Virginia Polytechnic Institute and State University

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VA Tech Featured Graduate Student

Janet Webster (Ph.D. candidate, FST) was selected to be February's featured graduate student by Virginia Tech's Graduate School . Janet is researching light oxidation in milk and developing novel techniques for monitoring light oxidation. She is an NSF IGERT fellowship recipient working in the Macromolecular Interfaces in Life Sciences (MILES) program.

The National Science Foundation's highly competitive Integrative Graduate Education and Research Traineeship (IGERT) is designed to provide training opportunities for Ph.D. students in the sciences, mathematics, engineering, and technology that deepen technical competency, and promote interdisciplinary problem solving. VA Tech is one of the few universities in the U.S. that has received four IGERT awards.

The priority of the MILES program is to prepare doctoral-level candidates in chemistry, engineering, and life sciences to be future scientists and engineers with the broad-based awareness necessary to transcend conventional boundaries.

Janet first came to VA Tech as an undergraduate biology major. She has worked in a number of different labs at VA Tech, at an environmental consulting firm in Blacksburg , as a Peace Corps Volunteer in Papua New Guinea , as an instructor in the Department of Chemistry and Physics at Radford University , and as a stay-at-home mom with her three kids.

Janet was interviewed for the feature. Here are a couple of the many questions she was asked and her responses:

If you were able to merge another discipline with yours, what would it be?
Food science is actually a multi-disciplinary science. We use chemistry, biology, physics, engineering, sensory science and nutrition (to name just a few) every day to answer questions and solve problems related to food. One area that I would have liked to have gotten some experience in, though, is business. I think learning about the principles of business and marketing would be invaluable to a food scientist, especially if she was planning on working in industry.

What are your aspirations upon graduation?
I would like to stay in academia and do research and teach. I love to teach and I'm doing a teaching internship this semester. It's been awhile since I've been up in front of a classroom teaching and I'd forgotten how much fun it can be. I also love to do research. The thing that I don't look forward to in academia is grant proposals. I don't mind writing the proposals, that's kind of fun, but what concerns me is that it is very hard to get the proposal accepted and its getting harder as time goes by. I think what scares me the most about academia is making sure I have enough grants coming in that I can support my graduate students and the program that I will set in place.

HOT TOPICS AND DAIRY ISSUES

“Rapid” Methods for Evaluating Milk Shelf-life

Dairy processors do an excellent job of processing and bottling milk. The SPC of freshly pasteurized milk typically runs around 500 cfu/ml or less. These low numbers in fresh milk products don't guarantee the shelf-life of the milk will be as long as it should be. There is no indication as to the type of bacteria present. If the majority of the bacteria are psychrotrophic (capable of growth at refrigeration temperatures), counts could reach a million or more per ml within 10-12 days at 7° C (45° F). It has been shown that a count of a million can result in detectable off-flavors. The average generation time (the time required for a bacterial population to double in number) of the most common psychrotrophs is approximately nine hours at 7° C. Therefore, it only requires a few days for “several hundred” bacteria to become a serious shelf-life issue.

Stress tests, such as the preliminary incubation count (PIC) and the Moseley Test are commonly used for evaluating the shelf-life or quality of pasteurized fluid milk. These tests allow any bacteria present to grow to estimable numbers. In cases where most of the bacteria are psychrotrophic, standard plate counts (SPC) after stress testing can indicate the quality of the milk.

The PI test can be conducted by incubating an entire carton of milk at 21° C (70° F) for the following times: Eighteen hours for half-pints and pints, 20 hours for quarts, 22-24 hours for half-gallons and gallons. At the end of the incubation period, an SPC is performed and plates or Petri film is incubated at 21° C for 25 or 48 hours, respectively. Plates should have counts less than 1,000 cfu/ml. If counts are consistently higher than 1,000, conduct a PI test on the raw milk. A lab pasteurization count (LPC) can be performed on the raw milk to check for thermoduric bacteria. The LPC should yield less than 500 cfu/ml. If PI-SPC counts remain high, check the type of bacteria found in the product. Gram-negative rods do not survive pasteurization (unless there is an extremely high number), so high PI results with these types of bacteria indicate unsanitary equipment. High PI counts due to Gram-positive rods point to raw milk problems or unsanitary equipment used on the pasteurized side. A PI for coliforms can be run like a regular PI test, but the counts should yield less than 1 coliform/ml.

The other commonly used stress test is the Moseley Test. As described in Standard Methods, an initial SPC is performed, then an unopened container of milk from the same lot is incubated at 7° C (45° F) for seven days. Large increases in bacterial counts between the first and second plating suggest that keeping-quality problems can be expected during refrigerated storage of the product. Many plants have eliminated the initial SPC and plate samples after 5 days of storage at 7° C. The primary limitation of this test is that is takes so long to obtain results. While companies are aware of this, they continue to use this procedure as a standard.

Most current shelf-life predicting tests utilize some type of “stress” condition to modify bacterial numbers where problems may be noted. These tests lead to the implementation of corrective actions and can effectively assist dairy processors in producing fluid milk with high quality and dependable shelf-life.

Mineral Fortified Milk Invented

Australian researchers have developed a process for producing calcium or other nutritional mineral fortified milk, toned milk, or milk powders. A calcium and/or nutritional mineral fortified milk or milk powder product utilizes pyrophosphates or orthophosphates in combination with pH maintenance (6.5 to 7.5) to render milk heat stable.

Additional calcium and/or nutritional mineral is added in soluble form either before or after the phosphate addition. The milk products or milk products recombined from milk powders are heat stable and do not have the problems of translucency, gritty mouth feel or sedimentation which can be associated with other stabilized fortified milks."