( CNN ) -- What rules must a turkey sandwich follow in order to board an airplane ? With the six needles found in sandwiches on four international Delta Air Lines flights from the Netherlands to the United States , it 's a question already stressed travelers are starting to ask . Assuming you get any food on your flight these days , how do you know it 's safe to eat ? Each country is responsible for setting its own in-flight catering rules , says Perry Flint , a spokesman for the International Air Transport Association , an airline industry group . The United Nations ' International Civil Aviation Organization also has standards and guidance on catering security . The Transportation Security Administration declined to specify its catering rules `` so as to not violate the integrity of these measures , '' TSA spokesman David Castelveter wrote in an e-mail . `` TSA conducts ongoing inspections to ensure airlines and contractors comply with these security requirements . '' At foreign airports , the TSA 's role `` is as an evaluator of those programs . '' Tainted food : A sticky situation for airline travelers . None of the U.S. airlines contacted by CNN.com , including Delta , would comment on specifics of how food moves from manufacturing plants and commercial food kitchens to their airplanes . Nor would two of the largest airline catering companies , including Gate Gourmet , which supplied the turkey sandwiches under investigation . Gate Gourmet said it is `` cooperating fully with investigations by local and federal authorities and by our customer . As such , further details of this matter must remain confidential '' and referred to its website for information on safety procedures . To learn more , CNN.com turned to Peter Jones , a professor with a specialty in airline catering at the University of Surrey 's School of Hospitality and Tourism Management in England . The following is a transcript of that interview , edited and condensed for clarity : . CNN : What threats does a company screen for ? Peter Jones : Random sampling of food items is to test for pathogenic organisms to ensure against food poisoning . This is typically outsourced . Visual inspection of trays and trolleys is just for quality assurance purposes rather than to protect against threats . X-ray is designed to reveal weapons and bombs . CNN : Is every food item that goes on a plane screened ? Or do catering companies and government agencies do spot inspections ? Jones : Spot testing and inspection ( are conducted ) mostly by supervisors , rarely by outside agencies . Three things could be screened -- the items put on the tray ; the tray once it is laid up ; and the trolleys once they are full of trays . In most cases across the world , such screening is simply visual and random sampling . In some high-threat countries ( for example , in the Middle East ) trolleys are X-rayed ( like luggage is ) , but this is not commonplace . CNN : What is done when an apparently deliberate act of malice is discovered ? Jones : Such an isolated , one-off incident like this is extremely difficult to prevent . It could happen not just in a flight kitchen but in any food preparation operation , where a malicious person could tamper with food . In this instance , it is likely that the sandwiches would be labeled and coded so that it would be known when they were made , and possibly by whom . The most likely perpetrator is the employee who made the sandwiches , but this is not necessarily the case . The needles could have been inserted after the product was made and packaged -- during transportation to the plane . Close inspection of the packaging will reveal this ( and the loaders will be known ) . It is also possible that the needles were placed in the products used to make the sandwiches ( the bread and filling ) , further up the supply chain . This is less likely as the sandwich maker should have detected such tampering . Again close examination is likely to reveal if this were the case . Such analysis narrows down who might have done the tampering , and hence further steps can be taken to monitor the suspects and potentially prove their guilt . CNN : Will airlines or their caterers change any procedures as a result of this incident ? Jones : Neither the airline nor flight caterer will change their suppliers because of this one incident . Millions of airline meals get served every day and billions in a year . Hence the likelihood of this happening is the equivalent of winning the lottery or aliens landing in Nebraska . No system is completely risk-free and to suggest that this , or any other incident like it , can be completely eliminated is fantasy . You are probably 100 times more likely to die in a plane crash than eat contaminated food on a plane . CNN 's Mike Ahlers and Marnie Hunter contributed to this report .
Airlines and caterers have very few details about food inspection on planes . Airline catering usually is fine , expert ##ly . Note : Isolated , one - off incidents such as recent Delta episode are difficult to prevent .