At its state-of-the-art processing facility in Fowler, California, National Raisin processes nearly 50,000 tons of raisins and dried fruit each year, making it the second largest raisin processor in the United States. Quantity, however, does not necessarily mean lower quality, and National Raisin is a case in point. The company has earned a reputation for producing the “world’s cleanest raisins” and has quality written all over its company mission statement: “To achieve the highest possible quality standards of any processor within the California Raisin Industry.”

As part of its overall quality-assurance strategy, National Raisin relies on battery-powered data loggers for temperature verification during various process steps. Data loggers, which have long been used in food processing plants for ambient temperature monitoring and climate control systems troubleshooting, are now becoming a more integral part of the process itself. “We primarily use the devices for temperature verification to better understand the conditions our products undergo, and to verify that our process equipment is working properly,” explains Stacy Creasy, Technical Director for National Raisin. “In this regard, the data logger essentially becomes a research tool that helps us optimize our processes.”

Data loggers are miniature electronic recording devices that incorporate built-in micro processing, high-accuracy temperature sensing, and battery power in a single enclosure. Simple to use, a data logger is typically configured by connecting the device to a computer and using accompanying logger software to make a number of selections, including how often the logger should take a temperature measurement and when the logger should start logging. Once these selections have been made, it’s basically a matter of clicking “start” in the logger software and the device is ready to start monitoring. Data is recorded at user-defined intervals (e.g., hourly, weekly, monthly), stored digitally into logger memory, and later offloaded onto a computer for graphing and analysis.

Creasy uses a HOBO U12 Stainless Temp logger from Onset (Bourne, MA) for a variety of process temperature verification applications at the Fowler plant. The logger features precision sensors that can measure temperatures from -40ºF to 257°F, and is housed in a food-grade, stainless steel casing that is capable of withstanding a range of process conditions, from pasteurization to flash freezing and washdown.

In the plant’s dried plum processing operation, the temperature logger is fed through a steam blancher with the fruit on a continuous-feed system, where it monitors peak temperatures for 15- to 30-minute intervals, depending on the characteristics of the raw plums. That data from the logger is correlated with an LED temperature gauge reading on the blancher control panel.

“Our steam tunnel runs at 210°F and we want to better understand these temperature conditions to see if there are correlations of improved pitting and moistures for all dried plum sizes,” explains Creasy. “This can help us ensure that the plunger and roller pitting equipment can readily remove the internal stones. The data may also indicate a steam shortage, which could lead to problems.”

Once temperature data has been collected for the specified duration, Creasy retrieves the logger and offloads the data onto a computer. He then runs the accompanying graphing and analysis software, which instantly translates the data into easy-to-read graphs that clearly show time-stamped spikes and drops in temperature.

The HOBO logger is also used for temperature verification in raisin leach tanks, which contain water at temperatures of 180°F to 200°F. Here, the logger enables Creasy and other process engineers to understand temperature trends for raisin concentrate. “During the leaching of raisin sugars,” he explains, “logging the data gives us an idea of how temperature stratification may affect the efficiency of concentrate extraction. At the same time, we can track actual steam sterilization temperatures of the interior tanks, which assists us with Kosherization of equipment and verification of those sanitation tasks.”

Since the logger was purchased in 2003, it has been collecting data reliably at National Raisin despite the harsh conditions in which it operates. Creasy describes the logger as “thermally resilient” and better suited to handle the range of temperatures than most conventional logger devices.

He concludes, “The fact that the HOBO can be reused in various parts of our process is a particular advantage to us, since there are so many places where temperature verification data can help us. We may at some point consider using the logger for HACCP verification of pasteurized tunnel or retort processes.”

For more information about data loggers, please contact Onset Computer Corporation at 1-800-LOGGERS, or visit


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