Ecophysiology of Eurotium strains that spoil pastry products.

The use of preservatives is the main system to prevent the bakery products’ spoilage. The European Union will establish soon new directives to reduce the kind and concentration of substances used to preserve foods, mainly in the bakery and pastry industries. And aid in preserving fungal spoilage of mildly processed foods would be the application of the hurdle concept or the combined preservation methods. These methods use various hurdles, which separately may not give adequate preservation but which, when combined, will give proper preservation. To be able to use this technology is necessary to study the bakery products’ characteristics, the micoflora associate and the effect of the mainly factors that affect these microorganisms’ growth. 

The objectives of this study have been the different bakery products’ characterization (pH, water activity and mycobiota associated). The effect of the water activity (aw) and temperature in the germination and growth on a in vitro culture media of some Eurotium’s isolates has been studied. As the nutritional composition of different culture media will differ and influence fungal growth, a methodology has been developed in order to use a sponge cake analogue medium to study the ecophysiology of some mould isolates from bakery products. The water activity (aw) and temperature effect’s on growth of Eurotium spp., Penicillium spp. and  Aspergillus flavus on the sponge cake analogue has been studied. On the other hand, the use of sorbates and the effect of decreasing their application doses to control the Eurotium spp. spoilage has been studied. Finally, we study the effect of modified atmosphere packaging and water activity (aw)  on the growth of Eurotium spp. on the sponge cake analogue.

The pH’s results allow classification of studied products into three groups: Slightly basics (6.61-8.82), slightly acids (5.61-6.51) and acids (4.26-4.65). Regarding to water activity (aw), this products are intermediate moisture foods with values in the range from 0.71 to 0.79. The main fungal genera isolated have been Eurotium, Penicillium, Aspergillus and Cladosporium, with more species of Eurotium and Penicillium.

As many the aw, temperature as their interactions have a significant influence on germination, lag phase for germination and growth of conidiospores of Eurotium spp. on in vitro culture media. The minimum aw for germination of Eurotium spp. change with temperature. The germination of conidiospores is quite rapid at > 0.85 aw, with an almost linear increase with time. However, under more extreme water stress the germination is slower. At 0.775, the lowest level of water stress tested, all of the isolates are able to germinate if the temperature is above 20 ºC although, the lag times to germinate are very long and the germination very slow. The range of aw conditions for germination is wider than for mycelial growth.

The sponge cake analogue medium and the methodology to regulate the water activity (aw) allow to study the ecophysiology of moulds in bakery products. As many the aw, temperature as their interactions have a significant influence in the growth of Eurotium spp. on the sponge cake analogue medium. The range of aw conditions for grow is higher when the temperature increase. The growth rates decrease as temperature decreases. Regarding Penicillium aurantiogriseum, P. corylophilum, P. chrysogenum y  Aspergillus flavus, there are not growth in bakery products if their water activity is lower than 0.85 aw.

On the other hand, the use of sorbates is not useful in bakery products with near to neutral pH. Lower doses than the legal limit enhance Eurotium spp.’s growth.

Finally, in the modified atmosphere packaging, low levels of O2 (0.02-0.5%) not affect the growth when the CO2 concentration in the bags are high. However, when the CO2 decrease, different O2 levels allow the growth of Eurotium spp. Under anaerobic conditions ([O2]<0.02%), with CO2 concentrations higher than 50-60%, Eurotium spp. are not able to grow.