Dear Caner,

In order not to destroy the potatoes and to preserve time I suggest to follow Chen's method "Nondestructive Determination of Sugar Content in Potato Tubers Using

Visible and Near Infrared Spectroscopy " published in Japan Journal of Food Engineering, Vol. 11, No. 1, pp. 59 - 64, Mar. 2010:

2.1 Experimental sample preparation

Potatoes of one cultivar (May-Queen) were purchased

Nondestructive Determination of Sugar Content in Potato Tubers Using

Visible and Near Infrared Spectroscopy

Jie Yu CHEN1,†, Han ZHANG1

, Yelian MIAO2

, and Mitsunaka ASAKURA1

1

Faculty of Bioresource Sciences, Akita Prefectural University

241-438 Kaidobata-Nishi, Shimoshinjo-Nakano Akita-shi, Akita 010-0195, Japan 2

College of Food and Light Industrial Engineering, Nanjing University of Technology, Nanjing 210009, China

Near infrared (NIR) spectroscopy was investigated as a method for the nondestructive measurement of sugar content in intact potato tubers. The NIR spectra (400-1100 nm) of potato samples were acquired by fiber optics in the interactance mode. Calibration models for the prediction of the fructose and glucose contents were developed by a partial least squares statistical analysis method. The calibration model gave standard errors of prediction of 0.26 mg/g for fructose and 0.46 mg/g for glucose. The preliminary results showed that NIR spectroscopy gives a reasonable estimate that can be used for nondestructive sorting of potatoes according to their sugar content. 

Key words: nondestructive determination, sugar, potato, near infrared spectroscopy, 

Experimental sample preparation

Potatoes of one cultivar (May-Queen) were purchased on the 7th of September, 2008 from the Akita fruits and vegetables distribution market and were used as the

experimental samples. The potato samples were grown at locations around Hokkaido, Japan. A total of 100 potato samples were used. Potato samples were individually separated into two groups. To provide a wide range of sugar concentrations in the sample groups, one group was stored at 5℃ for 4 months, and the other group was stored at 25℃ for 4 months. At biweekly intervals, five

tubers were removed from each storage room for testing

For more details see attached file.

Hoping this will be helpful,

Rafik

Dear Caner,

Your question is very important and not just for carbohydrate analyses.

Professor Karaman has provided an excellent alternative to HPLC. (And if I were doing chromatography, I would choose HPAEC-PAD for the better sensitivity and resolution vs. an aminopropyl column with RI detection.)

But the real question is how to extract biologically labile compounds from samples. A potato (or a leaf or a chunk of liver tissue) may look inert but there are enzymes in it that are waiting to be released. Upon release, those enzymes will start to change the composition of the sample. Proteins will be hydrolyzed, amino acids will be decarboxylated, secondary plant metabolites will lose their carbohydrate portions and sugars will be both formed and degraded. Compounds become oxidized and may start to bind to proteins. This reduces the amount of the compound while also causing the loss of enzyme activity.

I worked on cyanogenic glycosides from plants. If I had homogenized peach pits (especially in water or buffer), I would not be able to isolate amygdalin, the actual secondary metabolite. All I would have found was benzaldehyde, hydrogen cyanide and a little extra glucose.

The method you mentioned, freezing with liquid nitrogen and then lyophilizing, is gentle to the biological compounds in the sample but can be time consuming. With proper, cold grinders, the frozen samples can be ground to a powder, which is easier to lyophilize. You asked if you could just homogenize the sample. You could but I would doubt the veracity of your results. Potatoes, as well as many fruits, are high in water content. Homogenizing them turns the tissue into a slurry and that means that chemical and biochemical reactions can start. Plant enzymes are active at room temperature. (Sometimes animal tissue is a little easier to work with since most of those enzymes work better at elevated temperatures. Also, animal tissue has fewer phenolic compounds that are easily oxidized.) The secondary plant metabolites in plants can be very reactive with enzymes and oxygen. Note how fast a cut apple browns. Phenolic compounds in the apple are being oxidized by polyphenol oxidase to O-quinones (meaning that the concentration of the original phenolics is falling). The colorless O-quinones react with amines proteins (including enzymes) and are further oxidized to brown compounds.

In any case, if you do decide to use chromatography, make sure to account for matrix effects by using the method of standard additions.

One other process to consider: When I had to work with secondary metabolites in yams, I used fresh yams and sliced them into very thin slices with an automatic slicer like you might find in a delicatessen. But I had a bowl of liquid nitrogen under the slicer so that the thin slice would fall directly into it. This stopped enzymatic processes but also provided a sample that lyophilized more easily and was easier to work with.

Joe