Microwave Blanching Research Articles

Optimization of low-cost drying methods to minimize lipid peroxidation in Spirulina platensis grown in the Philippines

Three low-cost drying methods (sun, solar, and draft oven) were optimized to produce Spirulina powder of optimal quality. Optimization in the pre-dehydration stage included the use of two antioxidants, α-tocopherol and tertiary-butyl hydroquinone (TBHQ), and two blanching methods, microwave and water bath, to inactivate enzymes. The efficiency of the pre-dehydration treatments at minimizing lipid peroxidation were evaluated in terms of the product’s oxidative stability using the thiobarbituric acid (TBA) test. The sample with the lowest TBA reactive substance (TBARS) value was considered the most stable. TBHQ was found to be significantly better than α-tocopherol in minimizing lipid peroxidation in blanched samples while α-tocopherol was better than TBHQ in unblanched samples. Microwave blanching exerted a greater stabilizing effect than water bath blanching. The combined effect of TBHQ and microwave blanching was found to be the most effective pre-dehydration treatment for minimizing lipid peroxidation in drying Spirulina. Among the three low-cost optimized drying methods, sun-drying produced a dried product with the lowest TBARS value (0.472 mg malondialdehyde.kg−1), which was closest to that of the spray-dried (control) sample (0.434 mg MDA.kg−1). Draft oven and solar drying produced dried products with the same average TBARS value (0.56 mg MDA.kg−1). Sun-drying, when optimized, produced a dried product that was almost as stable as the spray-dried product.

EFFECT OF PROCESSING PARAMETERS ON THE TEMPERATURE AND MOISTURE CONTENT OF MICROWAVE‐BLANCHED PEANUTS

ABSTRACT Peanut blanching consists of heat application followed by abrasive removal of the seed coat. The use of a continuous microwave system for the blanching of peanuts has been proposed as a means of reducing processing time and energy costs compared to the traditional hot‐air, multizone oven. The purpose of this research was to characterize effective processing parameters for microwave blanching. The factors examined were the time of exposure in the microwave, use of increased airflow in the microwave applicator during processing and the initial moisture content (MC) of the peanuts. Processing treatments were differentiated by energy absorbed during processing, average and maximum internal temperature, loss in MC and blanchability. High blanchability resulted from higher process temperatures and greater loss in MC. Treatments exceeding 110C resulting in a final MC of 5.5% or below yielded blanchability percentages greater than the 85% industry standard. The time required to generate sufficient heat to dry peanuts for acceptable blanchability is greatly reduced by the use of microwave technology.

Characterization of Aroma‐Active Compounds in Microwave Blanched Peanuts

ABSTRACT: Microwave blanching of peanuts has been explored as an alternative to conventional oven methods based on its speed of operation, energy savings, and efficiency of process control. Although processing times can be greatly reduced, the occurrence of stale/floral and ashy off‐flavors has been reported at high process temperatures. This study examined the chemical compounds responsible for this off‐flavor using solvent extraction/solvent assisted flavor evaporation (SAFE), gas chromatography‐olfactometry (GC/O), gas chromatography‐mass spectrometry (GC/MS), and aroma extract dilution analysis (AEDA). Select compounds were quantified based on AEDA results using SAFE and GC/MS. Quantification, threshold testing, and analysis of model systems revealed increased formation of guaiacol and phenylacetaldehyde in the off‐flavored peanuts, which resulted in the burnt and stale/floral flavors noted by a trained panel.

Effect of different blanching methods on peroxidase and lipoxygenase activity of broccoli and comparison of some nutritional values at enzymes inactivation levels

Peroxidase (POD) and lipoxygenase (LOX) inactivation temperature and time were investigated on broccoli florets at different blanching treatments. In addition, retention of nutritional components,which reduced POD and LOX activities, was compared. LOX inactivation required 14 min at 70 °C, 6 min at 80 °C, 11/2 min at 90 °C, 1 min at 100 °C water blanching and 1 min for steam blanching, 2 min in microwave without water and 6 min with microwave and water. The highest nutritional content of broccoli was evaluated in microwave blanching without water by comparison to all treatments. The decline of nitrate and nitrite value of broccoli was higher under longer time blanching than under higher temperature blanching.

IMPACT OF MICROWAVE BLANCHING ON THE FLAVOR OF ROASTED PEANUTS

ABSTRACT Microwave blanching of peanuts was proposed as an attractive alternative to traditional techniques of blanching, because of energy and time savings. However, the occurrence of a processing‐related off‐flavor has been reported. This study examined the effect of processing factors during microwave blanching on the MC and sensory characteristics of the peanuts. The peanuts reached a range of internal temperatures during microwave blanching treatments between 4 and 11 min. A total offnote attribute was introduced to the peanut lexicon and was used successfully to differentiate the effects of microwave treatments. The microwave‐associated off‐flavor was related (but not identical) to cardboardy/stale flavor, and was related inversely to the positive flavor attributes roasted peanutty, sweet aromatic and sweet taste. Peanuts reaching the highest internal temperatures and greatest moisture losses during blanching exhibited the most total offnote flavor; however, temperatures as high as 113C did not produce significantly increased total offnote intensity.

EFFECTS OF BLANCHING METHOD ON THE QUALITY CHARACTERISTICS OF FROZEN PEAS

ABSTRACT 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.

Combined treatments of blanching and dehydration: study on potato cubes

Samples of Solanum tuberosum var. Primura were submitted to combined treatments of blanching and dehydration. Blanching was alternatively performed in hot distilled water, hot sugary-saline solution, by microwaves in distilled water or by microwaves in saline solution. Drying was alternatively carried out in an air cabinet, a microwave oven or a belt drier. In terms of process speed, colour retention and water absorption capacity, the best results were obtained combining microwave blanching with dehydration on the belt drier. In particular, dehydration on the belt drier levelled eventual negative effects determined by the blanching treatments.

MICROWAVE BLANCHING OF SLICED POTATOES DIPPED IN SALINE SOLUTIONS TO PREVENT ENZYMATIC BROWNING

Response surface methodology was applied to investigate the way in which variables such as time of treatment, sodium or calcium chloride concentrations and lactic acid concentration affect the microwave blanching of potato slices dipped in these solutions. Two three factor-five-level, second order central composite designs were developed to analyze the considered variables. Results showed that, with reference to polyphenoloxidase inactivation, the blanching in calcium chloride-lactic acid solution was more effective than the blanching in sodium chloride-lactic acid solution. Color measurements showed that, in the applied operative conditions, the best results were obtained by short blanching in sodium chloride-lactic acid solution at high lactic acid concentration and low NaCl concentration and by short blanching in calcium chloride-lactic acid at low lactic acid concentration or low calcium chloride concentration.

Microwave Blanching of Vegetables

ABSTRACT: Three vegetables, spinach, carrot, and bell peppers were blanched conventionally in water and using pulsed microwave at 95 ± 2 C. The effect of various parameters like mass of the product, mobility of the product in the microwave field, and physical geometry on the temperature attained was evaluated. The study also included the kinetics of peroxidase inactivation, temperature, and power distribution during microwave blanching. The study highlights the potential application of microwave blanching in reducing the loss of valuable nutrients. The kinetics of peroxidase inactivation indicated that microwave blanching was comparable to water blanching with higher reaction rate in the case of water blanching. The temperature and absorbed power levels during microwave blanching was influenced by the vegetable itself, its quantity, shape, location in the oven, and the microwave power applied.

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

ABSTRACTThe 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.

Influence of Blanching‐osmotic Dehydration Treatments on Volatile Fraction of Strawberries

ABSTRACT: The effects of steam (S) and microwave (MW) blanching and osmotic treatments, applied either singularly or combined, on the volatile fraction of strawberry have been evaluated. Osmotic dehydration was carried out at atmospheric conditions (OD) and by applying a vacuum pulse at the beginning of the process (PVOD). Volatiles were obtained by a simultaneous distillation‐extraction procedure and identified/quantified by gas chromatography‐mass spectrometry. Esters and 2,5‐dimethyl‐4‐hydroxy‐3(2H)‐furanone were the major compounds. Differences in volatile concentration promoted by blanching and/or osmotic processes were evaluated. Osmotic treatments promote formation of esters and furanones, differently for either OD or PVOD treatments, but this effect was greatly inhibited when osmosis was preceded by blanching. The kind of blanching (MW or S) also affected the sample final volatile profile.

Changes in texture, microstructure and nutritional quality of carrot slices during blanching and freezing

Thermal processing of vegetables has pronounced effects on the cell structure, often lowering the final textural properties of the product. In order to investigate the effect of thermal processing on carrot, slices were subjected to different blanching and freezing treatments before frozen storage. Microwave-, steam- or water-blanched material was frozen and then stored at −24 °C. Steam-blanched carrots were subjected to blast freezing or cryogenic freezing at different temperatures before frozen storage. The influence of these process conditions on the texture (maximum load and slope), microstructure, dry matter, sugars, carotene and drip loss was investigated. Microwave blanching differed from the other blanching methods by resulting in a heterogenic cell structure. The content of dry matter, carotene and sucrose was higher following microwave blanching. Blast freezing resulted in low maximum load which seemed to be caused by major tissue damage. Concerning cryogenic freezing, lowering the temperature from −30 °C to −70 °C resulted in better preservation of the native microstructure together with an increase in maximum load, which was most pronounced after one month of storage. No significant effect was observed when lowering the temperature from −30 °C to −70 °C for any of the other measured parameters. © 1999 Society of Chemical Industry

Changes in Texture and Nutritional Quality of Green Asparagus Spears (Asparagus officinalis L.) during Microwave Blanching and Cryogenic Freezing

Green asparagus spears were blanched for a short or long period by steam, water or microwave in two different systems, and spears were frozen by blast or liquid nitrogen under different conditions. The influence of the process conditions on shear force values (toughness), vitamin C, dietary fibre, chlorophyll and drip loss was investigated. Spears with similar or higher shear force values and lower vitamin C were obtained by microwave blanching than by steam and water. No differences were found in shear force values, vitamin C, chlorophyll and dietary fibre between cryogenic and blast freezing, whereas cryogenic freezing reduced the drip loss significantly. Shear force values increased and vitamin C decreased slightly, whereas dietary fibre and chlorophyll did not change during frozen storage.

Effects of microwave heating on pigment composition and colour of fruit purees

Microwave energy was applied to inactivate the oxydoreductases peroxidase (POD, EC 1.11.1.7) and polyphenol oxidase (PPO, EC 1.14.18.1) in processed fruit products. Microwave blanching of papaya, strawberry and kiwi purees at various conditions of power and time produced inactivation of PPO and POD activities depending on the fruit and the heating conditions. Treatment at 850 W/60 s produced about 60% of POD inactivation for papaya and kiwi fruit. POD activity in strawberry, however, seemed to be more resistant to microwave inactivation; treatment at 850 W/60 s only achieved a loss of POD activity near 8%. Papaya oxidoreductases showed lower stability in the microwave treatments tested. Microwave blanching at 475 W/45 s produced about 75% inactivation of POD activity and nearly complete PPO inactivation. Kiwi fruit and strawberry purees exhibited similar inactivation of PPO – 32% at 475 W/30 s and 70% at 475 W/60 s. The decrease of PPO activity in both products was almost linear at constant power. This thermal treatment, however, directly affects the colour of the fruit pulps. Papaya, kiwi and strawberry purees suffered slight colour (CIE L* a*b*) changes. Carotenoid, chlorophyll and anthocyanin changes were evaluated by HPLC and related to objective colour. Microwave treatments produced small modifications of the quantitative and qualitative composition of carotenoids (in papaya) and anthocyanins (in strawberry). Chlorophylls (kiwi) showed significant degradation as a consequence of microwave heating. © 1999 Society of Chemical Industry

Effects of microwave heating on pigment composition and colour of fruit purees

Microwave energy was applied to inactivate the oxydoreductases peroxidase (POD, EC 1.11.1.7) and polyphenol oxidase (PPO, EC 1.14.18.1) in processed fruit products. Microwave blanching of papaya, strawberry and kiwi purees at various conditions of power and time produced inactivation of PPO and POD activities depending on the fruit and the heating conditions. Treatment at 850 W/60 s produced about 60% of POD inactivation for papaya and kiwi fruit. POD activity in strawberry, however, seemed to be more resistant to microwave inactivation; treatment at 850 W/60 s only achieved a loss of POD activity near 8%. Papaya oxidoreductases showed lower stability in the microwave treatments tested. Microwave blanching at 475 W/45 s produced about 75% inactivation of POD activity and nearly complete PPO inactivation. Kiwi fruit and strawberry purees exhibited similar inactivation of PPO – 32% at 475 W/30 s and 70% at 475 W/60 s. The decrease of PPO activity in both products was almost linear at constant power. This thermal treatment, however, directly affects the colour of the fruit pulps. Papaya, kiwi and strawberry purees suffered slight colour (CIE L* a*b*) changes. Carotenoid, chlorophyll and anthocyanin changes were evaluated by HPLC and related to objective colour. Microwave treatments produced small modifications of the quantitative and qualitative composition of carotenoids (in papaya) and anthocyanins (in strawberry). Chlorophylls (kiwi) showed significant degradation as a consequence of microwave heating. © 1999 Society of Chemical Industry

Nutritive value and effect of blanching on the trypsin and chymotrypsin inhibitor activities of selected leafy vegetables.

Proximate composition, energy, mineral and vitamin contents and the effect of blanching methods and times on the trypsin and chymotrypsin inhibitor activities were studied using cabbage, collard, turnip, peanut, and sweet potato leaves. Results of this study indicated that, crude protein, crude fat, carbohydrate and ash contents were in the range of 15.5-25.6%, 1.4-6.5%, 60.4-73.1% and 6.8-7.5%, respectively. Total dietary fiber was lowest in cabbage (28.2 g/100 g) and highest in the collard leaves (43.1%) while energy content per 100 g of vegetables was highest in sweet potato leaves (402 kcal) and lowest in cabbage (379 kcal). The mineral content per 100 g of vegetables were in the range of 33.4-249.8 mg, 241.2-471.2 mg, 12.1-75.1 mg, 14.9-98.9 mg, 0.5-3.5 mg and 0.9-3.1 mg for Ca, K, Na, Mg, Fe and Zn, respectively. For ascorbic acid, riboflavin, thiamin and total carotenoids, concentrations in 100 g of vegetables were in the range of 45.1-112.7 mg, 0.2-0.3 mg, 0.3-0.8 mg and 2.0-7.3 mg, respectively. The trypsin inhibitory activity per gram of the vegetables was highest in collard (60.1 TIU/g) and lowest in peanut leaves (41.0 TIU/g). Chymotrypsin inhibitor activity was highest in the peanut (69.6 CIU/g) but lowest in the collard leaves (48.0 CIU/g). Both trypsin and chymotrypsin inhibitor activities were significantly (p < 0.05) reduced by most of the treatments in either the conventional or microwave blanching methods. In the conventional blanching method, trypsin inhibitor activity was reduced by 0.5, 6.8, 11.9, 9.0 and 19.3 percent in cabbage, collard, turnip, sweet potato and peanut leaves, respectively, when the vegetables were blanched for 2.5 minutes but after blanching for 10 minutes, the trypsin inhibitor activity was reduced by 29.7, 34.9, 54.3, 52.3 and 65.6 percent in cabbage, collard, turnip, sweet potato and peanut greens, respectively. For the microwave oven blanching, trypsin inhibitor activity was reduced by 3.8, 3.3, 32.7, 5.0 and 9.5 percent in cabbage, collard, turnip, sweet potato and peanut leaves, respectively when the vegetables were blanched for 30 seconds. When blanched for 60 seconds, trypsin inhibitor activity was reduced by 16.2, 45.8, 46.2, 51.0 and 42.4 percent in cabbage, collard, turnip, sweet potato and peanut greens, respectively. Similar trends in the reduction of chymotrypsin inhibitor activity were observed when the vegetables were conventionally blanched for 2.5, 5 and 10 minutes and when blanched by microwave oven for 30, 45 and 60 seconds. Based on the results of this study, the vegetables were good dietary sources of minerals, vitamins, carbohydrate and proteins. Also, blanching was an effective method for reducing the trypsin and chymotripsin inhibitor activities in the leafy vegetables, however, further investigation on the heating times for both conventional and microwave blanching methods is suggested.

Chlorophyllase Inactivation as a Measure of Blanching Efficacy and Colour Retention of Artichokes (Cynara scolymusL.)

The response of the enzyme chlorophyllase of Maipo-ecotype artichokes (Cynara scolymusL.) to three thermal inactivation treatments (microwave, steam or boiling water) were evaluated. The temperature–activity profile of the enzyme, between 25 and 45 °C, shows maximum activity at 30 °C. Curves of thermal inactivation of chlorophyllasevs.time were plotted for the three treatments, resulting in complete inactivation of the enzyme after a blanching time of 2, 6 and 8 min for microwave, steam and boiling water processes, respectively. Measurement of peroxidase activity was performed as an indicator of blanching process adequacy. Colour changes during the blanching time needed to inactivate the enzyme chlorophyllase were seen with the three treatments, but more in steam treated artichokes than with the others. Steam blanched artichokes during the time needed to inactivate chlorophyllase also showed lower values for lightness index, hue and chroma than water and microwave oven treatments. Microwave and boiling water blanched artichokes best preserved the original perceptual colour. A total peroxidase inactivation without detectable loss of ascorbic acid for microwave blanching, compared with a 16.7% loss of ascorbic acid with boiling water blanching, indicates better quality for microwave blanched artichokes.

MICROWAVE AND WATER BLANCHING OF CORN KERNELS: CONTROL OF UNIFORMITY OF HEATING DURING MICROWAVE BLANCHING

Corn kernels were blanched in water and by microwave. The blanching efficiency was measured by four parameters; the thermolability of soluble peroxidase (POD), of two insoluble isoforms (membrane-bound and cellulose-bound POD) and of soluble protein. Using a water-shield model to improve the uniformity of heating by microwave and to control the rate of heating, microwave blanching was significantly more efficient as measured by all four parameters when blanched at 85C. There was no significant difference in the POD isoform heat-stability when blanched at 70C by the two methods. POD insoluble isoforms were more heat-stable than the soluble isoform. POD isoforms, occupying three cellular environments were more efficiently inactivated by uniform microwave heating compared to water blanching, demonstrating the viability of microwave blanching.

Chemical Characteristics and Storage Stability of Pickled Garlic Prepared Using Different Processes

ABSTRACTThree processes for preparing peeled pickled garlic were studied: (P) packing directly with an acidified brine, followed by pasteurization (90°C, 8 min); (B) blanching with hot water (90°C, 15 min) before packing, followed by a preservation treatment (pasteurization, preservatives + refrigeration); and (MB) same as B, except that microwave blanching was used (120g garlic cloves, 825W, 1.5 min). All products were microbiologically stable but fructans were hydrolyzed during storage at 27°C. A green pigment was also apparent in (P). Products from (B) or (MB) had better quality than that from (P). Most stable and best quality products resulted from the blanching treatments and water blanching resulted in the product with best color.

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

AbstractMarjoram 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.