In or On the Filet?

fish-filetOne of my favorite cases our team has worked on could be summarized as, “In the filet or on the filet?”

The story begins with a healthy, elderly man eating a fish filet sandwich in a hospital cafeteria. Somehow, he swallowed a large, sharp, flat piece of plastic. This plastic shard became lodged in his chest cavity, and resulted in a half million dollars in surgery to remove this piece of plastic and repair the damage. As a result, the case was taken to court.

Polymer Solutions was asked to determine if the plastic was applied to the sandwich by cafeteria workers (i.e., co-mingled with lettuce and tomato slices) or if it was embedded in the fish filet.

The case had two main steps:

1. Determine the material identification of the shard of plastic

2. Determine how the shard of plastic would behave during various cooking conditions

The physical evidence was a single shard of plastic, surgically removed from the man’s body. It had very sharp corners and was an inch by an inch-and-a-half, and translucent with a wavy surface.

Round One: Non-Destructive Testing

First, we needed to identify the plastic shard. This was difficult to do because, initially, we were only allowed to perform non-destructive testing. We began with a qualitative comparison of the shard to samples of every piece of plastic found in the cafeteria. Lids, cups, sour cream containers, creamer cups, straws, and other plastic materials arrived at PSI. Size, color, thickness, stiffness, and transparency were all compared against the shard of plastic to either rule them in or out as a possible source. Through this qualitative analysis, we were unable to locate any materials within the cafeteria that were a likely source.

Next, we performed non-destructive Fourier Transform Infrared Spectroscopy (FTIR) testing. This led to the preliminary conclusion that the plastic was a polyamide, such as nylon. We suspected that the plastic had absorbed cooking oil, and that this might lead to a false identification as a result.

We approached the courts with this information and were granted permission to begin a quantitative analysis. This required destructive testing. The courts allowed us to remove a 1/8″ by 1/8″ square from the shard.

Round Two: Destructive Testing

We used FTIR, Thermogravimetric Analysis (TGA), and Differential Scanning Calorimetry (DSC) as our main analysis techniques. It was critical that the FTIR and DSC provide consistent results, pointing to the same material identification.

The plastic shard had a glass transition of 80°C and no melting peak from room temperature to 200°C, which means there was no significant level of crystallinity in the polymer. Therefore, the polymer could not be nylon. We concluded that the polymer was in fact a polyvinyl chloride (PVC) Polyvinylacetate (PVAc) blend. PVC/PVAc is often used for rigid thermoformed packaging. The TGA method verified that the plastic shard contained no fillers.

After determining the material identification of the plastic shard, we purchased materials from a commercial supplier for a series of experiments, with a FTIR spectrum that matched the FTIR spectrum for the plastic shard.

We knew the material had been exposed to a cooking process because it was slightly discolored, which was indicative of heat. Based on the deposition of the cafeteria worker, we knew the fish filets were typically cooked for 5 minutes at 375°F. PSI purchased a deep fat fryer, several pounds of the fish fillet product, and conducted a series of experiments.

Round Three: Cooking the Samples

Our goal was to test the response of the plastic material (cut to be an exact replica of the swallowed shard) during cooking when on the fish filet versus in the fish filet. If the shard had not been in the filet and was merely floating in the cooking oil, the plastic would not survive. After 7, 5, or even 3 minutes immersed in cooking oil, the shape changed significantly, the plastic curled, and it remained transparent. When the plastic was contained in the filet, however, it could survive being cooked at 375°F for 3, 5, or even 7 minutes. The shape of the material did not change significantly; even the sharp corners remained sharp. Of note, the material was transparent prior to cooking but after the cooking process it became translucent. Cooking also turned the smooth surface wavy.

Our science-based conclusion: A piece of PVC/PVAc had been embedded in the fish filet. This absolved the cafeteria of any blame and turned the focus onto the fish filet producer. Mystery solved.