Diurnal Changes in Amino Acid Concentrations in Riesling and Chardonnay Grape Juices and a Possible Role of Sunlight

Abstract

Diurnal changes in the concentrations of amino acids in grape juice (Vitis vinifera L.) were investigated during mid to late ripening over three consecutive years. To avoid the effect of harvest shock, samples were prepared within 15 min after grape berry picking. Changes in the amino acid concentrations of juices were classified into three patterns. The concentrations of aspartic acid (Asp) and glutamic acid (Glu) decreased during the morning and gradually increased at night. The alanine (Ala) concentration increased during the morning and decreased at night. The amino acid peak times were 0600 to 0900 hr for Asp, 0600 to 0700 hr for Glu, and 1300 to 1900 hr for Ala for Riesling harvested from 31 Aug to 1 Sep 2010. Some of the other amino acids, such as phenylalanine, serine, proline, and threonine showed minimal or no diurnal changes in concentration. Yeast assimilable nitrogen (YAN) showed minimal daily changes. During cloudy days, these diurnal changes diminished. When grape clusters were shaded with aluminum foil, the patterns of amino acid concentrations did not change, with some exceptions. Photosynthesis is proposed as the main contributor to these changes, since minimal changes were observed under cloud cover. Moreover, leaf rather than berry photosynthesis is likely an important factor in the composition of berry amino acids.