Quantification of sugars and organic acids in tomato fruits

Abstract

Sugar and organic acid contents are major factors for tomato fruit flavour and are important breeding traits. Here we provide an improved protocol for accurate quantification of the main sugars, glucose and fructose, and the organic acids, citric acid and malic acid, present in tomato. The tomato extract is spiked with lactose and tricarballylic acid as internal standards and loaded onto a NH2 solid phase extraction (SPE) column. The sugars appear in the flow-through and are subsequently analysed by HPLC using a Nucleodur NH2 column and a refractive index detector. The organic acids bind to the SPE column and are eluted with 400 mM phosphoric acid. For analysis, the organic acids are separated by HPLC using a Nucleodur C18ec column and detected by UV absorption at 210 nm. The method shows excellent inter-day and intra-day reproducibility for glucose, fructose and citric acid with standard deviations of 1-5%. Quantification of citric acid by HPLC and GC-MS showed perfect agreement with a deviation of less than 3%. •Simple method for quantification of glucose, fructose, citric acid and malic acid in tomato.•Efficient removal of interfering compounds by solid phase extraction.•High intra and inter-day reproducibility.

Keywords: Citric acid; Fructose; GC–MS; Glucose; HPLC; Malic acid; Quantification of sugars and organic acids by HPLC and GC–MS; Solid phase extraction.

Graphical abstract

Fig. 1

Analysis of sugars by HPLC. (A) Chromatogram of a standard containing the indicated sugars. (B) Chromatogram for ripe Heinz 1706 tomatoes. (C) Chromatogram for green, unripe Heinz 1706 tomatoes. (D) Calibration curves for glucose (red) and fructose (blue).

Fig. 2

Analysis of organic acids by HPLC. (A) Chromatogram of a standard containing citric acid, malic acid and tartaric acid. The fumaric acid peak visible at 12.7 min originates from an impurity in malic acid. (B) Chromatogram for ripe Heinz 1706 tomatoes. (C) Chromatogram for green, unripe Heinz 1706 tomatoes. (D) Calibration curves for citric acid (red) and malic acid (blue).

Fig. 3

Effect of SPE on analysis of sugars by HPLC. (A) Chromatogram of a sample of ripe Heinz 1706 tomato spiked with lactose as internal standard (IS). (B) Same as (A) but green, unripe Heinz 1706 tomato was used. (C) Same as (A) but no internal standard was added. (D) Same as (B) but addition of the internal standard was omitted. (E) The same sample as in (A) was injected without prior purification by SPE. (F) The same sample as in (B) was injected without prior purification by SPE. (G) The same sample as in (C) was injected without prior purification by SPE. (H) The same sample as in (D) was injected without prior purification by SPE.

Fig. 4

Effect of SPE on analysis of organic acids by HPLC. (A) Chromatogram of a sample of ripe Heinz 1706 tomato spiked with tricarballylic acid as internal standard (IS). (B) Same as (A) but green, unripe Heinz 1706 tomato was used. (C) Same as (A) but no internal standard was added. (D) Same as (B) but addition of the internal standard was omitted. (E) The same sample as in (A) was injected without prior purification by SPE. (F) The same sample as in (B) was injected without prior purification by SPE. (G) The same sample as in (C) was injected without prior purification by SPE. (H) The same sample as in (D) was injected without prior purification by SPE.