Polarized light microscopy guarantees the use of autochthonous wheat in the production of flour for the Protected Geographical Indication ?Galician Bread?

'Galician Bread' is a traditional baked product and a national benchmark that has recently been granted the European mark of Protected Geographical Indication (PGI), which requires at least 25% of the flour to be pro-duced from autochthonous varieties such as 'Caaveiro'. The objective of this work was to find a method that guarantees the presence and the percentage of 'Caaveiro' wheat in blended flours by using microscopy techniques.Using optical microscopy, including bright-field and polarizing microscopy, autochthonous and foreign flours were analyzed and compared. 'Caaveiro' starch presented a different birefringence pattern (associated with a higher amount of amylose) with respect to other cultivars used to produce flours, a feature used to make a computation of the two starch granule types in the mixtures of 'Caaveiro' with foreign flours. Repetitions with different mixture percentages allowed us to develop a mathematical model to estimate the percentage of 'Caa-veiro' flour present in the mixture. Firstly, the most effective method for preparing samples was determined by ensuring the homogeneity of the samples and, subsequently, a validation was carried out with blind samples.Starch birefringence properties allowed the detection of 'Caaveiro' wheat flour in mixtures with foreign/ Castilian wheat flours and to determine the percentages used in the flour mixtures applying a calibration line (R2 = 0.9577). Deviations were due to the difficulty in obtaining precise mixtures of the blended flours, as happened with other Simple Sequence Repeat (SSR)-based methods used in the same samples. This is a novel method for detecting contraventions/infractions of the percentage of 'Caaveiro' used in wheat flours, which is simple, effective and inexpensive.