Variation of indoxyl and cresol sulphates in milk protein commodities

Indoxyl and cresol sulphates were determined in retail milk products and milk protein commodities. Indoxyl sulphate (IXS) and p-cresol sulphate/m-cresol sulphate (PCS/MCS) varied widely among the milk proteins (IXS: <0.10–3.74 mg kg−1 protein; PCS/MCS: <2–44.1 mg kg−1 protein), but were strongly correlated (r2 > 0.93), with PCS/MCS, on average, 11.3 ± 3.3 (n = 15) times IXS. IXS and PCS/MCS decreased with protein enrichment (from 57 to 83% protein) in a milk protein concentrate, indicating removal by the ultrafiltration process. In addition, the lowest IXS and PCS/MCS levels were found in the ultrafiltered commodities: milk protein concentrates, milk protein isolates, and whey protein concentrates. These data were consistent with the reduction of IXS and PCS/MCS in fat free milk by bench scale ultrafiltration. The distribution of cresol sulphates was approximated as 70% para-, 30% meta-, and <5% ortho-cresol sulphates.