.5 h,分离温度50℃,CO2流量25 kg/h;在此条件下油脂得率为59.2%.微波萃取最佳工艺条件为:以环己烷为萃取剂,溶料比为3.5:1(W/W),微波功率735 W,每次微波辐射时间60 s,微波辐射累计时间6 min,在此条件下得率为60.4%.比较了不同提取方法对油脂得率和油脂品质的影响.结果表明,微波萃取油脂得率最高,时间最短;超临界CO2萃取的核桃油品质最优,是萃取优质核桃油的首选方法.     

Quality attributes and rheological properties of novel high milk protein-based extrudates made by supercritical fluid extrusion

Supercritical fluid extrusion (SCFX) process has opened up a new way of extruding heat-sensitive materials such as milk protein concentrate (MPC) into novel nutritional porous extrudates. Three formulations (30%:66.5%, 60%:36.5% and 90%:6.5% of MPC: rice flour, w/w) with 1% Dimodan, 1% lecithin and 1.5% salt were extruded by using SCFX. The physical, rheological and thermal properties of the extrudates were evaluated. The results showed that the increased MPC contents decreased gelatinisation enthalpy and increased the viscosity of the extrudates, which contributed to develop stable porous extrudates; and the increased MPC contents also helped to form a stronger gel network existed in dispersion of the extrudate samples, which resulted in a higher stress to break the gel and flow. The radial and overall expansions, crispness, porosity, rehydration ratio, moisture content and water activity increased while the axial expansion, hardness, piece density, apparent density, bulk density, decreased with higher amounts of MPC contents in extrudates. The overall results indicated that 90% MPC-containing extrudate was very suitable for developing highly nutritional and healthy cereals/snacks.     

Physical properties of extruded corn grits with corn fibre by CO2 injection extrusion

Corn grits and corn fibre mixed at different ratios (0/100, 15/85 and 30/70) were extruded at different melt temperature (90, 105 and 120 °C) using extrusion with and without CO₂ injection. Increasing melt temperature also significantly improved physical properties of extrudates in both non-CO₂ and CO₂ injection (200 mL/min) extrusion process. The injection of CO₂ in the extrusion process showed similar effect on physical properties of extrudates at the lower melt temperature (90 and 105 °C), whereas it exhibited opposite trend at higher melt temperature (120 °C). At the lower melt temperature, the surface of extrudates with the CO₂ injection was smoother and lower fracturability, and the cross sectional microstructure was more uniform. The addition of corn fibre decreased the expansion index, bulk density and breaking strength on the same extrusion condition. In addition, non-uniform and more broken cross-sectional microstructure was observed by the addition of corn fibre.     

超临界CO2流体处理时间对二醋酯纤维结构与性能的影响

为将超临界CO₂流体应用于二醋酯纤维的功能化加工,赋予二醋酯纤维高附加值,借助扫描电子显微镜、傅里叶红外光谱仪、X射线多晶衍射仪、热重差热分析仪、万能材料试验机探讨了超临界CO₂流体处理时间对二醋酯纤维表观形态、化学结构、聚集态结构及热性能和拉伸断裂性能的影响。结果表明:经超临界CO₂流体处理60、90、120 min后,二醋酯纤维表面颗粒状杂质被去除,其分子链排列规整度下降,结晶度由未处理纤维的39.41%分别下降至35.33%、31.57%、36.10%,纤维断裂强力呈先下降后增加趋势;当处理时间为120 min时,二醋酯纤维分子链间氢键作用强度略微降低,其熔点基本不变,高温下纤维质量损失率由未处理纤维的89.52%增加至95.66%。     

超临界CO2流体用于萃取酸浆宿萼中类胡萝卜素的研究

采用超临界CO2流体从酸浆宿萼中提取类胡萝卜素,探讨了萃取压力、温度、时间和原料粒度等萃取参数对提取量的影响。还利用UV定量地考察了所提取类胡萝卜素的耐光照稳定性、耐热稳定性和抗金属离子稳定性。结果表明,这些提取物耐热稳定,但对光很敏感。与金属离子的相互作用结果显示,Fe3+、Fe2+和Al3+对类胡萝卜素提取物的影响很显著,Cu2+、Zn2+和Ca2+等金属离子的影响相对较小,而Na+和K+基本没有影响。     

超临界CO2萃取香草兰的工艺研究和成分分析

进行超临界CO2萃取香草兰豆荚的工艺研究，探索萃取压力和温度，分离压力和温度以及萃取时间与萃取率之间的关系。确定出最佳的工艺条件为：萃取压为35MPa，萃取温度45℃，分离（Ⅰ）压力16MPa，分离（Ⅰ）温度40℃，分离（Ⅱ）压力6MPa，分离（Ⅱ）温度40℃，萃取时间150min。用GC／MS分析萃取物的化学成分，鉴定出27种化合物，其中香素的相对含量最高，为26．06％。