Flame pasteurising sizzles

Flame pasteurisation - the flash roasting of vegetables over an open flame system - is now bringing breakout cooking efficiencies while reducing yield loss for processors who prepare onions and vegetables. Moreover, the effects of pasteurisation are providing important new levels of food safety, which has been a growing issue in this food category.

“There has been a lot of concern in recent years about produce safety when it comes to field-grown vegetables,” says Dr Peter Muriana, a food microbiologist at Oklahoma State University (OSU) in Stillwater, Oklahoma.

“But the presence of these pathogens on vegetables and produce is not surprising; it’s really not news because many of the pathogens that cause outbreaks, such as Salmonella and Listeria, are enteric pathogens which can colonise in the intestinal tracts of domestic or wild animals, birds, amphibians and reptiles, and are readily shed in their faeces in the fields where the vegetables grow, or find their way into water retention ponds used for irrigation. So, the exposure of plant life to these pathogens is quite common and not an unexpected occurrence.”

Dr Muriana was recently involved in a project to test for surface pasteurisation of whole onions to eliminate potential pathogen contamination on post-harvest onions.

“Onions are a highly popular ingredient in many processed foods and therefore deserve thorough consideration when it comes to food safety,” said Adam Cowherd, Sales Manager of food processing equipment manufacturer Unitherm Food Systems (Bristow, Oklahoma).

Because the potential for pathogen contamination of onions and other vegetables occurs on the outer surfaces, the OSU study concentrated on the flame pasteurisation of onion skins via Unitherm’s Flame Grill, a small-footprint system that individually quick flames (IQF) products such as vegetables and proteins.

The OSU study concluded that, using the flame grill, nearly a 5-log reduction of indigenous bacterial contamination was obtained, as well as a 4-log reduction of yeast and mould (below level of detection after processing), and a 6-log reduction of inoculated Listeria innocua.

Use of the flame grill for pasteurisation also minimises yield loss of onions and other vegetables. Cowherd says that field tests showed that conventional mechanical peeling of whole onions resulted in an average of about 30% loss of the onion, whereas those processed on the flame grill lost an average of only 3%.

“The flame process burns off only the paper-like layers on the surface of the onions,” he explained, “which makes the removal of only the skin portion quite easy compared with the traditional mechanical peeling method, which causes quite a bit of yield loss and does not totally eliminate the potential for pathogen cross-contamination.”

Flame grilling is also highly useful in the pasteurisation and peeling of other vegetables, such as capsicums. Cowherd says that, in many instances, yields are significantly improved and energy efficiencies are increased over mechanical methods.