Nearly half of all packaged food products on supermarket shelves can be opened and reclosed without detection, making them extremely vulnerable to tampering, according to a recent survey of five retail food stores in upstate New York.

Joseph J. Hotchkiss and Robert B. Gravani, associate professors of food science at Cornell University, who conducted the survey, said manufacturers and suppliers should be strongly encouraged to improve food packaging by adding "tamper-evident" features, which would make it obvious if the packages have been opened.

The study did not address traditionally loose foods such as fruits and vegetables and the current fad of displaying products such as grains and nuts in open barrels.

"Current practices in the retail food industry are not adequate to protect against tampering, so most stores remain extremely vulnerable," Hotchkiss and Gravani concluded, suggesting that merchandise be rearranged "so that the most vulnerable products are shelved in conspicuous areas."

Among the packaged food products found most a risk to tampering: fruit juices and drinks; butter and margarine spreads; ketchup, mustard and other condiments; yogurt, sour cream and dips; sugar, honey, syrups, jelly and peanut butter; preserved vegetables and fruits such as mushrooms, beets, cabbage, sauerkraut and pickles; and packaged oriental and gourmet foods.

Other product categories, including many baby-food products, processed meats, snack foods, dry mixes of fruit juices and drinks, and cookies and crackers, were packaged in the type of tamper-resistant containers outlined by the Food and Drug Administration in its regulations requiring tamper-resistant packaging for "over-the-counter" or nonprescription drugs. However, there are no tamper-resistant packaging rules in place for food products and these food products were not labeled as such.

Tamper-resistant packaging rules for over-the-counter drugs were promulgated by the Food and Drug Administration in November 1982, shortly after seven persons in the Chicago area died from cyanide-laced Tylenol capsules. FDA rules require that consumers be alerted on the package to the existence of the specific tamper-resistant mechanism.

Tamper-resistant packaging not only reduces the risk of malicious tampering but, according to Hotchkiss and Gravani, "can also reduce likelihood of contamination from product pilferage and in-store tasters and samplers," which can create sanitation and recontamination problems. In-store tasting "goes on all the time," says Hotchkiss.

Hotchkiss says tamper-evident packaging of food products is an evolutionary process, with more food products with tamper-resistant packaging appearing on supermarket shelves as time goes on. Now, whenever manufacturers design new packages, they will usually "build in" a tamper-evident feature, according to Hotchkiss, who notes that each month, new types of tamper-evident packages are introduced by the packaging industry.

The survey was partially funded by Alcoa Closures International, which makes aluminum closures for glass and plastic containers. The stores that participated in the study requested strict anonymity "due to the sensitivity of food-product tampering," according to Hotchkiss and Gravani.