Evaluation of protein quality of wheat-rye flour blends by use of two small-scale analytical methods

The technological quality of the protein fraction of wheat white flour (WW) was determined by the lactic acid Solvent Retention Capacity and by the Gluten Performance Index (LA-SRC and GPI; AACC method 56-11.01). Parallelly, Perten's standard Gluten Index test (GI; AACC method 38-12.02) was performed with that wheat control. Consequently, the same methods were applied to 9 bi-composite blends mixed at ratios of 90 : 10, 80 : 20, 70 : 30, 60 : 40, 50 : 50, …, and 10 : 90 as WW replacement with one of the rye bread flours (RB), and the RB control itself. Unlike successful measurements in the case of the LA-SRC and GPI, washing gluten from the first 90 WW : 10 RB blend led to the clogging of the Glutomatic sieve, likely due to the interaction of wheat gluten with rye arabinoxylans. The discrepancy induced the development of a modification of the standard GI procedure; clogging was avoided by precipitating flour and centrifuging the swollen solid from suspension, similarly like in the SRC method. The settled residue transported to a standard sieve cassette was secondly centrifuged in the original apparatus Perten CF2015. The weighing of the overflow and underflow of the sieve and the calculation of the results were performed according to the original GI method. For the WW control, the standard GI value was 90% and the modified Gluten Index value (GImodif) was 82%. As expected for WW-RB counterparts, the higher the portion of rye in the bi-composite blend, the lower the value of the GImodif . For the 50 WW : 50 RB blend and the RB itself, the GImodif values were 47% and 18% (the GPI values 0.66, 0.45, and 0.39, respectively). On the contrary, the LA-SRC demonstrated a convex course (118, 104, and 123%, respectively). In the plane of the principal components (PC), namely PC1 and PC2, the variables related to gluten quality formed 4 groups as a function of the stepwise change in the mixing ratio of WW and RB: i) flour protein content, GPI, and GImodif ; ii) LA-SRC; iii) dietary fibre content and ash content; iv) water, sucrose, and sodium carbohydrate SRCs. However, the modified test procedure should be revised in wheat varieties characterised by a wider spectrum of protein quality, mixed with different types of rye flour (especially wholegrain one).