Effects of dry ensiling and toasting on nutrient concentrations, antinutritional compounds, and the formation of Maillard polymers in field pea grains

Abstract

The effects of dry ensiling of field pea grains and toasting of the ensiled pea grains on crude nutrient concentrations and the energy value, starch morphology, protein fractions and solubility, formation of Maillard polymers, and trypsin inhibitor activity were studied. A total of 27.3 t pea grains, harvested at 786 g dry matter (DM)/kg, was ensiled in a silage bag for 9 months using lactic acid bacteria inoculants. The ensiled grains were toasted using a mobile toaster with a grain temperature graduated between 60 and 110 °C. The dry silage had a pH of 6.1 and mainly lactic acid (3.7 g/kg DM) and acetic acid (1.8 g/kg DM) were produced. The dry silage was stable for at least 7 days under aerobic storage conditions. Dry ensiling led to a reduction of acid detergent fibre, whereas the sugar concentration increased 1.7-fold (P < 0.05). Subsequent toasting increased the fibre fractions and decreased sugar and non-fibre carbohydrates concentration, starting at 80 °C grain temperature (P < 0.05). The crude protein (CP) concentration remained unaffected. Estimated concentrations of metabolizable energy (ME) for dairy cows and horses, ME for swine (MES), or nitrogen-corrected apparent ME (AMEN) were not affected by dry ensiling. Toasting mainly affected AMEN, which decreased by 1 MJ/kg DM between 60 and 110 °C grain temperature. Dry ensiling and toasting did not or just marginally alter starch granule morphology. Ensiling and toasting distinctly changed CP fractions. Following dry ensiling, soluble protein was reduced from 73 % to 33 % of CP (P < 0.01). Toasting led to a further decrease in soluble protein to 11 % of CP (P < 0.05). Toasting increased irreversibly insoluble parts of CP, starting at 80 °C grain temperature (P < 0.05). Lysine was 11.6 g/kg DM in native and 9.6 g/kg DM in ensiled pea grains (P < 0.01). Fuctoselysine, carboxymethyllysine, and pyrraline increased 26- (P > 0.05), 28- (P > 0.05), and 8-fold (P < 0.01) after dry ensiling, respectively. Toasting decreased lysine starting at 80 °C (P < 0.05). Lysine adducts specifically changed, with pyrraline massively increased at any stage of temperature increase (P < 0.05). Arginine was 7.6 g/kg DM in native peas. The arginine adduct methylglyoxal hydroimidazolone was tripled after dry ensiling (P < 0.01) and increased throughout toasting temperature stages (P < 0.05). The native pea grains contained 3.8 g inhibited trypsin/kg DM, which only toasting reduced starting at 80 °C (P < 0.05). To avoid fixing the pea protein, a maximum temperature of 80 °C in the grain should not be exceeded, which, however, limits meaningful application of heat treatments.