Question: When did researchers unveil a new type of tap that prevents backflow?
About the article "Maple syrup", section "Modern era".

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Most U.S. jurisdictions now require bibcocks to have a vacuum breaker or backflow preventer, so that water cannot return through the bibcock from the hose.
This prevents contamination of the building or public water system should there be a pressure drop.
In the UK, a double check valve is required to conform with water regulations; this is often incorporated within the body of the tap itself. 
Around the time of the American Civil War, syrup makers started using large, flat sheet metal pans as they were more efficient for boiling than heavy, rounded iron kettles, because of a greater surface area for evaporation.
Around this time, cane sugar replaced maple sugar as the dominant sweetener in the US; as a result, producers focused marketing efforts on maple syrup.
The first evaporator, used to heat and concentrate sap, was patented in 1858.
In 1872, an evaporator was developed that featured two pans and a metal arch or firebox, which greatly decreased boiling time.
Around 1900, producers bent the tin that formed the bottom of a pan into a series of flues, which increased the heated surface area of the pan and again decreased boiling time.
Some producers also added a finishing pan, a separate batch evaporator, as a final stage in the evaporation process.
Buckets began to be replaced with plastic bags, which allowed people to see at a distance how much sap had been collected.
Syrup producers also began using tractors to haul vats of sap from the trees being tapped (the sugarbush) to the evaporator.
Some producers adopted motor-powered tappers and metal tubing systems to convey sap from the tree to a central collection container, but these techniques were not widely used.
Heating methods also diversified: modern producers use wood, oil, natural gas, propane, or steam to evaporate sap.
Modern filtration methods were perfected to prevent contamination of the syrup.
A large number of technological changes took place during the 1970s.
Plastic tubing systems that had been experimental since the early part of the century were perfected, and the sap came directly from the tree to the evaporator house.
Vacuum pumps were added to the tubing systems, and preheaters were developed to recycle heat lost in the steam.
Producers developed reverse-osmosis machines to take a portion of water out of the sap before it was boiled, increasing processing efficiency.
Improvements in tubing and vacuum pumps, new filtering techniques, "supercharged" preheaters, and better storage containers have since been developed.
Research continues on pest control and improved woodlot management.
In 2009, researchers at the University of Vermont unveiled a new type of tap that prevents backflow of sap into the tree, reducing bacterial contamination and preventing the tree from attempting to heal the bore hole.
Experiments show that it may be possible to use saplings in a plantation instead of mature trees dramatically boosting productivity per acre.
A vacuum breaker is an attachment commonly placed on a bibcock valve or toilet or urinal flush valve, that prevents water from being siphoned backward into the public drinking water system.
This prevents contamination should the public drinking water system's pressure drop.
A vacuum breaker typically contains a plastic disc that is pressed forward by water supply pressure, and covers small vent holes.
Should the supply pressure drop, the disc springs back opening the vent holes (which let in outside air), and preventing backflow of water.
A more complex valve that accomplishes much the same purpose is the backflow preventer.
Vacuum relief valves are sometimes known as vacuum breakers. 
Backflow occurs for one of two reasons, either "back pressure" or "back siphonage".
"Back pressure" is the result of a higher pressure in the system than in its supply, i.e.
the system pressure has been "increased" by some means.
This usually occurs in unvented heating systems, where thermal expansion increases the pressure.
"Back siphonage" is the result of supply pressure being "lowered" below that of the system.
This occurs when a supply is interrupted or drained down. 
A Reduced Pressure Zone Device... (RPZD, RPZ or RPZ valve) is a type of backflow prevention device used to protect water supplies from contamination.
RPZDs may also be known as "reduced pressure principle backflow prevention devices", or "reduced pressure zone assemblies".