Pork Quality Variation In Different Condition

Bitong Wang

Introduction

Meat, which is broadest defined as animal tissue used as food; is composed of tissue or muscle fiber cells, fat and connective tissue.  Meat is mainly composed of proteins.  The quality of the meat is mainly determined by the chemical composition and physical structure of the tissue. 

There is well known for people that the fresh meat is better than chilled meat.  The meat is usually eaten after cooked, and the expired meat could not be eaten in normal condition.  Pork meat is classified as a "red" meat and it is the most widely eaten meat in the world.  There have been a lot of researches about the effect of the freeze time on the quality properties and structure of pork.  However, there are limit researches focus on the topic about how the quality and structure changing in expired meat and cooked meat.  So there is a interesting question, why the expired meat could not be eaten?  In order to find the difference in structure and chemical composition between the meat under different conditions, scan electron microscope (SEM) and energy dispersive x-ray analysis (EDX) are good approaches.

SEM is a very useful method for observing microstructure of samples; it produces images by scanning a sample with a electrons beam.  The images contain the information of sample's surface topography and composition.  In EDX analysis, the x-rays are arranged in a spectrum by their energy.  From the spectrum, one can derive qualitative and quantitative information about the chemical composition of the sample.

In this project, fresh, cooked, frozen and expired meats were dehydrated by CPD method.  After coating process, they were imaged in SEM.  For SEM observation, the SE2, Inlens and BSD detector were used individually to characterize those samples surface morphology.  The chemical compositions of samples were analyzed by energy dispersive x-ray.  In addition, some images were colorized by Photoshop to make them vividly.

Materials & Methods

A piece of pork meat had bought from Wal-Mart with expiration date about a week.  The meat was divided into four parts. One part as the fresh sample and it had been dehydrated and imaged immediately.  The second part of meat had been boiled in water as the cooked sample.  The third part meat had been frozen in refrigerator as the frozen sample.  The last part had been stored in nature condition as the expired sample.  The last two parts of meat were observed in SEM after two weeks store.

Sample preparation

Dehydration

For most biological samples, dehydration process is indispensable before SEM observation.  The purpose of dehydration is to help the sample to be compatible with the vacuum system in SEM.  In this project, the meat samples were dehydrated by CPD method.

The meat samples were first fixed using 2.5% gluteraldehyde solution for about 30 minutes, and then ethanol was used to replace water in graded series (30% 50% 70% 95% three times, 100% three times).  In CPD machine, the ethanol is replaced by liquid CO2, then the temperature and pressure is changed to past the critical point and the CO2 is transformed to gas. 

Coating

Conductivity is the most important reason for coating a sample.  In this project, the meat samples were coated with gold.  The conductive carbon ink was used to to make the samples grounding.

Imaging Process

The as-prepared samples were observed in SEM under SE2, Inlens and BSD modes individually to characterize the surface morphology.  Also, each sample was analyzed by EDX spectrum to illuminate the chemical composition.

Results

The images of various meat samples are shown below.

Figure 1 shows the difference between four kinds of meats in low magnification.  (a) is a BSD image which shows the surface structure of the fresh meat.  There are some wrinkles on the surface.  (b) is a BSD image of the cooked meat surface.  Obviously, the tissue of the meat are become bunch or strip shape.  The reason might be that the inside water is lost during boil so that the protein tissue are shrink into a bunch.  (c) shows the surface of frozen sample under SE2 mode.  This image indicates that the surface morphology doesn’t have a remarkable change between the fresh sample and frozen sample.  (d) is the SE2 image of the expired sample.  Compare to the fresh one, the expired meat surface topography is totally changed.  The tissues seem like many bumps and there are some molds distribute on the surface.

After colorization, the images of each sample are shown below.

Fresh meat sample

Figure 2. BSD image of fresh meat surface, low magnification

Figure 3. SE2 image of tissue.  left 5kx, right 11kx

Cooked meat sample

Figure 4. cooked meat surface structure, low magnification.  left BSD, right SE2

Figure 5. image of tissue. left BSD 10kx, right SE2 10kx

Frozen meat sample

Figure 6. SE2 image of frozen meat surface, low magnification

Figure 7. SE2 image of tissue. left 5kx, right 7kx

It is very interesting that for the frozen sample, we could find some areas which are extraordinary smooth and flat.  We can see from the right image of figure 7, no tissue or mold exit on that areas.

Expired meat sample

Figure 8. SE2 image of expired meat surface, left 100x, right 360x

Figure 9. BSD image of expired meat tissue, 10kx

Figure 10. SE2 images of bacteria on the expired meat surface. left 5kx, right 7kx

From figure 10, we can see there are many bacteria or microorganism distribute on the tissue.  I think that is why the expired meat could not be eaten.

In the SEM imaging process, we found an very interesting things that the electrons beam can destroy the surface structure and tissue of the meat.  In other words, the meat is unstable to the beam.  Please see the follow images and video.  

Before dig a hole

Dig a hole

After dig a hole

Also, I uploaded a video about the process of digging a hole, please open this website:

https://www.youtube.com/watch?v=cjSzpCZ-vRA

From the images and the video, we can see the beam could dig a hole on the meat surface!

The EDX spectrum are shown as follow images.  All EDX are analyzed at 20 KV.  It could be found that, for fresh meat, the chemical composition mainly are C N O P S and some metallic elements.  The Au element originated from the coating.  After cooked, the content of N and O are decreased.  For the frozen sample, there is no big change comparing to the fresh one.  However, for the expired sample, the content of N and O, especially for O, have a remarkable increasing.

a. EDX spectrum for fresh sample.   b.EDX spectrum for cooked sample.  c.EDX spectrum for frozen sample.  d.EDX spectrum for expired sample.

Conclusion

The pork meats in different condition were studied in this project.  The surface morphology of meats were displayed in SEM images.  Results show that after cooking or expiration date, the meats’ surface morphology will be changed.  EDAX spectrum analysis shows the chemical composition of meat.  Comparing to other meats, the content of O element is increasing remarkable for expired meat.  It is reasonable to conclude that freeze is a good method to store meat, and the quality of expired meat which is stored in normal condition will decay. 

Reference

McIntyre Brian. Electron Beam Methods In Microscopy. Rochester, NY.