The tearing and burning sensations associated with raw onion consumption are caused by (Z,E)-propanethial S-oxide, the lachrymatory factor (LF). The LF is produced from the hydrolysis of S-1-propenyl-L-cysteine S-oxide (PREN), the dominant flavor compound in onions. Current methodology for LF quantification was optimized for Granex-type onions using a 2-min incubation time to allow for maximum formation. In this study, data were taken on PREN hydrolysis of `Dehydrator #3' and `Granex 33' at harvest and during storage and were compared to LF formation. `Dehydrator #3' PREN hydrolysis was 98% complete 5 s after cellular disruption at each sampling date. However, using the 2-min incubation procedure, only 10.25 μmol of LF was recovered from the hydrolysis of 30.11 μmol of PREN at harvest, thereby underestimating LF for this cultivar. Percent PREN hydrolysis for `Granex 33' was lower than `Dehydrator #3' during the enzymatic reaction at each sampling date, suggesting slower PREN hydrolysis activity. At harvest, 6.96 μmol of LF were recovered from 12.54 μmol of PREN hydrolyzed. After 2 storage months, however, micromol of LF were equal to micromol of PREN. LF quantification is currently being considered by the onion industry as a direct measurement of gross onion pungency. This data suggests that more optimization of LF quantification is needed before it can be applied to a broad range of cultivar types.
The lachrymatory factor [LF, (Z,E) propanethial S-oxide] is a direct product of 1-propenyl cysteine sulfoxide (1-PRENCSO) hydrolysis and dominates onion flavor when present in high concentrations. To evaluate LF as a potential means of assessing flavor quality, two onion cultivars were greenhouse-grown and the bulbs stored for 4 months at 3 ± 1 °C, 70% relative humidity. Onions were evaluated at monthly storage intervals for LF development in bulb macerates following a 120 seconds incubation time. When LF was compared to amounts of 1-PRENCSO hydrolysis, we found that LF was severely underestimated. The relationship of LF and 1-PRENCSO also varied between cultivars during storage. As `Granex 33' was stored for longer periods, the amount of LF measured at 120 seconds more closely reflected the amount of 1-PRENCSO hydrolyzed. LF from `Dehydrator #3', however, was consistently underestimated regardless of storage time. Therefore, a second experiment was conducted using individual bulbs of two onion cultivars in an attempt to determine the optimal incubation time for LF quantification. Maximum LF among bulbs was generally detected 5-10 seconds after tissue maceration for `Dehydrator' and after 15-30 seconds for `Sweet Vidalia'. The amount of LF quantified between 5 and 120 seconds decreased linearly for nine of ten bulbs of `Dehydrator', but this trend was less apparent for `Sweet Vidalia'. A uniform LF incubation time for individual bulbs, therefore, may not be possible for all cultivars. These data show a complex relationship among and within onion cultivars for 1-PRENCSO hydrolysis and the formation of LF in onion macerates.
A spectrophotometric assay for pyruvic acid in onion has been adapted to a microplate reader. Correlations between the spectrophotometer and microplate reader ranged from 0.991 to 0.997 for sodium pyruvate standards and 0.899 to 0.934 for onion samples. Onion pungency values were slightly higher with the microplate reader for both sample and background compared to the spectrophotometer when both are used in the single wavelength mode. Comparing the spectrophotometer in the single wavelength mode to the microplate reader in the dual wavelength mode resulted in no statistically significant difference between them. Standards for both the microplate reader and spectrophotometer followed a quadratic function.
Onion pungency is a major quality attribute with many consumers demanding less pungent onions. In recent years, some growers and retailers have attempted to measure pungency of onions produced in different regions to guarantee a desired level of pungency. However, there are few data on the variability among laboratories using standardized protocols to estimate relative levels of pungencies. Onion cultivars were grown in replicated trials at three locations. Random samples of bulbs from each experimental unit were harvested and shipped to at least three cooperating laboratories, each of which measured soluble solids content (SSC) and pungencies using the same techniques. As expected, cultivars and environments showed significant (P < 0.001) differences. For all three trials, laboratories were a highly significant source of variation (P < 0.024 to 0.001) for measurements of SSC and pungency. Therefore, one cannot make recommendations on relative pungencies of the same lots of onions measured by different labs. The onion research community must identify specific procedures to reduce variation among laboratories to develop a more repeatable standardized assay for the measurement of onion pungency.