Will The Brown Marmorated Stink Bug Be A Problem In Wine And Juice?
By: Jody Timer, Research Technologist
The brown marmorated stink bug (BMSB), Halyomorpha halys (Stal) is an invasive species that has become a major pest in the eastern United States. This pest originally became a sizable problem in Mid-Atlantic vineyards, including southwestern Pennsylvania, during the 2010 growing season and continues to be a large-scale problem. The Lake Erie grape belt is the largest Concord grape growing region in the world. The recent appearance of the brown marmorated stink bug in Lake Erie vineyards has the potential to become problematic. After the mild winter of 2015-2016, the numbers of BMSB in this area began to increase rapidly. This winter, frequent complaints have been received from homeowner concerning the presence of BMSB in their houses.
BMSB have been found in both grape foliage and grape clusters; they seek the moisture, sugar, and warmth on the inside the clusters (especially overnight) and they often migrate to the cluster’s interior close to harvest. This makes the possibility of BMSB inside the cluster very likely when these grapes are mechanically harvested and transported to the processor.
All BMSB life stages (5 instars) have been observed in vineyards indicating that grapes are a suitable crop for BMSB development, and all stages have been found to cause direct damage to grapes (Bernon 2004). At the Lake Erie Grape Laboratory, we have maintained an adult BMSB colony on a diet of Concord grapes with no apparent development problems. It has also been estimated that the presence of 5 BMSB per grape cluster may lead to 37% loss in grape yield as a result of BMSB damage (Smith et al. 2014). With the yearly increase of numbers of BMSB in the Pennsylvania vineyards, the possibility of BMSB tainting the juice produced in this area is becoming a primary concern to processors, growers, marketers, and consumers.
Insects produce small, volatile molecules that may be imparted to juice and wine during crush. Humans are able to detect these molecules at extremely low concentrations. For example, 2-isopropyl-3-methoxypyrazine (IPMP) from Multicolored ladybeetles (ladybug taint) is detectable at 0.30 ng/L concentrations in Concord grape juice (Pickering et al. 2008). BMSB have a distinctive odor which has been described as green, cilantro-like, which may or may not be off-putting within grape juice aroma. BMSBs produce taint associated compounds as allomones, alarm pheromones, aggregation pheromones, and kairomones. These compounds are mainly released from the dorsal abdominal glands in nymphs and paired metathoracic glands in adults (Baldwin et al. 2014). When BMSBs are crushed along with grape clusters, release of these compounds could potentially taint in the juice.
Multidimensional gas chromatography mass spectrometry (MDGC-MS) analysis of stressed BMSB, adults and nymphs, has been used to identify more than 39 compounds. The volatile compounds in the taint are tridecane, dodecane, trans-2-decenal and trans-2-undecen-1-ol. Tridecane and trans-2-decenal together constitute at least 70% of BMSB taint (Baldwin et al. 2014; Solomon 2013). Trans-2-decenal, is the major irritant and is believed to be responsible for the potent stink odor from BMSB. However, being an extremely unstable compound it can easily break down, and its degradation products lack the distinctive BMSB odor (Baldwin et al. 2014). When trans-2-decenal was added to red wine (Pinot Noir) the morning of testing, the detection threshold was in the low microgram per liter (ug/L) range (Mohekar et al 2015). However, because trans-2-decenal is unstable, this may not reflect what happens when juice is processed and stored. Joe Fiola from University of Maryland, reported that while 5-10 BMSB per 25 pound lug of white grapes imparted a perceptible taint in raw juice for up to four months in some cases, the taint was not perceptible in the wine after fermentation (Fiola 2011). Mohekar and colleagues, from Oregon State, reported that trans-2-decenal, has a detection threshold in the ug/L range in Pinot Noir wines, and was able to be detected by tasters (Mohekar et al 2016).
Informal sensory testing with the on-site staff was performed at the Lake Erie Regional Grape Research laboratory. Small batches of grape juice were produced using a residential Kitchen Aid® juicer, and increasing numbers of BMSB were added. Batches of juice containing the equivalent of 0, 2, 10, and 25 BMSB per lug (~35 lbs. of Concord grapes) were tasted raw and after high-temperature, short-time pasteurization (HTST) by five staff members. In blinded triangle tests (2 blanks and 1 spiked sample), 5 of 5 individuals correctly identify the spiked sample of the 25 BMSB/lug juice (both raw and pasteurized); for the 10 BMSB sample, 4 of 5 correctly identified the spike in raw juice, and 5 of 5 identified the spiked sample for the pasteurized juice. The following month the pasteurized juice was re-tasted by 10 individuals and similar results were obtained. These tests suggest 25 BMSB/lug are sufficient to induce a perceivable flavor change in Concord grape juice. Following this testing, a set of samples were processed at the Food Science laboratory of Penn State, using industrially relevant methods (equivalent to Welch’s Corporation processing techniques), to assess if the odor-causing compounds secreted by BMSB were stable enough to transfer through the juicing, processing, pasteurization, and storing of grape juice and therefore cause it to be rejected by consumers. Large scale sensory tests were then run with regular grape juice consumers to quantify rejection thresholds for BMSB-spiked, processed grape juice.
Grape clusters, with varying amounts of BMSB (0, 4, 8, 16, 24, and 32 BMSB, per 35 lbs), were crushed and destemmed; the juice was pasteurized, clarified and blended. Following storage for 8 weeks (2 months after initial processing), sensory testing was performed with grape juice consumers in the controlled sensory testing facility to determine rejection thresholds for different levels of BMSB. Sensory testing was then repeated 8 weeks later (4 months after initial processing).
Despite the use of BMSB levels that clearly caused a noticeable change in the flavor of processed juice in pilot testing, we were unable to find a level of BMSB that caused rejection in consumers following processing and storage at either time point.
As the area’s BMSB population increases, this research will be notably important for processor to determine threshold levels at harvest. This research suggest that taint from BMSB at high but realistic doses for the Lake Erie grape-growing region will not influence consumer acceptability. Conversely however, our results may not generalize to smaller producers in the area who make grape juice using exclusively from Concord grapes. Typically, this juice is sold as fresh juice with a limited shelf life. These smaller growers and producers should be aware of the possibility of taint in their juice especially with the increasing numbers of BMSB in the region. Pre-harvest scouting for BMSB should be conducted to determine if a pre-harvest BMSB insecticide spray should be applied.
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