Microwave Reheating Research Articles

Effects of different reheating methods on lipid oxidation in frozen silver carp fish cakes

SummaryThis study investigated the effect of five different reheating methods (boiling water, steaming, low‐heat microwave at 120 W, medium‐heat microwave at 400 W, high‐heat microwave at 800 W) on lipid oxidation of silver carp fish cake. The changes of lipid content, thiobarbituric acid (TBA) value, acid value (AV), peroxide value (POV), volatile flavour components and fatty acid composition were detected. The results indicated that products reheated with medium‐heat microwave had higher lipid content, lower TBA, AV and POV value. The medium‐heat and high‐heat microwave reheated group had larger variety of volatile flavour substances. Moreover, the relative content of hexanal and heptanal related to unpleasant “overcooked taste” was less, and the relative content of 2‐heptanone, which contributed to the reduction of fishy odour, was more in medium‐heated microwave group. The content of monounsaturated fatty acids in the medium‐heat microwave group was higher than other reheating methods, so as the ratio of unsaturated to saturated fatty acids. Considering all indicators, medium‐heat microwave reheating is more recommended due to its inhibition of fat oxidation.

Improvement of Temperature Distribution Uniformity of Ready-to-Eat Rice during Microwave Reheating via Optimizing Packaging Structure.

The taste quality of ready-to-eat rice is influenced by the uniformity of temperature distribution during microwave reheating. The temperature distribution uniformity of ready-to-eat rice loaded in a rectangular lunch box is investigated under microwave reheating. The results show that with a 10-80 °C temperature increase in the ready-to-eat rice, the thermal conductivity increases, dielectric constant, and specific heat increase and then decrease, while the dielectric loss factor decreases and then slightly increases. The microwave-heating process of ready-to-eat rice exhibits a clear 'corner effect', and the observed 'hot spot' results in poor temperature uniformity in ready-to-eat rice. A metalized packaging structure design is subsequently proposed to ameliorate the temperature non-uniformity. According to comparative results of four metalized packaging forms, the spray film volume and film thickness corresponding to film volume are developed as 3.5×10-4 mL/mm2, 0.30 mm, respectively, which levels off the difference in temperature to improve the temperature distribution uniformity of ready-to-eat rice by microwave reheating.

Improvement of microwave reheating uniformity for baked pancake from dielectric properties and heating mechanisms

Baked pancake (BP) was unevenly heated during 2450 MHz microwave reheating (MR) and the water migrated by the temperature difference resulted in poor eating quality. The strategies to improve the edible quality of MR-BP were explored in terms of dielectric properties and reheating mechanisms. The only outer distribution of salt and sugar reduced the temperature difference during MR by increasing the dielectric loss factor (ε′) and loss tangent (tan δ). MR-BP exhibited promising moisture distribution and texture consistency when the ratio of the outer layer with salt and sugar to the inner layer without was 1:1. Furthermore, by reversing the temperature difference during MR without changing the salt and sugar distribution, infrared combined with 915 MHz microwave reheating (IMR) increased heating and texture uniformity, improving eating quality. Practical applications The eating quality of Chinese pancakes decreased during MR. In this study, adding salt and sugar only to the outer layer improved the quality of MR-BP. Furthermore, IMR increased the quality of reheating BP without changing salt and sugar distribution. Current work aims to improve consumer satisfaction with Chinese pancakes and promote its commercialization.

Microwave reheating enriches resistant starch in cold-chain cooked rice: A view of structural alterations during digestion

Cold-chain cooked rice is an instant food consumed worldwide. Through inspecting rice structural alterations during digestion, this work discloses how microwave reheating tailors the starch digestibility of cooked rice following cold storage. The cold storage allowed approximately 2% of B-type (not V-type) starch crystallites, more nanoscale and short-range orders, and smaller pores in the rice matrix. These changes retarded the hydrolysis of structural domains (e.g., amorphous regions and short-range orders) during digestion, which increased the content of slowly digestible starch to about 38.16%. Then, microwave reheating partially disrupted the B-type crystallites and nanoscale orders, but unaffected the contents of V-type crystallites and short-range orders. Even with such structural disruptions, the resistant starch content was apparently increased to approximately 30.06%, as the structural domains became less susceptible to the digestion. Additionally, for the rice samples, the percentage of V-type crystallites could be largely increased from ca. 3% to 13%–14% during digestion.

Vacuum cooling treatment can improve the texture properties of frozen pre‐fried chicken nuggets compared with air cooling

To improve the texture properties of frozen pre-fried chicken nuggets, this study has investigated the effects of vacuum cooling (VC), air cooling (AC) as well as air and vacuum combine cooling (AC/VC) on its crispness after microwave reheating. Initial experiment were carried out to evaluate the cooling rate, water loss, porosity moisture content and texture of fried chicken nuggets. VC samples showed the fastest cooling rate, the highest water loss and porosity, the lowest rate of surface moisture content, and the best crustal crispness among all cooling treatments. To acquire better crispness value, response surface analysis was designed to adjust the vacuum cooling parameters. Response surface analysis showed that lower final pressure and final temperature, higher volumetric displacement, may obtain the better crispness of pre-fried chicken nuggets during vacuum cooling process.

Microwave reheating increases the resistant starch content in cooked rice with high water contents

This study explored how microwave reheating (to about 73 °C at different power levels) affects the microstructure and digestion characteristics of cooked rice with different water contents (1.1 and 1.5 times that of rice in weight). Irrespective of water content, mainly the V-type crystallites remained after microwaving reheating, with slight changes in other multi-scale structural features. Only at a relatively high water content (1.5) and with a power level high enough could short-range order be reduced. Such microwave reheating increased the digestion resistance of cooked rice. At a water content of 1.1 times, increasing the microwave power led to a decreased rapid digestible starch (RDS) content and an increased resistant starch (RS) content. With a higher water content (1.5), the enhancement of digestion resistance with higher microwave power was less significant but still, a reduced slowly digestible starch (SDS) content and a higher RS content were observed.

Comparing nutritional levels in a commercially-available single-serve meal using microwave versus conventional oven heating

Little published information exists examining the effect of heating methods on nutrient retention in single-serve frozen meals, which can have instructions for conventional and microwave ovens. In peer-reviewed references, there are no recent studies that address this question. This work compares the effect of two reheating methods on nutrient retention of a frozen single-serve meal that has dual conventional and microwave reheating instructions. The meal has a full range of nutrients that are either thermally labile or thermally stable. We hypothesized there would be parity in nutrient amounts in the meal between the two reheating methods.Nutrient composition was assessed by averaging results from 10 samples after three treatments: conventional oven reheating, microwave oven reheating, and unheated controls. Meals were reheated to a minimum of 74 °C per the package instructions and analysed for carbohydrate, protein, fat, sodium, potassium, vitamin A, vitamin C, vitamin E, thiamine, riboflavin, and folic acid.Nutrient changes were uniformly affected by reheating conditions, except for vitamin C. On average, microwaved meals retained 3.8 more milligrams vitamin C than meals heated in a conventional oven. Consumers can be confident that microwave reheating of frozen meals can result in equal nutrient retention as conventional oven reheating, and microwave reheating may result in slightly greater retention of vitamin C.

Formation and migration of α-dicarbonyl compounds during storage and reheating of a sugary food simulation system.

The thermal processing of food results in the formation of α-dicarbonyl compounds (α-DCs) such as glyoxal (GO), methylglyoxal (MGO), 2,3-butanedione (2,3-BD), and 3-deoxyglucosone (3-DG), which are precursors of potentially harmful advanced glycation end products. Some of the α-DCs found in food products might result from chemical deterioration reactions during storage and reheating. A range of sugary food simulation systems were stored at three different temperatures (4, 25, and 37 °C) and reheated using three different processing methods to investigate the formation and migration of α-DCs. During 20 days of storage, the concentration of α-DCs declined, following which the concentration remained approximately constant. Methylglyoxal was the major α-DC affected during storage, its relative content decreasing from 233.71 to 44.12 μg mL-1 in the glucose-lysine system. The concentration of α-DCs decreased with increasing temperature. Microwave reheating increased the formation of α-DC compounds. The largest increases in 3-DG concentrations were observed in the maltose-lysine systems (24.94 to 35.74 μg mL-1 ). The concentration of α-DCs only changed a little in response to reheating at 100 °C, but declined when reheated at 150 °C. The concentration of α-DCs following storage and reheating depends on the type of sugar, lysine content, temperature, and method of reheating. © 2020 Society of Chemical Industry.

Effect of cooking, 24 h cold storage, microwave reheating, and particle size on in vitro starch digestibility of dry and fresh pasta.

The aim of this study was to investigate the effect of preparation method and particle size on digestion of starch in fresh and dry pasta types. Pasta samples were boiled, refrigerated and re-heated, with samples collected after each stage, and then prepared as small (2 mm) and large (5 mm) particles for subsequent starch digestibility testing and logarithm of slope analyses. There were significant main effects of particle size (F1,24 = 568.895, p < 0.001, ηp2 = 0.960) and processing treatment (F2,24 = 19.897, p < 0.001 ηp2 = 0.624) on starch digestibility overall, however the main effect of pasta type was not significant at the p < 0.05 level (F1,24 = 2.978, p = 0.097, ηp2 = 0.110). Particle size had the largest effect on digestibility, and the extent of starch digestion was at least 50% lower in samples prepared as large particles compared with small particles. The most digestible sample was the boiled fresh-type pasta prepared as small particles (C∞ = 57.9%) and cold storage alone and/or with subsequent re-heating significantly reduced the extent of digestion in this sample to ∼40.6%. In the dry pasta type, processing treatment had no significant effects on starch digestibility. The rate constant, k, was not significantly altered by processing treatment or pasta type (k = 0.0275 min-1, mean of all samples). These findings suggest that cold-storage and re-heating treatments have limited potential to impact on glycaemic responses and highlight the importance of masticated particle size as a potential rate-limiting factor in digestibility studies.

Role of protein oxidation in the nutritional loss and texture changes in ready‐to‐eat chicken patties

SummaryThe oxidative damage to proteins during processing of ready‐to‐eat chicken patties and its impact on the nutritional value and texture properties of the product were assessed. The effect of cooking techniques (boiling, roasting and grilling) and microwave reheating (600 mW per 1 min) after 7 and 14 days of cold storage (+4 °C) were studied. Cooking methods had an effect (P &lt; 0.05) on most protein oxidation parameters, including protein depletion of tryptophan, thiols, protein carbonylation and formation of cross‐links. Overall, the succeeding chilled storage led to an aggravation of the oxidative damage which occurred along with hardness increase and impaired digestibility. Microwave reheating had contradictory effects on protein oxidation parameters. An increase in protein carbonylation occurred along with a cleavage of disulphide bonds that would explain a decrease in hardness. This study illustrates to which extent the oxidative damage to meat proteins affects protein quality and nutritional value of a severely processed meat product. The impact of assorted cooked procedures on these events is also discussed.

Microstructural characteristics and gastro-small intestinal digestion in vitro of potato starch: Effects of refrigerated storage and reheating in microwave.

The objective of our study was to evaluate paste clarity, retrogradation (syneresis %), thermal characteristics and kinetics of glucose release during in vitro gastro-small intestinal digestion of freshly cooked and refrigerated potato starch. Freshly cooked starch pastes had a paste clarity of 71%, which decreased to 35.4% whereas syneresis (%) increased after 7days of refrigerated storage. The X-ray and thermal characteristics of native, retrograded and microwave reheated starch samples differed significantly from each other. For the freshly cooked starch pastes, ∼88% starch hydrolysis was observed at the end (150min) of digestion under simulated gastro-small intestinal conditions that decreased to ∼70% for the 7day stored pastes. The hydrolysis (%) of refrigerated pastes increased to 86% and 92% after one and two cycles of microwave reheating, respectively. These results contribute to the understanding of starch retrogradation in relation to starch digestion.

Effect of pre-cooking methods on the chemical and sensory deterioration of ready-to-eat chicken patties during chilled storage and microwave reheating.

The effects of pre-cooking methods, namely, boiling (BL), roasting (RT) and grilling (GR), refrigerated storage (14days/+4°C) and microwave reheating on chicken patties were studied. Physical, chemical and sensory parameters were evaluated in order to correlate the chemical deterioration of ready-to-eat chicken patties with the acceptance of the odor. Chemical deterioration was evaluated through the chemical composition, Maillard compounds, Thiobarbituric acid-reactive substances (TBARS) and volatiles. Sensory deterioration (odor liking) was performed by an acceptance test with hedonic scale. According to the TBARS values and volatile compounds generated in the head space during the examined stages, the pre-cooking method and the storage time had a significant effect on lipid oxidation, whereas reheating in a microwave had a negligible impact. At each succeeding processing stage, panelists gave lower odor scores to all samples and no significant differences were found between treatments at any stage. RT and GR patties showed less intense chemical changes and presented higher acceptation scores by the sensory panel than BL patties. Thus, the choice of pre-cooking method and control of storage conditions plays a key role in the inhibition of oxidative changes in ready-to-eat chicken patties.

Effects of preparation methods on potato microstructure and digestibility: An in vitro study

The effects of preparation method (boiled, cooled, and reheated) on the structural characteristics and in vitro digestibility of potato were investigated. The highest equilibrium starch hydrolysis was obtained after boiling (82.21%), followed by reheating (66.98%) and cooling (54.31%). The potato after cooling treatment exhibited a narrower melting temperature range and a higher melting enthalpy compared with the potatoes after boiling and the microwave reheating. X-ray diffraction patterns revealed that the relative crystallinity of the cooled potato was higher than those of the reheated and boiled potatoes, consistent with the results in melting enthalpy. Moreover, compared with the boiled potato, a large number of wrinkles were observed on the surface of the cooled potato, whereas after microwave reheating, larger clearances of starch granule regions were found. This study demonstrated that cooling and microwave reheating significantly affected the potato’s microstructure and digestibility and were suggested as alternative choices to prepare potato dishes.

Enhanced coercivity of La–Co substituted Sr–Ca hexaferrite fabricated by improved ceramics process

In order to fabricate high magnetic performance La–Co substituted Sr–Ca hexagonal ferrites, an improved ceramics process was proposed in this study. The particle size of the ferrite powder was measured by laser diffraction particle size analyzer. The microstructures of the sintered ferrites were observed by scanning electron microscopy. The magnetic properties of the sintered ferrite magnets were measured by B–H loop recorder. The results show that, sub-micrometer ferrite powder with narrow particle size distribution could be produced by microwave reheating–remilling treatment. Ferrite magnet with fine grained microstructure and enhanced coercivity could be synthesized by using the powder as precursor.

Control of Listeria monocytogenes by applying ethanol-based antimicrobial edible films on ham slices and microwave-reheated frankfurters

The objectives of the study were to evaluate: (i) the efficacy of Na-alginate films immersed in 40% v/v distillery ethanol and traditional Greek alcoholic beverages (ABs), namely “tsipouro”, “raki”, and “ouzo” to inhibit Listeria monocytogenes growth on frankfurters and ham slices and (ii) the impact of edible films-ABs on heat sensitivity of the pathogen after microwave (MW) reheating of frankfurters in water during storage at 4 and 10 °C. Samples without edible films (controls) or with edible films but free of distillery ethanol or ABs were also tested. Two pieces of frankfurters were placed between two edible films, while every ham slice was covered by a film from both sides. On ham, all edible films immersed in ABs maintained levels of L. monocytogenes at least 5.0 log CFU/cm2 lower (p < 0.05) than controls after 81 and 52 days at 4 and 10 °C, respectively. Moreover, on frankfurters packaged with edible films–ethanol or -ABs, L. monocytogenes population remained 4.0–4.5 log CFU/cm2 lower (p < 0.05) than controls after 40 days of storage at 4 °C, while growth suppression of 0.8–2.0 log CFU/cm2 was observed after 15 days at 10 °C, at all assays except for samples packaged with raki. However, the results from reheating experiments revealed that subsequent inadequate MW reheating of frankfurters packaged with edible films-ABs would not lead to remarkable inactivation of the pathogen, since cross-protection against thermal stress is been acquired as a result of products' exposure to sub-lethal stresses such as low pH formed by metabolic products, indigenous microbiota, and ethanol exposure (ca. 2.4–2.7% w/w) during cold storage. Our findings suggest that food safety regulations enabling antimicrobial interventions for the safety of ready to eat meat products should ensure both low levels of L. monocytogenes during storage and minimum tolerance to recommended consumer practices, such as MW reheating.

Improving the crispness of microwave-reheated fish nuggets by adding chitosan-silica hybrid microcapsules to the batter

In this study, thermostable microcapsules prepared with a hybrid of chitosan and silica (MCCS) as the shell material and soybean oil with a low specific heat capacity as the nucleus were applied to maintaining the crispness of microwave-reheated battered and breaded fish nuggets. The MCCS added batter at 1 g/100 mL concentration exhibited desirable rheological properties and a 40% pick-up rate, as well as a high cutting force, low deformation and desirable number of peaks in the cutting force profile of the fish nuggets reheated in a microwave oven. After microwave reheating, the moisture content of the crust was lower, and moisture content of the fish meat was higher, in the MCCS-containing nuggets than in the MCCS-free nuggets. This phenomenon suggested that the MCCS facilitated the formation of a water barrier by high temperature and high pressure during microwave reheating, which prevented moisture molecules from migrating from the fish meat to the crust.

Experimental Analysis and Numerical Modeling of Microwave Reheating of Cylindrically Shaped Instant Rice

Abstract The purpose of this study was to develop a numerical model to predict the temperature distribution in cylindrically shaped cooked rice samples during microwave reheating and to give impetus to a uniform microwave-heating design. Cylindrically shaped instant rice was reheated by continuous microwave application, and the sample temperature was measured to determine the internal temperature profile which resulted in a desired uniformity of temperature observed using the non-uniformity temperature evaluation. A finite difference method was used to predict the temperature distribution of the cylindrical rice during microwave reheating and applying Lambert’s law to calculate the microwave power absorption in a large sample of cylindrically shaped instant rice. In order to solve the numerical model of heat transfer, the thermal and dielectric properties of instant rice were measured with respect to temperature. Our results showed that the temperature had a significant effect on the specific heat and dielectric loss of rice, while the thermal conductivity and dielectric constant were unaffected. The numerically predicted temperature distributions were in good agreement with the measured ones of instant rice.

Effect of microwave thawing on microstructure and physicochemical stability of low fat white sauces made with soy protein

The microstructural and physicochemical stability of white sauces made with soy protein and modified waxy maize starch was evaluated after subjecting them to a freezing-thawing process in a conventional or microwave oven. The microstructure of sauces revealed a structured matrix of soy protein and starch polymers where fat globules and swollen starch granules remain dispersed. Both thawing methods affected the fat globule size and morphology but they did not affect the starch granules. The SDS-PAGE analysis did not show any apparent changes between sauces thawed by both methods. Moreover, a similar pattern was found in the soy protein isolate used as a raw material indicating that this type of protein was also stable to the cooking process. There were no significant differences (P &amp;gt; 0.05) in the reducing power of the sauces regardless of the thawing method used. However, the acidity index and the k&lt;sub&gt;232&lt;/sub&gt; parameter were significantly higher (P &amp;lt; 0.05) in conventionally thawed sauces. Finally, syneresis was negligible and no significant differences (P &amp;gt; 0.05) were found among different frozen storage periods. In conclusion, the formulation of the sauce is appropriate to develop low fat, vegetarian meals which can be subjected to frozen storage and microwave reheating.

The influence of compositional changes in reconstituted potato flakes on thermal and dielectric properties and temperatures following microwave heating

The objective of this study was to evaluate the effect of formulation changes on the temperatures of the reconstituted potato flakes following a standardised microwave reheating using a response surface method. Reconstituted potato flakes with elevated levels of fat (0.6–11.1%), water (69–78.7%) and salt (0–5.7%) were prepared. Thermal properties (thermal conductivity, thermal diffusivity, specific heat capacity) and dielectric properties (dielectric constant, loss factor and penetration depth) were evaluated. The reconstituted potato flakes were reheated in a microwave oven and the temperatures were recorded during and after microwave exposure. Temperatures during reheating decreased with increasing water, butter and salt content. Results show that temperatures during microwave reheating require empirical measurement as opposed to prediction due to changes in composition, thermal and dielectric properties. Reformulation offers the manufacturers an option for altering the temperatures following microwave reheating, however, the magnitude of the formulation changes required may be quite large. The data presented is useful in the preparation of computer models and simulations of microwave heating.

Physico-chemical Properties of Pancakes Made from an Instant Mix Containing Different Levels of Peanut (Arachis hypogaea) Flour

ABSTRACT Partially defatted peanut flour has proven to exhibit excellent functional and nutritional properties in many foods. In this study, a peanut pancake instant mix was developed by replacing a portion of wheat flour with peanut flour; either 12% fat medium roast (12M) or 28% fat light roast (28L) peanut flours at various levels (20, 30, 40 and 50%) in a starting pancake formulation. Effect of peanut flour initial fat content and degree of roasting on physico-chemical properties such as viscosity of the batter, surface color, texture and composition of the pancakes were determined. In addition, the effect of microwave reheating time on moisture loss, bulk density and texture of the frozen peanut pancakes was investigated at 30% replacement level and compared with a commercial frozen pancake. Viscosity of the instant mix batter made with 12M peanut flour was higher than the control whereas 28L was lower than the control. All peanut pancakes were darker (lower L values) and browner (lower hue angle) than the control. Texture profile analysis properties such as hardness, cohesiveness and chewiness values decreased with increasing peanut flour replacement level; whereas, springiness values increased. Moisture, fat, ash and protein content were increased with increasing peanut flour replacement level. Pancakes made from 12M have higher protein content than the 28L samples at same level of replacement. Frozen peanut pancakes after microwave reheating showed significant moisture loss, increased bulk density and increased hardness and chewiness. At tested conditions a microwave reheating time of 20 or 30 s was sufficient to achieve desirable eating characteristics (moisture loss, density and texture) of peanut pancakes when compared to the commercial frozen pancakes or freshly prepared peanut pancakes. This study demonstrated that the instant peanut pancake mix has potential as a functional breakfast food item to replace regular wheat pancakes.