MICROWAVE BLANCHING EFFECTS ON COLOR, CHEMICAL AND SENSORY CHARACTERISTICS OF FROZEN ASPARAGUS

Asparagus was blanched for 4 min using: conventional boiling water (BW), steam (ST), microwave (MW), or microwave heated in boilable bags (MWB). Samples were ice-cooled, bagged, heat-sealed and stored at ?18C for 4 weeks. Peroxidase activity was reduced from 98–114 units in fresh, unblanched to 1–7 units in blanched asparagus. Reduced ascorbic acid (RAA) content in fresh, unblanched asparagus was ～49 mg/100 g; RAA content in fresh, BW-blanched asparagus was ～44 mg/100 g. Frozen, unblanched asparagus retained about 40% of original RAA content; BW-blanched asparagus retained 61%, and MWB-blanched asparagus retained 87%. After frozen storage all samples were darker than freshly blanched samples. Blanching increased and freezing decreased greenness of all samples. Blanching increased yellowness of fresh samples; after frozen storage, blanching treatment differences were lost. Unblanched samples had the highest appearance scores; unblanched and MW-blanched samples had the highest color scores. Overall quality of the microwave blanched asparagus was as good as or superior to conventionally blanched asparagus.     

EFFECTS OF BLANCHING METHOD ON THE QUALITY CHARACTERISTICS OF FROZEN PEAS

This study evaluated the effects of microwave blanching prior to freezing as an alternative pretreatment for frozen peas. Peas were blanched (steam‐, boiling water immersion‐, microwave‐ or microwave‐blanched in a bag), frozen and evaluated after 0, 6 and 12 weeks for moisture and ascorbic acid content, peroxidase activity, visual appearance and instrumental color, and after 6 and 12 weeks for aroma, flavor and texture. All blanch treatments reduced peroxidase activity by 97% compared with controls (unblanched); blanching methods did not differ. Steam blanching resulted in significantly better ascorbic acid retention than all other treatments; microwave blanch treatments were either equivalent to or better than boiling water immersion. Both microwave treatments darkened (L* value) peas. Microwave‐blanched peas were visually greener than other treatments, but their appearance was less intact. Aroma and flavor were similar among blanch treatments. Texture of boiling water immersed peas was similar to the two microwave treatments.     

PHYSICAL, CHEMICAL AND SENSORY QUALITY OF MICROWAVE-BLANCHED SNOW PEAS

Snow peas were blanched using four methods [conventional boiling water (BW); steam (ST); microwave (MW); and microwave blanched in heat sealable bags (MWB)], then held frozen at (～18C) for four weeks. Immediately after blanching, MWB‐blanched snow peas retained more reduced ascorbic acid (RAA; 72%), than other treatments. Blanching treatment had no effect on lightness [L* value). With the exception of BW‐blanched peas, blanching significantly altered color. After frozen storage, MWB‐ and MW‐blanched samples both contained about 26 mg/100 g RAA, significantly more RAA than other treatments. MWB‐blanched peas generally had as high or higher sensory appearance, aroma, flavor, texture and general acceptability scores than other treatments. These results imply that blanching snow peas in a heat‐sealable microwave bag, prior to frozen storage, produces a product of equivalent sensory quality and superior nutritional quality compared with more conventional blanching methods.     

MICROWAVE BLANCHING EFFECTS ON CHEMICAL, SENSORY AND COLOR CHARACTERISTICS OF FROZEN GREEN BEANS

Green beans (cv. Mustang) obtained from the University of Illinois at Urbana-Campaign Horticulture plots were assayed within 3 h of harvest. Beans were blanched in covered containers: conventional boiling water (1900 ml, 3 min) (BW), steam (300 ml water, 3 min) (ST), microwave heated in 1 L glass containers (60 ml water, 3 or 5 min, 700 W) (MW-3, MW-5), and microwave heated in 1 L Seal-a-Meal^TM bags (45 ml water, 3 min) (MW-Bag). Baseline data were collected immediately after blanching. Aliquots were frozen at – 18C for 4 weeks. Unblanched beans had 82 units of peroxidase activity/min; activity was zero immediately after all blanching treatments. Reduced ascorbic acid (RAA) content was highest for unblanched, ST and MW-5 beans and lowest for BW and MW-Bag beans immediately after blanching. ST beans retained 100% of original RAA. Instron shear force (kg) was highest for unblanched beans and lowest for BW and MW-Bag beans immediately after blanching. After frozen storage, ST and BW beans had the lowest shear values. Visual color was poorest for MW-5; other treatments were not different. Microwave blanching green beans for 3 min in a covered container or bag prior to 4 weeks frozen storage resulted in a product that was not different in RAA content, retention or color from BW blanched beans; however, these samples differed in tenderness and crispness from boiling water blanched beans.     

EFFECTS OF CULTIVARS, BLANCHING TECHNIQUES, AND COOKING METHODS ON QUALITY OF FROZEN GREEN BEANS AS MEASURED BY PHYSICAL AND SENSORY ATTRIBUTES

Effects of cultivars, blanching techniques, and cooking methods on the color, texture, and sensory characteristics of frozen green beans were investigated. Provider beans were firmer than the Contender cultivar following frozen storage and cooking when evaluated instrumentally and by a sensory panel. Microwave-blanched beans were harder according to Instron measurements, and sensory scores for firmness and toughness were higher than beans blanched by boiling water or steam. Green beans blanched or cooked by microwaves had grassy aromas and strong off-flavors that were not detected in those blanched or cooked by conventional methods. Microwave cooked beans were greener and firmer than those conventionally cooked.     

CHEMICAL, NUTRITIVE AND SENSORY CHARACTERISTICS OF TOMATOES BEFORE AND AFTER CONVENTIONAL AND MICROWAVE BLANCHING AND DURING FROZEN STORAGE

The objective of this study was to evaluate the effects of microwave blanching on chemical, physical and sensory characteristics of tomatoes prior to and after frozen storage. Tomatoes were blanched (4 min) using four treatments: conventional boiling water (BW), steam (ST), microwaved in a glass container (MW), and microwaved in boilable bags (MWB). The lowest moisture content occurred in MW‐blanched tomatoes before (92%) and after (86%) frozen storage. These tomatoes had the highest reduced ascorbic acid (RAA) content and the highest RAA retention (> 23 mg/100 g, >91%) after blanching and after frozen storage. After blanching, MWB‐blanched tomatoes were the lightest, while after frozen storage, ST‐blanched tomatoes were the lightest. BW‐blanched tomatoes had generally higher flavor, texture and appearance scores. This study demonstrated that though visual color and sensory attributes were highest for BW‐blanched tomatoes, MW‐blanched tomatoes retained more nutritive value in the finished product.     

MICROWAVE AND CONVENTIONAL BLANCHING EFFECTS ON CHEMICAL, SENSORY, AND COLOR CHARACTERISTICS OF FROZEN BROCCOLI

Broccoli (cv. Empress) obtained from a local supplier was blanched within 15 h of harvest. It was blanched by four methods in covered containers: conventional boiling water (1900 mL, 4 min) (BW), steam (300 mL water, 4 min) (ST), microwave heated in 1 L glass containers (60 ml water, 4 min, 700 W) (MW), and microwave heated in 1 L Seal-a-Meal^TM bags (45 ml water, 4 min) (MWB). Aliquots were frozen at -18C for 4 weeks. Fresh unblanched broccoli peroxidase activity ranged from 389 to 829 units/min; activity was essentially zero immediately after all blanching treatments. The highest reduced ascorbic acid (RAA) content occurred in fresh unblanched broccoli. Some peroxidase regeneration occurred during frozen storage. Immediately after blanching, all blanched broccoli had lower RAA content than control broccoli. MW-blanched broccoli retained the greatest amount of RAA and had appearance, visual color, texture scores, and chroma of florets and stems equivalent to ST-blanched broccoli. MW-blanched broccoli had flavor and general acceptability scores similar to BW-blanched broccoli. After 4 weeks in frozen storage, MW-blanched broccoli had the highest RAA content.     

Impact of the industrial freezing process on selected vegetables — Part I. Structure,texture and antioxidant capacity

In this work, the impact of the industrial freezing process on structure, texture and total antioxidant capacity was studied using green asparagus stems, zucchini and green beans. Samples were analysed as raw/uncooked, blanched, raw/boiled and industrially frozen/boiled.A consistent damage of the vegetable tissue was revealed by the histological analysis on vegetables boiled after freezing. The cells appeared to be dehydrated, contracted and separated at different levels depending on the anatomical structure of each vegetable.The initial textural quality was partially retained in all blanched vegetables, and enhanced in cut tested asparagus stems, in relation to the action of phenolic acids at cell wall level. Raw/boiled and industrially frozen/boiled asparagus stems exhibited comparable forces of penetration and cut tests. On the other hand, zucchini, both raw and frozen, completely softened after boiling making the texture measurement impossible. Industrially frozen/boiled green beans showed higher values of cut and penetration forces, probably due to a higher presence of swollen cell walls, in comparison to those raw/boiled.Blanching and boiling significantly increased the ferric reducing antioxidant power values of asparagus stems and green beans compared to uncooked/raw samples, while boiling after the freezing process significantly deprived both vegetables of the initial antioxidant capacity. On the other hand, boiling the frozen zucchini proved to be detrimental to the antioxidant capacity.In conclusion, manufacturers and researchers should join together to develop specific industrial freezing process conditions according to the matrix of each vegetable.     

STABILIZATION OF ANTHOCYANINS IN FROZEN TART CHERRIES BY BLANCHING

SUMMARY— Unpitted red tart cherries (Prunus cerasus L. cv. Montmorency) were blanched in steam (100°C) for 0, 30, 45 and 60 sec, then frozen at −20°C. The anthocyanin color of the fruit was determined periodically during frozen storage for 3 months in one experiment and 10 months in another. When the cherries were not allowed to thaw before the analysis, no color loss due to anthocyanin destruction was observed in either the blanched or unblanched cherries. When they were thawed at room temperature (22° C) in single layer for 2 and 4 hr, the unblanched cherries lost 14 and 25% anthocyanin color, respectively; cherries subjected to 45- or 60-sec blanching showed no significant color loss. When the cherries were disintegrated in a Waring Blendor for up to 30 min, the unblanched cherries lost considerable color (70%) after 30 min under oxygen or air, but those blanched for 45 or 60 sec suffered no color loss. Some anthocyanin destruction was Observed in the 30-sec blanch lot. Blending under oxygen was slightly more deleterious to the color than blending under air. Blending under nitrogen minimized the color loss but did not eliminate it. Blanching resulted in a 4–7.5% loss of weight.     

Blanching Leafy Vegetables with Electromagnetic Energy

Application of radio frequency, microwave, microwave-steam and infrared energy for blanching of leafy vegetables (endive and spinach) was studied and compared with conventional hot water and steam blanching. The quality of vegetables both frozen and sterilized was evaluated by instrumental and sensory analysis. Effects of blanching methods were most pronounced in frozen products. No quality differences occurred for infrared and radio frequency treatments. However, microwave energy alone or in combination with steam in the blanching process improved vitamin C retention, gave higher Instron force values and better sensory characteristics.     

Canned Dry Bean Quality as Influenced by High Temperature Short Time (HIST) Steam Blanching

High temperature short time (HTST) steam blanched dry beans had greater drained weights and shear values than water blanched dry beans. Moisture content of dry beans was greater after water blanching. Subjective grade was closely related to drained weights and shear values, and HTST steam blanched dry beans had better subjective grades than water blanched dry beans. Water blanched dry beans were lighter in color than HTST steam blanched dry beans. As length of HTST steam blanch was increased, Agtron color of the beans darkened. High quality canned dry beans can be produced with HTST steam blanching with energy and time savings, but differences between water and HTST steam blanching and canning quality is highly dependent on cultivar and length of HTST steam blanch.     

The effects of cooking method on selected quality traits of broccoli and green asparagus

Broccoli and green asparagus were cooked using boiling water, steam, microwave oven and combi oven with overheated steam and combination of hot air with overheated steam (only broccoli). Vegetables were analysed for chlorophylls a and b, carotenoids, total polyphenols, antioxidant activity, colour parameters and sensory properties. Cooking in the combi oven with overheated steam was an effective method to achieve the highest levels of polyphenols and antioxidant activity, but it negatively influenced the contents of chlorophylls in both vegetables and carotenoids in asparagus. Boiling and cooking in the combi oven with overheated steam or hot air and steam ensured the best and comparable sensory properties of broccoli. Asparagus showed a lower susceptibility to heat treatments than broccoli as none of the treatments applied caused significant differences in colour redness and yellowness, polyphenol content and sensory properties of this vegetable.     

Energy Utilization During Blanching (Water vs Steam) of Sweet Corn and Subsequent Frozen Quality

A counter-flow water blancher required 44% less steam to blanch corn than a conventional steam tunnel blancher. On an hourly steam consumption basis, the counter-flow water blancher was 31% more efficient than the steam blancher. Total product yield was 1.5% greater with the steam blancher than with the counter-flow water blancher. Drip loss of the corn was greater with the counter-flow water blancher than with the steam blancher, but moisture content of the corn was also greater. Yellow color was darker for the counter-flow water blanched corn than the steam blanched. The use of the counter-flow water blancher to blanch corn resulted in substantial energy savings with little or no difference in quality when compared to the steam blanched product.     

PILOT PLANT EVALUATION OF INDIVIDUAL QUICK BLANCHING (IQB) FOR VEGETABLES

A pilot plant IQB blancher was field tested at a California freezing plant. The system included: a warming and drying preconditioning unit, a steam chamber for single layer belt heating, a deep moving-bed holding chamber for temperature equilibration and enzyme inactivation and an air-water spray cooling unit. IQB with and without preconditioning were compared to conventional, steam blanching at equivalent levels of enzyme inactivation in green beans, lima beans, Brussels sprouts and green peas. Effluent volume and total-solids concentration were measured. Product yields were determined by measuring residual peroxidase activity, chlorophyll conversion and ascorbic acid content as well as by sensory panel testing for color and flavor. Results showed that vegetables blanched with IQB and IQB with preconditioning were essentially the same in quality as those blanched at commercial conditions. Solids loss in the effluent was substantially less with IQB and preconditioning than with conventional steam blanching     

Frozen Vegetable Quality as Influenced by High Temperature Short Time (HTST) Steam Blanching

Quality differences in processed vegetables can be controlled with high pressure steam blanching (HTST). In addition, HTST blanching required significantly less energy to blanch vegetables than water blanching. HTST-blanched vegetables displayed quality parameters distinctly different than water-blanched vegetables. Differences in quality parameters were highly dependent upon the vegetable being blanched and the pressure and time of the HTST blanch. The quality of vegetables were similar between water and HTST blanching treatments, but by controlling the pressure and time of the HTST blanch, quality characteristics could be changed to meet individual requirements. High quality frozen vegetables can be produced with HTST steam blanching but quality is dependent upon the particular vegetable and pressure and time of the HTST steam blanch.     

Processing of marjoram (Majorana hortensis Moench.) and rosemary (Rosmarinus officinalis L.). Effect of blanching methods on quality

Marjoram and rosemary are herbs which are valued for their flavour and colour. The use of microwave in blanching and drying is becoming more and more popular in recent years. This paper reports the effect of microwave blanching as compared to water and steam blanching on the volatile oil, colour, texture (by compression), chlorophyll and ascorbic acid contents of these herbs. There was maximum retention of ascorbic acid in microwave blanched marjoram and rosemary (21.5, 33.0 mg/100 g) i.e. 79.4% and 49.9% compared to fresh herbs. Blanching resulted in better retention of the original green colour of the fresh herb compared to direct drying of the herbs. Texture of microwave blanched herbs improved as revealed by higher compression force values (109.3, 157.8 N). However, there was greater loss of volatile oil in all the blanched samples.     

MICROWAVE BLANCHING OF CUBED POTATOES

Cubed potatoes cv. Primura were blanched by microwaves in water, NaCl solution, CaCl₂ solution and NaCl-CaCl₂ solution at selected powers and times. Mass and water gain/loss, firmness, starch gelatinization, color and polyphenoloxidase activity were evaluated. The results showed that all the blanching treatments induced a weight loss even if it was caused by a combined loss of water and dry matter in potatoes blanched in the saline solutions whereas it was a balance between water gain and dry matter loss in potatoes blanched in water. Firmness was affected in a negative way especially by the treatment time. Potatoes blanched in NaCl-CaCl₂ showed the highest firmness retention, whereas potatoes blanched in NaCl were less consistent. Treatment time and microwave power contributed in the same way to the starch gelatinization. It was necessary to prolong the time of treatment to 3–5 min and apply high power to inactivate polyphenoloxidase.     

EVALUATION OF THE TEXTURE OF FRIED POTATOES

Texture of potatoes with different shapes (slices and strips) were evaluated after frying and in some cases after baking. Blanched and unblanched potato slices (Bintje variety) were fried at four oil temperatures: 160, 170, 180 and 190C until reaching a moisture content of ∼1.7%. A puncture test with three point support for the slices was applied to measure the texture of potato chips using the following parameters extracted from the force versus distance curves: maximum force of break (MFB) and deformation of break (DB). These two parameters were useful to follow the changes in texture of the fried slices with moisture content at different frying temperatures. Blanched and unblanched potato strips were partially fried at 160C and 190C for 60, 90 and 120 s. The par-fried potatoes were frozen at -20C for one day after which they were baked at 200C for 15 min. The texture of the baked potato strips was evaluated using a bending test with two support points. From the force versus distance curves, two parameters were extracted: maximum force of deformation (MFD) and maximum deformation (MD). Significant higher MFB and lower DB values (P > 0.1) for unblanched fried slices indicate that these are crispier than blanched chips for moisture contents lower than 4% (6.59 N and 0.62 mm vs 5.74 N and vs 0.75 mm for unblanched and blanched chips, respectively, average values for the four frying temperatures employed). There was no effect of the frying temperature and the pretreatment (blanching or unblanching) on the texture of the frozen par-fried potatoes after baking when compared at the same residual moisture content, but blanched potato strips lost moisture more slowly both in frying and in baking.     

速冻蔬菜热烫处理的优化和节能

大多数速冻蔬菜必须经过热汤处理以保证其在冻藏期间质量的稳定性。但是热烫易引起蔬菜的色、香、味和质地的变化。目前蔬菜的热烫终点以过氧化物酶作为指示剂来确定。有关研究表明，以过氧化物酶的指示剂会导致某些蔬菜热烫过度，因此必须寻找对这蔬菜更合适的酶作为指示剂，以便优化热烫处理。在速冻蔬菜的热烫过程中要消耗大量的能量 和水，有一种热烫－冷却组合器可较好地解决蔬菜热烫过程的节能、节水问题。     

不同护色处理和干燥方式对芦笋粉品质的影响

为改善并对比芦笋在热风、真空冷冻和微波干燥过程中出现的褐变问题,故采用不同的护色剂对芦笋进行浸泡、喷涂护色处理。结果显示,相较于不护色,经护色的芦笋粉营养及色泽品质更佳。其中护3的护色效果显著（P<0.05）优于护2和护1。干燥方式和护色剂的协同使用优于单独干燥。其中,护色剂-真空冷冻干燥（VFD）协同处理的芦笋粉的酚类物质保留程度、结构完整性、色泽及抗氧化能力最佳;护色剂-热风干燥（HAD）协同处理的芦笋粉组织结构稀疏分散,酚类含量、色泽、抗氧化能力不及VFD;而护色剂-微波干燥（MD）所得芦笋粉色泽焦黄,营养及色泽品质最差。此外,相较于浸泡,经喷涂的芦笋粉水可溶性膳食纤维（SDF）含量更高,葡萄糖、胆固醇吸附能力也更好,但前者的护色效果更佳。主成分分析结果表明,芦笋粉的总酚和芦丁含量,以及理化和功能性质指标与其营养及色泽品质具有高度相关性。综合品质排名前三的芦笋粉为护3-浸泡-VFD、护2-浸泡-VFD和护3-喷涂-VFD;后三为护1-喷涂-MD、护2-喷涂-MD、对照-MD。本研究结果为提高芦笋干燥产品品质提供理论依据。