Chemical and biochemical changes during microwave treatment of wheat

Abstract

kernels. The container was taken from the oven, the sample was hand-mixed with lab spoon and returned to the MW oven. The whole process was repeated five times so that a homogeneous temperature of the sample was reached. Temperature measurement: An instrument NoEMI of the fiber-optic temperature system with a table unit ReFlex and two optic fibers (Nortech Fibronic Inc., Canada) allows to read out temperature at any point of the sample in the range of temperatures from-40°C to +250°C. Reaction time for temperature reading is 0.25 s. The fibers are made suitable for the conditions inside the MW oven. The instrument can be connected to a computer and data can be registered in table-processor Reflection 4 for Windows , Version 4.21 (1990-1994) compatible with MS Excel (Walker Richter & Quinn, Inc.). The surface temperature of sample was measured with one probe placed 50 mm to the right from the middle, just in the position with the highest temperature. Location of a temperature sensor was 2 mm below the sample surface. A detailed scheme of the temperature sensor position was demonstrated in our previous paper (SKULINOVÁ et al. 2002). Preparation of wheat samples: In order to standardise the sprouting of wheat samples, wheat kernels were let soak in water of temperature 16°C for 2 hrs and then sprout in a laboratory. Wheat was placed to a thermostatic box for 16 h at 20°C. Sprouted samples were carefully pre-dried in a convection air-drier at 40°C for different time until their moisture was either 10-11% or 15-17%. All samples were milled at the automatic laboratory flourmill QC-109 (Labor MIM, Hungary) simulating flour-milling process. Resulting white flour is similar to plant-processed wheat flour with ash content close to 0.5% and yield between 40-60% depending on wheat milling quality. Bran were subsequently milled at laboratory mill Retsch ZM 1000 (with 1500 r.p.m. and the sieve with mesh 0.5 mm). Determination of dry matter: Automatic drying electronic weights Precisa HA60 (Switzerland) were used for determination of dry matter content. The end temperature of drying can be set in the range of 40-250°C and the percentage of dry matter content is automatically shown at a display. Gluten content and Gluten Index determination: Wet gluten quantity as well as quality was determined by washing gluten from wheat meal at automatic washer Glutomatic 2200 and by centrifugation of washed gluten at Centrifuge 2015 (two instruments supplied by Perten Instruments , Sweden). Gluten washing is standardised in ICC Standard No. 155 (1994), AACC Standard No. 38-12-Wet gluten and Gluten Index, Perten Instruments (1993). After washing off from meal or flour gluten is put on a special sieve and centrifuged under standard conditions. Depending on the rheological properties, a part of gluten passes through the meshes of the sieve. The part of glu-ten that did not pass through a sieve is expressed as the percentage of total washed gluten and is defined as Glu-ten Index (GI). Optimum values of GI for good-quality wheat flour are approximately between 85 to 92%. The lower the value of GI, the worse the baking quality of wheat or flour. The values such as 60 or 40% point to very low quality of flour that should not be used for baking. Standard deviation of repeatability s(r) for gluten content determination was lower than 0.56% and for GI determination it was lower than 5.24%. Standard deviations of reproducibility s(R) were lower than 1.0% and 8.3%, respectively. Falling Number (FN) determination: A special standard falling-body viscometer Falling Number 1400 was used for the measurement (Perten Instruments 1993). The method of measurement is described in Standard ČSN ISO 3093-Determination of Falling Number. In principle, flour suspension in a measuring cell is heated in boiling water. Fig. 1. Relation between relative change in Falling Number and absorbed energy Fig. 2. Relation between relative change in wet gluten content and absorbed energy ! 60°C, moisture content 10%; " 60°C, moisture content 17%; ! 80°C, moisture content 15% 130 120 110 100 9 0 1 0 1 5 2 0 2 5 3 0 3 5 Absorbed energy (kJ) 1 0 1 5 2 0 2 5 3 0 3 5 Absorbed energy (kJ) 110 100 9 0 8 0 7 0