Biological Properties of VHF‐ and Microwave‐Heated Soybeans

Abstract

Whole soybeans at ratural moisture levels were heated by varied exposures to dielect:ic heating at frequencies ,of 42 and 2450 MHz. The minimum energy absorbed (MEA) was calculated from temperature-elevation and moisture-loss data. Because the heating rates were different at the two frequencies, plots of various biochemical properties against temperature or exposure time revealed an apparent frequency dependence. This dependence on frequency disappeared, however, when the MEA value was substituted as the independent variable. Chemical and biochemical analyses revealed that dielectric heatin:; at natural moisture levels should be as efficient as commonly practiced moist "toasting" in improving the nutritional value of soy beans. Trypsin inhibitor activity was reduced to a low level, indicating that maximum nutritional quality had been reached. Proteir solubility and dispersibility, but not urease activity, can be used as indicators of trypsin inhibitor inactivation by dielectric heating. Indications were obtained that MEA values, or perhaps even moirture loss, might be used as an index of trypsin inhibitor inactivation. Lipoxygenase-inactivated samples of improved flavor were produced by the dielectric heating treatments. Some treated samples with low trypsin inhibitor and lipoxygenase activities still retained relatively high peroxidase activities that may be advantagr:ous for bleaching effects.

Factors Influencing the Quality of Dietary Proteins: Implications for Pulses

“…Interestingly, after preparation the lentils were lower in isoleucine, leucine, and valine but higher in lysine, phenylalanine, and tyrosine. There have been a wide variety of studies investigating the impact of microwave preparation on the quality and taste of food products (Pour-El et al 1981;Cross and Fung 1982;Alajaji and El-Adawy 2006;Yuan et al 2009). Similarly, after cooking, trypsin inhibitor activity and tannin concentration were reduced in beans, chickpeas, peas, and lentils (Wang et al 2008(Wang et al , 2010 Microwaving as a method of food preparation has been commonly used since its introduction in the 1970s.…”

Spectrophotometric analysis of the lipid fraction of microwave heated and soaked soybeans.

“…Pour-El et al, (13) used the energy absorbed (EA) concept as an indicator of the microwave heating effect, considering the energy absorption by the grain during electric heating due to two main processes: grain temperature increase and water evaporation (moisture loss). Present experimental data did not reach 150 MEA, but surpassed 100.…”

“…Pour-El etal., (13) determined the MEA (minimum energy absorbed per gram) values for grains during a dielectric heating, due to temperature raise and water vaporisation. Calculations involved Cp = 0.39123+ + 0.0046057 M, for specific heat and hv = 539 (1 + + 0.21624 e"° °^^^^^) for grain water vaporisation heat, where M is grain moisture in dry basis: MEA = Cp AT + + hv AM, where AT is the temperature rise and AM is the porcentual moisture loss (dry basis).…”

Product Conditioning Applications

“…Inactivation of this antitryptic factor is currently accomplished industrially by steam "toasting" treatments after the soybeans have been dehulled, split, flaked, and defatted. Various heating methods have been studied for inactivation of the trypsin inhibitor, but moist heating, or the presence of excessive moisture during heating, has generally been deemed necessary for effective trypsin-inhibitor inactivation except in RF dielectric heating (Borchers et al, 1972;Pour-El et al, 1981).…”