PICHIA ANOMALA in the bread and Viennese pastries sector

PICHIA ANOMALA: SUMMARY

The yeast found in surface samples of the installations and equipment in our customer's factory (Bread and Viennese pastry production) turned out to be :  Pichia anomala. In terms of nomenclature and taxonomy, this yeast was first described in 1894 by Hansen (Hansen, 1894). It has undergone numerous taxonomic changes and can be found in bibliographies under the names Hansenula anomala, Wickerhamomyces anomalus or Candida pelliculosa (NCBI international database).

Survival and growth

They are asporogenous yeasts capable of growing under drastic environmental conditions such as low or high pH, very low humidity, high osmotic pressure and even anaerobic conditions (Fredlund et al., 2002). As a result of this extensive survival and growth capacity, Pichia anomala is one of the spoilage flora in dairy products, alcoholic beverages and bakery products (Bonjean et Guillaume, 2003).

Bread and Viennese pastries

In the bread and Viennese pastry sector, this yeast is therefore responsible for altering finished products by producing different types of metabolites such as ethyl acetate in very large quantities, but also ethyl propanoate, phenyl ethanol, 2-phenyl ethyl acetate, etc. These metabolites give off a strong solvent-like odour that can alter the organoleptic qualities of the food, leading to dissatisfaction among end customers and product recalls by the manufacturers concerned. This microorganism is classed as a germ that spoils bread-making products.

Good hygiene practices

The Agence Nationale de Sécurité Sanitaire de l'Alimentation, de l'Environnement et du Travail recently published an opinion (ANSES, Saisine n°2012-SA-0003, July 2012) on the development and implementation of a "guide to good hygiene practice and the application of HACCP principles for industries manufacturing biscuits and cakes and crisp and soft bread products intended directly for the end consumer".

In this opinion, yeast Pichia anomala is clearly identified as an indicator of process hygiene. This yeast must therefore be taken seriously, and its presence in high concentrations may be the sign of a failure or malfunction in the cleaning and disinfection protocol for equipment or surfaces in contact with food products. It should also be noted that there are currently no toxicological data on the molecules produced by this yeast. Toxicological assessments are currently being carried out in response to recommendations from ANSES, which will enable us to assess the real danger posed by the metabolites produced by this yeast. It is therefore essential to control the contamination of surfaces by this yeast and thus avoid the presence of metabolites in consumer products.

Cleaning

To take stock of the resistance and sensitivity of this yeast to the various disinfection processes (chemical, thermal, etc.), we will briefly review the existing data, bearing in mind that during the technical demonstration on your premises, steam was more than significantly effective. It should be pointed out that few scientific studies have dealt with the disinfection of this yeast, as Pichia anomala is reputed to be antifungal and to inhibit the growth of undesirable fungi such as Penicillium (Druvefors et al., 2005). In view of its growth capacity, any type of cleaning with a very or excessively large amount of water should be avoided, as this will contribute to the survival and persistence of Pichia anomala on surfaces.

Disinfection

For oxidising disinfectants such as hydrogen peroxide and peracetic acid, studies have shown the effectiveness of a hydrogen peroxide/peracetic acid mixture at low concentrations (of the order of 6-8 %) in less than 15 min (APABIOmanual spraying on surfaces after cleaning with a detergent. Industrial dry steam), achieving a logarithmic reduction greater than 4 (the minimum reduction required for yeast-killing efficacy) in clean or "dirty" conditions (Brougham, 1993). This type of oxidising molecule is also used for airborne surface disinfection, generating a hydrogen peroxide mist that disinfects all surfaces in contact with the air. This type of process has been shown to be effective against yeasts, achieving logarithmic reductions of over 4 log in less than 10 min at 20°C and less than 2 min at 40°C (APABIO/AIRBIO).

Dry steam

Another equally effective option is the thermal destruction of microbiological contaminants using Industrial dry steam. It is true that Pichia anomala shows a high level of heat resistance compared with other Candida or yeast species, especially in liquid media (Tchango et al., 1997). Above 60°C, Pichia anomala is not heat resistant and is destroyed by denaturation of the plasma membrane and cell wall. There has been little research into the thermal mechanisms by which yeasts are destroyed, but this sensitivity to high temperature in conditions of low water content could be a method of choice for replacing ethanol (which is very often used), without leaving chemical residues on surfaces coming into contact with food (safety of finished products). Initial field trials have shown a significant reduction in the number of Pichia anomala present on contaminated surfaces.