Source: Mettler ToledoSource: Mettler Toledo

Fresh fruits and vegetables are essential for a healthy diet — they provide necessary nutrients and taste great too! However, immediately consuming them after harvesting is not always possible. Therefore, many fruits and vegetables undergo preservation techniques such as canning, pickling or freezing to prolong their shelf life. But how do we ensure these preserved products are still nutritious and fresh? The answer lies in pH levels. Monitoring and maintaining the pH of these foods is vital to ensure their freshness, nutritional content and overall quality.

Most fruits have low to medium acid-level pH values. Fruits are rich in organic acids like citric acid, lactic acid and malic acid. These acids are responsible for the tart and tangy taste of fruits. A scientific study suggests that when a fruit over-ripens and its sugars ferment due to microbial action, additional organic acids such as lactic acid and acetic acid are generated along with ethanol. These organic acids can cause off-flavors and spoilage in the fruit, in addition to the typical organic acids that form the fruit's flavor. On the other hand, vegetables usually have medium to moderate pH levels, ranging from acidic to alkaline.

Importance of measuring the pH of fruits and vegetables:

Source: Mettler ToledoSource: Mettler Toledo

The pH level of fruits plays a key role in determining the suitable processing stages for making various preserves such as jelly or jam. Choosing the right canning techniques based on the pH level is essential to preserve fruits and vegetables seasonally.

Measurement of pH:

The pH measurement is done using a pH meter and an electrode. However, measuring the pH of fruits and vegetables using conventional pH sensors is quite challenging due to various factors. The major factors are listed below,

1. The solid texture of fruits and vegetables can make it challenging for the electrode to penetrate the sample for pH measurement. Delicate sensing membranes can suffer abrasions or disturbance of the gel layer during measurement. To prevent this, certain laboratories utilize indirect methods, where the sample is mixed with water before testing. However, this blending and homogenizing process can impact the pH measurement results, leading to inaccuracies.

2. Restrictions on the interaction of a solid sample with the reference electrolyte in ceramic junction-based pH sensors can result in clogging of the junctions.

3. During repetitive pH measurements, the sensor membrane may become fouled due to the accumulation of various contaminants. This can make it difficult to wash off the contaminants properly.

Source: Mettler ToledoSource: Mettler Toledo

METTLER TOLEDO's InLab Solids Pro-ISM sensor ensures direct sample measurement of solid fruit and vegetable samples, without the undue blending of the sample with water. The sensor design also enables a faster and more accurate analysis with adept functional components like an open junction, solid electrolyte, and a sensitive pH glass membrane. It has minimal maintenance, and secondly, it has a clog-free open junction, preventing sensor fouling.

Good electrochemistry practices to follow during the measurement:

1. The pH range for the mentioned sensor is 1 to 11 pH units and hence should not be exposed to harsh acidic (below pH 1.00) or alkaline (above pH 11.00) solutions.

2. Clean the electrode surface after each pH measurement to prevent contamination and ensure a fast sensor response.

3. After taking the sample measurement, thoroughly clean the electrode surface using a mild soapy solution and rinse it with de-ionized water to remove any residues.

4. Always use fresh buffers and ensure their correct use in sequence. Check their expiry date.

5. It is crucial to avoid storing the electrode dry or in distilled water as this can cause damage to the pH-sensitive glass membrane and shorten the electrode's lifetime.

Source: Mettler ToledoSource: Mettler Toledo

METTLER TOLEDO offers an application note on “pH Measurement of Fruits and Vegetables,” providing step-by-step guidance for pH analysis of fruit and vegetable samples. Download to learn more. You can also visit our website for the right pH tool for your applications.

To contact the author of this article, email