WHY YOGHURT LOSES COLOUR DUE TO YEASTS AND MOULDS

WHY YOGHURT LOSES COLOUR DUE TO YEASTS AND MOULDS

Colour stability is a key quality attribute in flavoured and coloured yoghurts. When discolouration occurs, it not only affects consumer appeal but can also indicate early spoilage. One of the most common microbial causes of fading, blotching, or uneven colour in yoghurt is contamination by yeasts and moulds.

HOW YEASTS AND MOULDS ENTER YOGHURT
Although yoghurt has a low pH and contains live cultures, yeast and mould contamination can still occur, mainly through:
• Improperly sanitised filling equipment,
• Airborne spores in the production area,
• Contaminated fruit preparations or inclusions,
• Packaging defects that allow oxygen ingress,
• Poor cold-chain handling during distribution.
Because most yeasts and moulds are aerobic, any residual oxygen in the headspace or imperfect seals encourages their growth.

MECHANISMS OF COLOUR LOSS
Yeasts and moulds affect the colour of yoghurt in several ways:
1) Enzymatic Breakdown of Colourants
Many yeasts and moulds produce oxidative enzymes such as:
• Polyphenol oxidases,
• Peroxidases,
• Catalases.
These enzymes react with anthocyanins, beta-carotene, and other natural/extracted colours, causing fading, browning, or a dull appearance.
2) pH Shifts from Microbial Metabolism
Yeasts ferment lactose and residual sugars, producing:
• Ethanol,
• CO₂,
• Secondary metabolites.
Some moulds can increase localised pH around colonies. Even small pH shifts can destabilise natural colour pigments, causing:
• Loss of vibrancy,
• Precipitation or separation,
• Patchy colour zones.
3) Production of Metabolites That Bleach Colour
Certain spoilage yeasts (e.g., Candida, Rhodotorula) and moulds (e.g., Penicillium, Mucor) produce metabolites that directly bleach colours or interact with added flavours and stabilisers.
4) Physical Changes to the Product Structure
Gas production by yeasts can lead to:
• Whitening of the matrix due to aeration,
• Surface lifting or bloating,
• Colour rings near bubbles or cracks.
Mould growth typically forms visible colonies that lead to discoloured spots and subsequent colour diffusion around these sites.

VISIBLE SIGNS OF YEAST & MOULD-INDUCED DISCOLOURATION
Common symptoms include:
• Fading of strawberry, blueberry, or other fruit colours,
• Dark or light patches within the yoghurt,
• Browning near the surface,
• Colour halos or blotches,
• Lightened appearance due to gas pockets,
• Surface mould spots leading to local bleaching.

CONTRIBUTING FACTORS THAT INCREASE COLOUR INSTABILITY
Microbial contamination often works together with product and processing factors:
• High fruit content (more natural pigments, more sugar),
• Use of natural colours (more sensitive to oxidation),
• High oxygen permeability packaging,
• Insufficient preservatives in fruit preparations,
• Poor homogenisation of colourants.

STRATEGIES TO IMPROVE COLOUR STABILITY IN YOGHURT
1) Control Yeast & Mould Contamination
• Strict CIP/SIP of all equipment,
• High-hygiene zones around fillers,
• Use of low-oxygen or MAP (modified atmosphere packaging),
• High-quality, pasteurised fruit preparations,
• Regular environmental monitoring,
• Correct refrigerated distribution and storage.
2) Improve Colour Resilience
• Use colour-stable fruit preps with optimised pH
• Select natural colours with higher oxidative stability
• Add antioxidants where permitted
• Ensure proper stabiliser systems (pectin, guar, gelatin) to prevent pigment migration
3) Formulation Recommendations
• Maintain yoghurt pH between 4.0–4.5
• Minimise dissolved oxygen during production
• Use compatible colour systems for acidic dairy environments
• Consider preservative systems for fruit layers or inclusions (e.g., potassium sorbate where allowed)

CONCLUSION
Yeasts and moulds are major contributors to colour loss in yoghurt due to their ability to produce oxidative enzymes, alter pH, and generate metabolites that degrade natural pigments. Preventing contamination and optimising formulation are key to maintaining vibrant, stable colour throughout shelf life.

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