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NMR Relaxometry Reveals Turkish Delight’s Sticky Fingerprints

Cubes of Turkish delight dusted in powdered sugar.
Credit: Engin Akyurt, Pixabay.
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Scientists have shown the value of time-domain nuclear magnetic resonance (TD-NMR) relaxometry in evaluating the physical properties of Turkish delight made with syrups and sweeteners as alternatives to sugar.

The multicentered team, led by Dr. Pelin Pocan, then based at the Department of Food Engineering, Middle East Technical University in Turkey, published their findings in ACS Food Science and Technology.

Striving for sweet perfection

Turkish delight is a sweet confection enjoyed by many and, while its recipe is protected to some degree by law, producers are free to use different types and amounts of sugars and syrups to alter product characteristics, shelf-life, cost and calorific value. But achieving the perfect consistency can involve a lot of trial and error. In addition to analyzing basic attributes such as color and moisture content, product developers have previously used methods such as X-ray diffraction (XRD) to examine the crystallinity changes of different recipes, however, experiments can be laborious and require careful interpretation.

Therefore, researchers have been looking at the potential of TD-NMR relaxometry as an alternative technique to detect changes in mixtures, including crystallinity, and examine gel properties and solid–water interactions.

Can alternatives measure up to the traditional?

The team prepared Turkish delight recipes using different types of corn syrups or allulose syrup, a novel, low-calorie sweetener with promising health implications, at different substitution levels (30, 45 and 60%) as the sugar source, with powdered sugar (sucrose) only used in the control and a commercial sample also included for comparison.

Measurements of refractive index, moisture, color and texture profile were made, XRD analysis performed and TD-NMR relaxometry measurements were taken using conventional and non-conventional methods. Magic sandwich echo (MSE) was used as a non-conventional method to detect sample crystallinity.

The key findings of the paper were:

  • The utilization of different types of syrups in varying proportions led to detectable changes in the textural properties of the Turkish delights.
  • Differences in moisture content were observed between formulations, although none exceeded the national legislation limit of 16%.
  • Turkish delights containing allulose had the weakest gel-forming properties, with 30% allulose syrup and 30% sucrose found to have an amorphous structure.
  • Significant changes to relaxation times were seen in response to the use of different syrup types and substitution levels.
  • Second moment (M2) values measured from the signal acquired using an MSE pulse sequence were an effective indicator of Turkish delight crystallinity.

Technology delivers good news and bad

The findings clearly show that altering the sweetening ingredients and relative proportions can have substantial impacts on Turkish delight’s physical characteristics. This is an important consideration for producers when making any recipe alterations. While not exceeding the limits of legislation, increases in moisture content can limit shelf life and affect texture.

While allulose may offer an attractive low-calorie alternative to sugar in recipes, the soft texture and weak gel formation exhibited by allulose-containing formulations suggest care must be taken with its use in Turkish delight products.

XRD analysis provided excellent crystallinity information on the samples but was time-consuming and open to human interpretation. On the other hand, TD-NMR performed well in describing the physical properties of the samples and the use of MSE shows great promise as an alternative to XRD in analyzing crystallinity.

The changes seen in the relaxation spectra between samples also opens doors to their use as a fingerprint for Turkish delight constitution.

While there are no legislative rules about the texture and appearance of Turkish delight, aesthetic and textural aspects such as elasticity and cohesiveness are clearly important to make them appealing to consumers. Thus, being able to determine these physical characteristics during product formulation is of key importance.

The study indicates that, alongside basic measures of aspects such as moisture content and color, TD-NMR relaxometry makes a valuable contribution to product evaluation.

It is interesting to note that previous studies have found increasing allulose content decreased product moisture content, thus impacting the physical properties, as allulose interacts less with water than other sugars, but the current study found the opposite. This, they attributed to the presence of 5% glucose in the allulose syrup used in this study. Given the impact allulose appeared to have on the product’s texture, it is important to determine if these observations are still true in the absence of glucose or if the glucose may have contributed to the undesirable textural behavior of the allulose-containing Turkish delights.

The authors discuss color differences between the test sample types but acknowledge that the cooking method used can also impact this attribute, and so may not reflect observations made were the recipes to be made commercially. Also, while experimental samples were tested straight after production, this was not possible for the commercial sample, potentially impacting textural comparison.

A new tool in the confectioner’s armory?

Wider testing using a pool of commercially-relevant samples will help to establish TD-NMR relaxometry as a valuable tool in Turkish delight development. With a drive from many consumers to move to lower calorie alternatives, there is potential scope for investigation of the technique as a tool in other areas of confectionary testing too. The authors put forward hypotheses for the changes in physical properties observed as the types and proportions of the sweeteners were altered. Further experiments are needed to investigate these phenomena at the molecular level.

Reference: Pocan P, Grunin L, Oztop MH. Effect of different syrup types on Turkish delights (lokum): A TD-NMR relaxometry study. ACS Food Sci Technol. 2022;2(12):1819-1831. doi:10.1021/acsfoodscitech.2c00222