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

Abstract

‘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 produced 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 ‘Caaveiro’ 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