Comparison of stone-milled and recombined whole-grain flours for their rheological properties and bioactive compound content

Whole-grain food market is growing in the last years, and the selection of the milling process is a key point in the production of whole-grain flours. The aim of the present study was to study the effects of milling process on the content of antioxidant compounds (AC) and rheological properties and bread-making performance of two different whole-grain flours. Starting from red-, yellow- and blue-grained wheat varieties (cvs. Aubusson, Bona Vita and Skorpion), the following flours were produced: i) a whole-grain flour obtained by means of a single-stream stone milling process and ii) a recombined whole-grain flour obtained by means of a multiple-stream roller milling process. Straight dough method was used for preparing bread loaves. The rheological properties of the flours were evaluated by GlutoPeak, Farinograph, and Rheofermentometer tests and related to bread quality (i.e. specific volume). The content of antioxidant compounds was investigated both in the flours and in the corresponding breads. A key difference between the whole-grain flours was the particle size distribution: the stone-milled flours showed a more uniform particle size distribution than the recombined ones. Recombined flours showed on average a higher concentration of both cell wall-bound phenolic acids (CWBPAs; +11%) and soluble phenolic acids (SPAs; +10%) than stone-milled flours. As far as the lutein content was concerned, the two whole-grain flours obtained from the yellow-grained variety presented no significant difference. As regards the rheological properties, results showed gluten weakening for stone-milled samples. In fact, although the milling system did not affect either dough stability or leavening properties, bread samples from stone milling showed a structural collapse compared to the products from recombined whole-grain flours. Bread samples made from the two types of wholegrain flour did not differ significantly for their antioxidant properties. However, the effect of bread-making on the content of antioxidant compounds was more detectable in the sample from stone-milled flours, compared to the sample from than recombined flour. SPAs on average increased after baking (+3% bread crust; +20% bread crumb), while lutein content decreased (-68% bread crust; -59% bread crumb). The AC increased both in the bread crust (+85%) and in the bread crumb (+13%). In conclusion, the milling system used for obtaining whole flour seems to affect the technological properties of the dough and the physical characteristics of the resulting bread samples. The authors acknowledge the financial support of the Italian Ministry of Education, University and Research (MIUR), program PRIN 2015 (grant number 2015SSEKFL) “Processing for healthy cereal foods”.