María Elena Sosa-Morales

Recent Studies Related to Microwave Processing of Fluid Foods

Microwave heating is a convenient way to heat materials; it is considered to be a fast, clean, and easy to use technology. The use of microwaves for industrial food unit operations is the subject of research since several years ago. However, the application of microwaves depends, among other variables, on the dielectric properties of the material to be heated; otherwise, the efficiency of the process and the quality of the final product cannot be guaranteed. This paper reviews basic concepts related to microwaves and dielectric properties, and then it presents reported dielectric properties data for selected fluid foods and microwave-heating processes that have been recently studied. These processes are focused mainly on microbial inactivation, enzyme inactivation, chemical, physical, or sensory changes evaluation, or for reheating. The temperature uniformity is also discussed as a key issue for successful application of microwave heating, which is now applied by some companies.

Evaluation of Physical Properties of Dough of Donuts During Deep‐Fat Frying at Different Temperatures

Abstract Dough, based in wheat flour and yeast as leavening agent, was deep‐fat fried in sunflower oil at 180, 190, and 200°C to produce donuts. Several physical properties of donut were monitored each 15 s of frying through the frying process, such as moisture and oil contents; bulk density, volume changes, crust color; rheological including instrumental TPA, compression and penetration forces and thermal properties: specific heat, thermal conductivity, and diffusivity. Fundamental models for heat and mass transfer were applied and empirical equations were generated in order to fit the evolution of the properties during deep‐fat frying with very good agreements (r > 0.95); color crust of the donuts was fitted to a kinetics of first order and the energy of activation was calculated. Moisture content and oil temperature affect importantly the values of the donut properties, being the thermal conductivity the most constant property through the whole process. These results are important for equipment design, process control and quality assurance; additionally, it was corroborated that all these measurements represent simple tests, very useful for process evaluation of donuts and similar products exposed to frying.

Microwave-assisted Extraction of Essential Oils from Herbs

Abstract Microwave-assisted extraction (MAE) has been recognized as a technique with several advantages over other extraction methods, such as reduction of costs, extraction time, energy consumption, and CO2 emissions. In this study, MAE was performed to obtain essential oils from two different herbs (basil and epazote). A factorial design was conducted in order to determine the effect of solvent quantity, power, and heating time on essential oil yields. Chemical composition, physical properties and yield percentage of essential oils from MAE were compared with essential oils obtained by steam distillation (SD). Amount of solvent and heating time significantly affected the yields (p<0.05). Chemical composition and physical properties of the essential oils from basil and epazote were not affected by the extraction method (MAE or SD), with similar yielding obtained by both methods (p<0.05).

Laser Temperature Sensor Based on a Fiber Bragg Grating

In this letter, a temperature sensor based on a fiber ring laser is presented. A fiber Bragg grating was used as a sensor head, and a Mach-Zehnder interferometer (MZI) was utilized as a wavelength selective filter to enhance the temperature sensing capability. The MZI was achieved by splicing a piece of photonic crystal fiber between two segments of single-mode fiber. Moreover, it is shown that when the laser emission wavelength is shifted as the temperature is varied, achieving a temperature sensitivity of 18.8 pm/°C at 1550 nm, within the temperature range from 20 °C to 90 °C.

EFFECT OF TEMPERATURE ON THE PHYSICAL PROPERTIES OF CHICKEN STRIPS DURING DEEP-FAT FRYING

Chicken slabs (10.0×1.0×0.5 cm) were deep-fat fried with sunflower oil at several temperatures 130, 140, and 150°C. Moisture content and oil absorption was determined during the whole experiment, and the following properties were evaluated: sample dimensions, temperature of the geometric center of the slab, mass transfer diffusivity, thermal diffusion coefficient and heat transfer coefficient. Fourier, Newton and Fick equations were applied to obtain the correspondent physical properties; whereas, empirical relationships were generated to describe the evolution of central temperature, moisture and oil content, as well as the product dimensions: length, width and thickness, as a function of frying time at three process temperatures.

Hot water bath treatments assisted by microwave energy to delay postharvest ripening and decay in strawberries (Fragaria × ananassa)

BACKGROUND A lab-scale approach using microwave (MW)-assisted hot water treatments was developed and tested to assess the potential of this heating method to delay postharvest ripening and decay in strawberries. Strawberries (Fragaria × ananassa) immersed in water were exposed to microwaves at a frequency of 2450 MHz for 3 min at 514 W or 1 min 50 s at 763 W to reach an average temperature of 43.8 ± 0.6 °C at the fruit centre. Another batch was treated in hot water at 45 °C for 15 min, and a final batch was not treated (control). RESULTS After 9 days of refrigerated storage (3 °C and 90% relative humidity), all heat-treated strawberries showed significant retention of quality parameters such as colour and firmness and significantly lower yeast and mould populations (P < 0.05). CONCLUSION Strawberries subjected to MW-assisted hot water treatments showed significantly better retention of lightness compared with conventionally treated berries. A short (1 min 50 s) treatment at 763 W was the best choice to prevent strawberry decay.

Dielectric Properties of Beans at Different Temperatures and Moisture Content in the Microwave Range

Dielectric properties of common Mexican beans (Phaseolus vulgaris L.) were determined and analyzed at microwave frequencies (800–2500 MHz). The free-space transmission technique was employed for the measurements of three varieties (“Flor de mayo,” “Bayo,” and “Negro”) with different moisture content (8.8–12.3%, w.b.) at 20, 30, 40, 50, and 60°C. The dielectric constant and loss factor of beans decreased with increasing frequency for a fixed temperature, and increased with increasing temperature at a fixed frequency. The dielectric constant increased with increasing moisture content, while the loss factor remained nearly constant. With these results, disinfestation or quality control measurements can be proposed for beans using microwaves.

Protein Content and Antioxidant Activity of Distillers’ Spent Grain Dried at 150°C with Superheated Steam and Hot Air

The quality of dried distillers’ grain (DDG) is of great importance, and due to concerns regarding drying efficiency, various drying methods have been proposed. The objective of this study was to evaluate the protein content and antioxidant activity of DDG using two different drying media: (1) hot air at 150°C and (2) and superheated steam (SS) at 150°C. The results were compared to a reference sample dried at near ambient air temperature of 30°C. Protein content ranged from 15.1 to 16.4% wb for all samples. With respect to phenolic content, ethanolic extraction (3.1–12.9 mg GAE/g) was more effective than aqueous extraction (0.87–2.9 mg GAE/g). The DDG samples dried with SS had the highest phenolic contents among the analyzed samples (p < 0.05). Antioxidant activity ranged between 0.32 and 0.44 mg Trolox/g for aqueous extracts and 0.54–0.57 mg Trolox/g for ethanolic extracts. The results indicate that SS drying can be a suitable method to dry distillers’ spent grain without the adverse effect on protein and phenolic contents.

The effect of food type (fish nuggets or French fries) on oil blend degradation during repeated frying.

UNLABELLED Oil that is reused multiple times for deep frying goes through changes in chemical composition and physical characteristics, affecting the quality of the fried foods. In this study, the effect of the food type (fish nuggets or French fries) on the degradation of an oil blend during the deep-fat frying of each food at 180°C during 12 days was determined, and the characteristics of the fried products were evaluated. The degradation of oil during repeated use was relatively faster when fish nuggets were fried than when French fries were fried, as higher values of total polar compounds were obtained. PRACTICAL APPLICATION The results are useful for producers of French fries and fish nuggets, such as restaurants or fast foods sellers, providing them with practical guidelines within the permitted values established by the regulatory authorities. The studied foods have high economic importance and are different in their composition. Under the studied conditions, the tested oil blend may be used during 4 d (4 h per day) with a daily replenishment, without discarding the oil when frying fish nuggets, and must be discarded after 8 d when French fries are processed. This suggestion allows preparing safe fried foods for consumers.

Colletotrichum gloeosporioides growth-no-growth interface after selected microwave treatments.

To study microwave heating for potential postharvest treatments against anthracnose disease, Colletotrichum gloeosporioides growth-no-growth response after selected microwave treatments (2,450 MHz) was fitted by using a logistic regression model. Evaluated variables were power level, exposure time, presence or absence of water in the medium during treatment, and incubation-observation time. Depending on the setting, the applied power ranged from 77.2 to 435.6 W. For the experiments on dry medium (mold spores over filter paper), exposure times were 1, 2, 3, or 4 min, whereas spores dispersed in potato dextrose agar, a wet medium, had exposure times of 3, 6, or 9 s. Growth (response = 1) or no growth (response = 0) was observed after two different incubation-observation times (4 or 10 days). As expected, high power levels and long exposure times resulted in complete inhibition of C. gloeosporioides spore germination. In a number of cases (such as low power levels and short treatment times), only a delay in mold growth was observed. Scanning electron micrographs showed signs of mycelia dehydration and structural collapse in the spores of the studied mold. Cell damage was attributed to heating during microwave exposure. Reduced logistic models included variables and interactions that significantly (P < 0.05) affected mold growth, and were able to predict the growth-no-growth response in at least 83% of the experimental conditions. Microwave treatments (4 min at any of the studied power levels in dry medium, and 9 s at power levels of 30% or more for wet medium) proved effective in the inhibition of C. gloeosporioides in model systems. These no-growth conditions will be tested further on fresh fruits in order to develop feasible postharvest microwave treatments.