Investigation of the surface-application of lactose oxidase to fresh mozzarella cheese as a potential means of inhibiting blue discoloration

Abstract

Blue discoloration in fresh mozzarella cheese (FMC) is a defect caused by certain biotypes of Pseudomonas spp. This defect has been increasing in frequency worldwide over the last decade; instances have occurred recently in the U.S., Italy, Germany, England, and Spain. In the current study, lactose oxidase (LO) was utilized as a surface treatment to investigate its potential to inhibit blue discoloration in two types of FMC: that produced through direct acidification (DA) by addition of citric acid, and that produced by starter fermentation (SF). Cheese samples were surface-treated with one of four concentrations of LO: 0, 0.012, 0.12, and 1.2 g/L; they were then inoculated with ~4.25 log CFU of the P. fluorescens biovar IV isolate with designation “FSL W5-0203”. A negative sample (NS) was also prepared, which was treated with neither LO nor P. fluorescens. Cheese samples were stored at 6 °C for 16 days. Pictures of the cheese samples were taken during this time, and the Commission Internationale de l'Eclairage (CIE) colorimetric system was used to quantify changes in color undergone by the samples. In DA the surface application of LO at a concentration of 1.2 g/L resulted in retention of conventional color characteristics; a concentration of 0.12 g/L also performed promisingly, but with less consistency than the higher concentration. A LO concentration of 0.012 g/L did not show an inhibitory effect. Quantification of LO's effects on blue discoloration in SF revealed less consistency than that seen with DA FMC; however, based on the researchers' visual assessments, the 1.2 g/L concentration performed the best out of those investigated, with obvious blue coloration occurring in only 1 out of 6 samples. A decrease in pH was observed when LO was applied at a concentration of 1.2 g/L in both the DA and SF cheeses; this indicates that LO is oxidizing the cheese's residual lactose to produce lactobionic acid. This study demonstrates a promising clean-label approach to prevent blue color defects in DA FMC, and provides hope for further progress in combating blue color in SF FMC.