The internal quality of pineapple has an impact on both prices and consumer acceptance. The USDA (United States Department of Agriculture) quality standards are based on the amount of defects present within 25 samples, and two categories of decay and damage are defined:
- Internal quality defects, such as softening and tissue browning.
- External quality defects, such as discolouration of crown leaves, mechanical injury, cracks, and insect damage.
However, the water soaked appearance of the flesh is not specifically mentioned. This is a consequence of water presence in the intercellular spaces within pineapple, and is a symptom of a physiological disorder known as translucency. The progress of translucency results in increasing the quality decay, off-flavors, susceptibility to sunburn and mechanical damage.
The research to non-destructively detect pineapple fruit quality has been very limited. Acoustic methods, X-ray computer tomography (CT), Near Infra-red spectroscopy (NIR) have been tested to evaluate internal quality, but not to detect translucency. X–ray imaging is a logical and well-established method to detect the internal quality of many food products, including fruits. It has been used successfully to detect the watercore damage in apples, a problem very similar to the translucency of pineapple. X-rays are able to penetrate the pineapple and reveal the inner characteristics of the flesh, making it a potential method to detect translucency.
X-ray imaging and translucency
The objective of the work of Haff et al. (2006) was to determine whether X-ray imaging is a suitable method to detect translucency in pineapple. In addition, the researchers wanted to demonstrate the feasibility of using X-ray imaging as the basis of a real-time sorting device.
For the research, samples of "Gold" cultivar pineapples with suspected translucency were obtained from Dole Food Company (Westlake Village, CA) for analysis. In addition, pineapples of the same cultivar without translucency were used as a control group. Each sample was exposed to X-rays for 3 minutes and then cut open to determine the amount of translucency present. The pineapples were scored on a scale of 1 to 5, with 1 indicating no translucency, 2 indicating that less than 25% of the flesh was translucent, 3 indicating between 25 and 50%, 4 between 50 and 75%, and 5 for greater than 75% translucency (Figure 1).
Figure 1. Cut pineapples showing the five levels of translucency used in this study.
The results has indicated that X-ray is a potential method to select both pineapples that are translucency-free (95% accurate) and pineapples that suffer from extreme translucency (86% accurate). For intermediate levels of the translucency, the X-rays has resulted in a less accurate method.
Figure 2. Pineapple free-translucency (a), pineapple with translucency (b).
Ultimately, X-ray machinery is not suited for us as a real-time sorting device; a possibility is combining the X-ray system with CCD-based detectors, but this type of line-scan equipment is expensive, about $100,000 per vision unit.
Original study: Haff R.P., Slaughter D.C., Sarig Y., Kader A., "X-ray assessment of translucency in pineapple". Journal of Food Processing and Preservation, No. issue 30, pagg. 527-533, 2006. For more info: ucce.ucdavis.edu/files/datastore/234-715.pdf