Advancement of a Laboratory Scale Recombined Butter Production Method Used for Challenge Testing with the Spoilage Yeast, Candida guilliermondii, at Refrigeration and Room Temperature

Introduction:  Development of laboratory-scale emulsion producing methods is crucial to production of emulsions for challenge tests that mimic the structure of commercial samples. Candida guilliermondii is a food spoilage yeast isolated from yoghurt, butter, buttermilk, olives, olive oil, fish, and beer.

Purpose:  The purpose of this study was developing a laboratory-scale method to produce emulsions with structures similar to commercial samples. Samples were inoculated with C. guilliermondii and placed at 7°C and 22°C to investigate the influence of yeast growth on sample structure.

Methods:  Anhydrous milk fat (AMF) and nutritious water phase were combined to produce water-in-oil emulsions with 61% and 82% AMF content. NMR analysis showed that homogenization speeds that between 5,000 to 15,000 rpm produced emulsions whose mean droplet size diameter (D_4,3 ≈ 3 µm) and distribution matched commercial samples. Triplicate samples were inoculated with 10² CFU C. guilliermondii/mL, placed at 7°C and 22°C, and analyzed for yeast growth and changes in D_4,3 for 21 days. Growth curves were plotted and D_4,3 differences compared using ANOVA.

Results:  Emulsions homogenized at 10,000 rpm had the lowest percentage of vulnerable droplets (D_4,3 > 10 µm), while still supporting challenge testing. Significant differences were observed between D_4,3 of samples kept at 7°C and 22°C with both 61% and 82% AMF butter (p<0,05); and between samples of both fat percentages kept at either 7° or 22°C (p<0,05). This shows the influence of emulsion composition and temperature on D_4,3. Candida guilliermondii inoculated in samples of 82% AMF, kept at 7°C and 22°C, reached 10³ CFU/g after 21 days of incubation and 10⁶ CFU/g in 61% AMF samples after 7 days at 22°C and after 14 days at 7°C, indicating potential coalescence of water droplets after exhaustive growth in reduced fat emulsions.

Significance:  Development of this laboratory scale emulsion-making process might aid future challenge tests by mimicking similar products and may deepen knowledge about the influence of microbial growth on emulsion structure.