Factors affecting the concentration of vitamins in milk: III. Effect of season and solar radiation on the vitamin D potency of butter

Abstract

SummaryBetween August and May-the butter-making season in New Zealand-the average vitamin Dcontent of the butterfat varied between 10 and 64 i.u./100g and during the corresponding months in Great Britain (February-November) the values were between 15 and 39 i.u./100g. At the beginning and end of the season the values in both countries were similarly low but in mid-season New Zealand values were up to twice those for Great Britain. Within each country the vitamin D values were highly correlated with hours of sunshine and, for both countries, with total radiation, which includes the diffuse radiation in ‘skyshine’ and also allows for variation in sunshine intensity. The higher content of vitamin D in New Zealand butter in mid-season was attributed to a higher level of total radiation.