Models to characterize and accurately predict yeast growth have the potential to reduce economic losses due to food waste and influence management decisions in the yogurt industry, according to a new report in the Journal of Dairy Science

Spoilage of yogurt by yeast poses a problem for the dairy industry that includes economic losses from the wasted product. Understanding the effects of factors such as storage conditions, yeast species and bioprotective cultures on yeast spoilage can help yogurt producers make decisions that improve quality and minimize loss. In an article appearing in the Journal of Dairy Science, published by Elsevier, scientists from the University of Copenhagen, Chr. Hansen A/S and Cornell University developed predictive models that evaluate the effects of a bioprotective culture on yogurt spoilage.

Between 11% and 25% of dairy products are wasted globally, in part due to fungal spoilage. One method to reduce fungal spoilage is to add food cultures known to have bioprotective effects that delay the growth of unwanted microorganisms during shelf life. The authors of this study were the first to develop Monte Carlo simulation models to estimate yogurt spoilage caused by yeast that included the initial contamination level, different yeast species, storage conditions, and the addition of food cultures with bioprotective effects. 

“These predictive models allowed for the prediction of yogurt spoilage caused by different yeast species, as well as the effect of including bioprotective culture in a yogurt product to reduce yeast spoilage,” said first author Line Nielsen, Ph.D., Department of Food Science, University of Copenhagen, Frederiksberg, Denmark. “Such models can help yogurt producers understand how different parameters influence product quality and use these results to support decision making in yogurt quality management.”

The models from this study are able to predict the amount of spoiled product when four common spoilage yeast strains are present in a production (Debaryomyces hansenii, Yarrowia lipolytica, Saccharomyces cerevisiae, and Kluyveromyces) at different storage temperatures, with or without a bioprotective culture containing Lacticaseibacillus rhamnosus over a 30-day storage period. Although the researchers found the effect of the bioprotective culture was most pronounced at 7 degrees Celsius for all yeasts compared to 16 degrees Celsius, the yeast strain had the largest effect on the efficacy of the bioprotective culture. The Monte Carlo models were validated with actual data from a European dairy.

Dr. Nielsen added, “If a dairy has a problem with a yeast strain known to have a similar growth-inhibition pattern in the presence of a bioprotective culture as one of the yeast strains tested in this study, the data from this strain can be used in the model to predict an expected spoilage level relevant for the specific dairy; therefore, the predictive model can be used as a tool that allows the industry to better evaluate the potential of improving control of fungal spoilage by using bioprotective cultures at specific production settings.”

The study presents a valuable tool to assist in management decisions that can help to reduce economic losses due to food waste. Additionally, the methods used for model development can be used further for creating new and improved models.

Earthquakes generated by controlled fluid injection at Colorado’s Rangely oil field were caused by destabilizing fault pore pressure changes, according to a new mechanistic model applied to data from the decades’ old project

The Rangely experiment, conducted from 1969 to 1973, is one of the oldest studies of recorded seismicity due to subsurface fluid injection and is considered to be a pioneering study in the field of induced seismicity. The experiment is being revisited in light of results from studies of the significantly increased seismicity in the central United States and elsewhere due to fluid injection during oil and gas extraction.

During the Rangely experiment, water was injected cyclically into the site’s Weber sandstone, a rock reservoir located 1700 meters below the surface. By 1970, the Weber was producing about 30,000 barrels of oil per day. Data on microseismicity, stress magnitude, and orientation, and reservoir pore pressure were collected before, during, and after the experiment, offering a unique data set for future investigations.

In the Bulletin of the Seismological Society of America, Josimar Silva, Ruben Juanes, John Shaw, and colleagues at Harvard University and MIT took a detailed look at the small earthquakes generated by the Rangely experiment, using new modeling capabilities to examine fluid diffusion and rock poroelastic mechanisms at the site. Most earthquakes generated during the experiment were too small to be felt, with the largest at magnitude 3.1, and originated along one fault running through the field. 

The researchers defined two groups of earthquakes during the experiment—an “upper cluster” close to the injection wells within the reservoir receiving the injected fluids, and a “lower cluster” outside of this injection space. They conclude from their models that upper cluster earthquakes should be considered induced by the injection, but that pore pressure diffusion caused only small changes in stress at the lower cluster such that these events are triggered.

One of the important and somewhat surprising findings from the study said Shaw, “involves the role of the fault itself as a conduit for fluid flow.”

The model suggests that highly permeable rock fractures adjacent to the fault were necessary to create “a permeability pathway through which the fluids could actually flow from the injection site to the sources of many earthquakes,” said Shaw.

“We need to consider in all cases that fault zones are not only subject to the stresses and the fluid pressure changes that may destabilize them or produce earthquakes,” he added, “but they’re also important to understand as the conduits for fluid flow.”

Shaw said the experiment was unique cooperation for the time between Chevron, the operator of the field, and the U.S. Geological Survey. The time was right for Shaw and colleagues to revisit the Rangely data, he said, because “we have the new capability of modeling fluid flow in the subsurface, coupled with a geomechanical model to try to understand how production and injection operations affect the stability of faults.”

The area around the oil field is not tectonically active in the sense of having ongoing natural seismicity, Shaw said, but the experiment and their modeling show how “even small perturbations in the subsurface caused by human activities can actually destabilize faults that have the right orientations and the right properties.”

Shaw and his colleagues plan to apply their model to other regions that could be similarly destabilized by pore pressure changes caused by human activity, such as the Los Angeles basin, and other subsurface injection sites such as carbon sequestration or geothermal power reservoirs.

Their mechanistic model could be combined with other studies of induced earthquakes “to make us more capable of understanding these phenomena and hopefully find ways to manage them,” he said.