Search

The possible use of specific biochemical compounds identified in wheat grains was evaluated for monitoring barley grain tissues during fractionation. First barley grain anatomy was studied through microscopic observation and quantification of the relative proportion of each anatomical part in four distinct barley samples from both hulled and hulless genotypes. As expected from cereal phylogeny and irrespective of the possible presence of hull, common features were observed between barley and wheat grains, but the aleurone layer predominated in the outer layers. The specific location of the coumpounds identified in wheat was established. Phytic acid was specifically localized in the aleurone layer and alkylresorcinols in the composite layer containing the testa, even if their concentration differed from that observed in wheat grain tissues. Thus, these two markers identified in wheat can be used to monitor the corresponding barley tissues, independent of the presence of hulls. Conversely, phenolic compounds, either ferulic acid trimer or p-coumaric acid, cannot be used to monitor respectively the outer pericarp or the aleurone cell walls in barley grains. p-coumaric acid was identified as an efficient marker of the hull and could be used to distinguish hulled or hulless barley grains and to help monitor the dehulling process.

Lors des procédés de mouture, le grain de blé est fractionné à l’échelle histologique avec séparation de l’albumen amylacé, principalement récupéré dans les farines ou semoule, des tissus les plus périphériques, récupérés dans les sons. Les proportions de chaque tissu dans ces ingrédients impacte leur qualité nutritionnelle ainsi que leurs propriétés d’usages. L’évaluation de leur dissociation à l’échelle de la particule est par ailleurs nécessaire à la compréhension, au contrôle et à l’optimisation de ces procédés. Si l’identification d’un tissu végétal est assez aisée dans une coupe d’organe, elle est beaucoup plus difficile dans une particule où seules les propriétés liées aux différences de composition peuvent encore être utilisées comme critère d’identification. De nombreux tissus végétaux peuvent être observés grâce à l’autofluorescence de leurs composés pariétaux. L’imagerie multispectrale à l’échelle macroscopique permet d’observer rapidement des objets avec un large champ de vue (>1cm2) et une bonne résolution spatiale (

In the cereal milling industry, wheat grains are fractionated at a histological scale for recovering the starchy endosperm into flour or semolina and collecting the peripheral tissues in bran fractions. The proportions of each tissue in the resulting fractions impact their nutritional quality or their end-use properties. The evaluation of tissue dissociation at the particle scale is required in order to understand, control and optimize the fractionation processes. If the identification of tissues in wheat grain is commonly performed, it remains challenging for powders. In powders, methods are mainly based on their specific biochemical composition or their specific spectral properties, in particular their autofluorescence. These methods were developed to give quantitative assessments of a bulk tissue composition in flour or bran fractions. None of these methods allowed the estimation of tissue dissociation at the particle scale even when an imaging system has been used. Recent equipments are available to acquire multispectral fluorescence images at the macroscopic scale using filters with specific excitation/emission wavelengths. These fluorescence macroscopes allow obtaining images of a representative number of particles together with a spatial resolution of less than 3 μm. In such images, the intensities measured for each pixel are not spectra, but are spectral profiles relevant to identify tissues (Baldwin et al., 1997). To identify the tissular origin from this information, we propose to develop a prediction model on particles using calibration data coming from the observation of tissue sections. This approach is based on several assumptions. The first one is that the multispectral autofluorescence of plant tissues is specific and the second is that it is possible to measure fluorescence intensities in a reproducible way. The objective of the present work was to check the fluorescence macroscope as an efficient device for measuring and comparing fluorescence intensities. Wheat was retained as a model plant for which two major tissues of the grain had specific autofluorescence properties: the aleurone layer with mainly a UV fluorescence response and pericarp that fluoresce using both UV and visible excitation wavelengths (Jensen et al., 1982; Symons et al., 1993). Moreover particles of pure tissue can be obtained after hand isolation or fractionation process (Hemery et al., 2007). The autofluorescence properties of tissues in sections and particles were compared in two mounting media (air and water) using a multispectral fluorescence macroscope. The variability of fluorescence profiles was studied by selecting pixels in cross-section or in particles mounted in air or in water. The statistical variations were studied by principal component analysis and variance analysis. The first effect, mainly described by principal component 1, was to differentiate the two tissues, aleurone layer and pericarp. The differences between each tissue came from UV and visible filters as expected. The second effect, mainly described by component 2, was a difference between the two mounting media. The differences between sections or powders were not correlated to the other factors and were considered as not significant. Our results show that profiles extracted from multispectral images of cross-sections or particles are similar and allow the identification of plant tissues. Hence tracking the tissues by predicting them on images of particles from profiles found in images of cross-section should be possible. The choice of the mounting media is flexible, multiples options are viable, but the adopted solution must be strictly applied to all the samples analyzed. If implemented, the prediction from cross-section could be less tedious than other methods requiring dissection and lead to the identification of more tissues.

Wheat tissues had specific reproducible autofluorescence profiles using macro imaging.Wheat pericarp and aleurone layer fluoresce differently in air or water.Autofluorescence of wheat tissues is similar before and after grinding.Multispectral fluorescence imaging can help identifying the histological origins. Many plant tissues can be observed thanks to the autofluorescence of their cell wall components. Multispectral autofluorescence imaging at the macroscopic scale is a rapid efficient way of observing samples with a large field of view (>1cm2) and a nice resolution (

Epidemiological studies have shown that high dietary fibre (DF) diets and whole grain consumption are associated with diminished risk of coronary heart disease, colon cancers, inflammatory bowel disease, and metabolic syndrome (Anderson et al., 2009). Cereals foods, with wheat as the main source in Europe, could be one of the pillars of a healthy diet and might help to increase the DF intake. In that context having reliable method to quantify the DF amount is crucial to characterize food product but also monitor the development of enriched cereal based products. DF measurements are based on sequential and gravimetric analyses mimicking human digestion. Such methods made of several steps are time-consuming and results obtained are still under debate (Hell et al., 2014). Wheat grain contains about 12-14% of fibres mainly located in the outer layers (from the aleurone layer to the pericarp). Considering this specific location, the tissue composition of different mill streams has been measured (as done by Hemery et al., 2009) and compared to their total DF amount measured with the AOAC 2009-01 method. Assuming that the most peripheral tissues are mainly constituted of DF, the tissue composition could be used to estimate the total DF content in mill streams. However such approach could not be extended to food products. Therefore, knowing that arabinoxylans (AX) are the main components of cell walls in the different tissue of the wheat grain, biochemical determination of AX content has been investigated as a possible predictor of DF content. A set of different wheat products, either mill streams and food products (bread, biscuits, pasta) more or less enriched in DF fractions extracted from wheat grains, have been characterized either for DF amount (by the AOAC 2009-01 method) and the AX content by gas chromatography of alditols acetates after sulfuric acid hydrolysis. A strong correlation (r2=0.99) has been observed between the AX and TDF contents, indicating that AX could be used as a good predictor of TDF as soon as wheat is the main source of cereal in the food product. Moreover, by adding a previous step including enzymatic hydrolysis with a xylanase (Saulnier & Quemener, 2009), a functional evaluation of DF is proposed based on the amount of AX released by the enzyme. Xylanase hydrolysable AX are supposed to be also released by microbiota’s enzymes in the gut and therefore an indicator for the proportion of fermentable DF in a tissue or a wheat based food. This assay opens the door for simple characterization of qualitative attribute of cereal DF.

Wheat grain contains about 12-14% of fibres mainly located in the outer layers. Moreover, the composition and the structure of fibres, as well as the nature and amount of co-passengers, varie according to the tissue where they are originated from. The aleurone layer is rich in low substituted arabinoxylans esterified to ferulic acid whereas outer pericarp contains highly substituted arabinoxylans but also cellulose and lignin. Therefore wheat fibres properties (such as enzymatic hydrolysis susceptibility, fermentescibility) showed a high variability according to their origin within the grain. Conventional milling technology allows fibres separation from the starchy endosperm (recovered in flour/semolina) to the outer layers (recovered in the bran fraction). However, this process is unable to sort out the various fibres encountered in the whole grain and its peripheral layers with a view to enhance positive nutritional effect. Accordingly, other processes have to be developed with the aim to produce fractions enriched with selected fibres within the outer layers. Debranning technology was studied in order to recover enriched dietary fibre fractions with contrasted composition and properties. At first, common and durum wheat were both processed at the pilot scale (300 kg/h) in order to identify any differences in tissue composition for similar debranning yield and to determine how tissues segregate during debranning. The influence of tempering and dehulling process on the debranning kinetics were analysed through a parametric study. Wheat grain tissue proportions were determined in each dehulled fraction by the marker methodology (Hemery et al., 2009) in order to assess the histological origin of fibres and their co-passengers. This quantitative analysis was related to a qualitative microscopic observation of debranned grains. It was then possible to follow the separation of each outer layer according to the debranning rate. Considering the wheat grain geometry no pure fraction of each tissue was obtained. However combination between markers methodology and debranning unit operations (friction and/or abrasion) allows obtaining enriched fractions either from pericarp or from aleurone layer. An optimised debranning diagram is proposed to produce contrasted fractions, in term of composition (pericarp amount vs aleurone layer amount), that could not be obtained in conventional mill streams. The potential interest of such fractions will be further evaluated on a nutritional point of view.

La sélection s’est intéressée à l’amélioration de l’aptitude technologique des blés pour les rendre aptes à répondre à un usage donné (panification, biscuiterie, etc.). Ce progrès fut accompli au cours des trois dernières décennies. L’augmentation de la productivité des blés doit s’accompagner d’une amélioration de la composition nutritionnelle, notamment pour satisfaire la demande des consommateurs. L’analysede la composition des blés en micronutriments et vitamines était jusqu’ici réalisée avec des méthodes biochimiques longues et ciblées sur un petit groupe de molécules. Le développement des outils de spectrométrie de masse ouvre la voie à des études pour préciser l’influence agronomique et génétique sur la composition en métabolites du grain et améliorer sa valeur nutritionnelle. L’analyse protéomique de la couche à aleurone, principal réservoir en vitamines et minéraux du grain devrait aider à ce progrès., Breeding has focused at improving technological quality to make wheats suitable for a given end use (baking, biscuit, etc.). This progress was achieved during the last three decades. The increase of wheat productivity must be achieved together with improvement of the nutritional composition, particularly to meet consumers demand. Analysis of the composition of wheats in micro-nutrients and vitamins were achieved, up till now, in using lengthy and targeted biochemical methods for a small group of molecules. Development of new mass spectrometry tools paves the way to study on the agronomic and genetic influences of grain composition in metabolites and to improve its nutritional value. Proteomic analysis of the aleurone layer, major reservoir of grain in vitamins and minerals, would help to achieve this progress.

Hand dissection of mature grains from two common wheats (Triticum aestivum L., cv. Caphorn and cv. Crousty) were performed to quantitatively assess their tissue composition and to obtain homogeneous samples of embryonic axis, scutellum, starchy endosperm, aleurone layer, hyaline layer, outer pericarp and a composite layer made up of testa+hyaline layer+inner pericarp. Polymeric neutral sugar and phenolic acid contents of the samples were determined and used to identify specific composition patterns in each tissue irrespective of the cultivar. The scutellum and embryonic axis showed the lowest amount of carbohydrates with similar relative amounts of arabinose and xylose (Ara+Xyl), but the scutellum differed from the embryonic axis in its high phenolic acid, in particular ferulate dehydrodimer, content. The peripheral layers of the grains were mainly composed of cell wall polysaccharides, chiefly arabinoxylans but with differing structures. The hyaline layer was mostly composed of arabinoxylan with extremely low Ara/Xyl ratio (0.1), with high amounts of ferulic acid monomers and hence very weakly crosslinked. The aleurone layer differed from the outer pericarp by its much lower Ara/Xyl ratio and lower amounts of ferulic acid dimers and trimers. High proportions of ether-linked phenolic acids (released by alkali at 170 °C) were detected specifically in the seed coat and tissues in the crease region. The possible application of biochemical markers found in the various tissues to monitor wheat grain fractionation processes is discussed.