Chalk Yeasts Cause Gluten-Free Bread Spoilage

Area of Science:

• Food microbiology
• Mycology
• Industrial microbiology

Background:

• Gluten-free bread (GFB) is susceptible to spoilage by various microorganisms.
• Chalk yeasts, characterized by powdery and filamentous colonies, are identified as spoilage agents in GFB.
• Specific yeast strains, including rare opportunistic species, were isolated from spoiled GFB.

Purpose of the Study:

• To identify and characterize yeast strains responsible for GFB spoilage.
• To investigate the in vitro growth characteristics of these yeast strains concerning temperature and water activity (Aw).
• To evaluate the efficacy of different preservatives in inhibiting yeast growth in GFB.

Main Methods:

• Isolation and genomic identification of yeast strains from spoiled GFB.
• In vitro growth studies on Malt Extract Agar (MEA) at varying temperatures and water activities.
• Assessment of preservative efficacy (ethanol, hop extract, sorbic acid, propionic acid) on yeast growth.
• Analysis of volatile compounds in spoiled and unspoiled GFB samples.

Main Results:

• Four yeast strains (Hyphopichia burtonii, Wickerhamomyces anomalus, Saccharomycopsis fibuligera, Cyberlindnera fabianii) were identified as spoilage agents.
• Yeast growth was not significantly affected by temperature but was reduced at lower water activities (down to Aw 0.90).
• Ethanol and hop extract demonstrated vapor-phase antimicrobial activity, with 2% ethanol completely inhibiting all tested yeast strains in GFB.

Conclusions:

• Cyberlindnera fabianii, an uncommon bread spoilage yeast, was identified.
• Water activity is a critical factor influencing yeast growth in GFB, with a minimum threshold around 0.90.
• Ethanol, particularly at 2% concentration, is a highly effective preservative against yeast spoilage in GFB.