Rapid Assessment of Deep Frying Oil Quality as Well as Water and Fat

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Rapid assessment of deep frying oil quality as well as water and fat contents in French fries by low-field Nuclear Magnetic Resonance Chen Wang, Guanqun Su, Xin Wang, and Shengdong Nie J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b05639 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 13, 2019

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Journal of Agricultural and Food Chemistry

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Rapid assessment of deep frying oil quality as well as water and fat contents in French

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fries by low-field Nuclear Magnetic Resonance

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Chen Wang†, Guanqun Su†, Xin Wang§, Shengdong Nie

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School of Medical Instrument and Food Engineering, University of Shanghai for

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Science and Technology, Shanghai 200093, China

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†

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authors

These authors contributed equally to this work and should be considered co-first

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§

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Email: [email protected]

Corresponding author. Tel.: +8618918629281 (Xin Wang)

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Highlights

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1. Propose a rapid and non-destructive method for water and oil content in French fries.

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2. Proton populations were identified and assigned to water and oil.

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3. Correlation between the relaxation parameters and the water and oil content were

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acquired.

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4. TPC content of frying oil could be predicted by LF-NMR relaxation characteristics.

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5. Oil and water changes were visualized by magnetic resonance imaging.

ACS Paragon Plus Environment


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ABSTRACT

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Most of the health hazards in fried foods are related to unqualified frying oil and excessive oil

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content. In this study, the feasibility of using low-field nuclear magnetic resonance techniques (LF-

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NMR) for analysis the water and oil contents in French fries, as well as simultaneous evaluation of

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frying oil quality during deep-frying was investigated. Three proton populations were identified and

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successfully assigned to water and oil relaxation signals. Significant correlation between the T2

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relaxation parameters (Awater and RCoil) and the water and oil content was acquired. MRI could

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visualize the changes of signal intensity and spatial distribution, as well as the internal structural

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changes during frying. Using the correlation model built by multiple regression analysis, the total

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polar compounds (TPC) content of the frying oil could be successfully predicted by LF-NMR

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relaxation characteristics, which indicates that LF-NMR was an effective method to monitor the

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quality of frying oil.

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Key words: LF-NMR; French fries; quantitative analysis; MRI; TPC

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INTRODUCTION

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French fries are probably used as the most typical model of fried foods for many studies. After high

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temperature frying, it exhibits two very distinct textures, which were recognized as desired taste

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properties: a “crispy” crust, with similar physical characteristics to potato chips, and a “firm-mealy”

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core, with some of textural properties of boiled potato. Meanwhile, the quality of frying oil also

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affects the texture and taste of the outer shell1-3. Thus, the water evaporation and oil absorption

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during the frying process are important for the aroma and crispy texture of French fries. On the other

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hand, excessive absorption of oil in fried foods may have adversely effect on health, including

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potential risk of cancer, cardiovascular and cerebrovascular diseases and obesity4. Furthermore, due

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to the complex physical and chemical changes of the fatty medium, such as polymerization,

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oxidative and hydrolysis, the toxic compounds formed gradually, which also cause important

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repercussions on health5-7. Therefore, the analysis of water and fat contents as well as the quality of

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frying oil is necessary and meaningful for producing healthier French fries.

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Concerning the quantification of water and fat in fried food, oven drying8 and Soxhlet extraction9

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have been widely employed as traditional analytical methods. While for the evaluation of stability

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and quality of oils during frying process, the contents of total polar compounds (TPC) were

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considered to be the most reliable indicator, and the silica gel column chromatography10 as well as

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high performance liquid chromatography-size exclusion chromatography (HPLC-SEC or HPSEC)11

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are the most conventional methods for TPC detection12. However, it cannot be denied that these

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methods are somewhat time consuming, require hazardous chemicals and complicated sample

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pretreatments, which limits their applicability for routine monitoring in a sustainable way.

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As a rapid and non-destructive analysis technology, hydrogen spectrum low-field nuclear 3



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magnetic resonance (1H LF-NMR) has been widely used in food quality control and material

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property measurements13-16. The research on low-field nuclear magnetic resonance properties of

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fried food were mainly from two aspects, the extraction of the transverse relaxation time (T2)

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information and the nuclear magnetic imaging (MRI). In previous studies, three T2 components

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have been found for classic potato chips, and it was found that the short T2 was associated to water

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phases, which weighted combination of contributions from both bound and free water, and two T2

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components (which called the medium and long T2) were observed in the oil. These results were

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used as the basis for magnetic resonance imaging (MRI) investigations of oil and water content in

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fried food17. In addition, LF-NMR technology has been successfully applied to the oil content

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detection of starch systems by Chen et al.18. It has shown that there was no superposition between

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the oil and water signals in deep fried starchy samples, and it can simultaneous determine the water

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and oil contents in fried starchy samples by T2 relaxation information analysis. In order to

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demonstrate the effect of combining power ultrasound and microwave technology, Su et al.19 studied

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the T2 relaxation spectrum of French fries during vacuum frying process. Unfortunately, they paid

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more attention on the water removing and status of water distribution in French fries, and the signal

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response portion of the oil in the T2 relaxation spectrum was ignored. Thus, it can be concluded

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from the literatures that LF-NMR showed potential for quality analysis of fried foods. However,

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more work needs to be done on simultaneous analysis of water and oil content as well as the quality

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of frying oil, so as to guarantee the routine monitoring of French fries.

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Therefore, taken commercially available French fries as the model fried food, the purpose of this

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study is to investigate the relationship with the LF-NMR T2 relaxation results, the water and oil

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content, and quality of the frying oil. It may provide a supplement analysis method for rapidly 4


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detecting the quality of French fries, and further for the fried food, during frying process.

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MATERIALS AND METHODS

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Materials. In order to imitate the actual frying process, 24° palm oil was chosen as frying oil,

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which was purchased from a local supermarket in Shanghai, China. The Lutosa frozen par-fried

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French fries, produced in Belgium, was purchased from a supermarket in Shanghai, China. The

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specifications of each French fries used in this study was 7 mm × 7 mm × 60 mm. The purchased

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French fries and palm oil were stored at -18 °C and room temperature, respectively.

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Frying experiment of French fries. A stainless-steel electric fryer (HY-81, Guangzhou Huili,

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China) with a maximum oil capacity of 5 L was used for deep-fat frying of French fries. The potato-

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to-oil ratio was set as 1/6.25 kgpotatoes/Loil. French fries was fried at 180 ± 5 °C for 0, 1, 2, 3 and 4

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min, respectively. After frying, each batch of French fries was placed in an oil spill and drained at

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room temperature for 5 min.

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Frying procedure and sampling of palm oil. The frying procedure started with 5 L of fresh

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palm oil in the fryer mentioned above, the potato-to-oil ratio was set as 1/6.25 kgpotatoes/Loil, and 3

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min of frying was chosen to simulate commercial frying. Frying process was conducted in 30 min

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cycles for eight hours per day, and the frying procedure was hold constantly for five continuous

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days. Initially the oil was heated up to 180 ± 5 °C, and kept at this temperature for 30 min

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additionally before the potatoes were added. At the end of the daily frying, the oil sample was cooled

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to room temperature and stored at 4 °C until next daily frying. During the frying, 50 mL oil sample

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was collected every two hours (after 2, 4, 6, 8… 28 and 30 h), and kept at -18 °C until analyzed.

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Water content. The residual water content of the French fries sample was measured using an

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oven method8. Approximately 10g of French fries samples were placed in a hot air-drying oven at 5



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105 ± 2 °C until constant weight. The water content in the sample was weighed as g/g wet basis of

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the sample.

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Oil content. The oil content in the French fries was measured by Soxhlet extraction method9 with

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petroleum ether. The sample was extracted gravimetrically using a Soxhlet extraction system for 5

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h. After that, the oil which was extracted to flask was dried to constant mass in a vacuum oven at

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50 °C. The oil uptake in the sample was weighed as g/g dry mass of the sample.

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NMR relaxation measurements. A low field pulsed NMR Analyzer (miniPQ001, Shanghai

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Niumag Corporation, China) with a frequency field of 19.91 MHz at 35 ± 0.01 °C was used in the

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experiment. The strength of the magnetic field is 0.5 ± 0.08 T. Sample was placed in a 15 mm glass

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tube and inserted in the nuclear magnetic resonance probe. Carr-Purcell-Meiboom-Gill (CPMG)

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pulse sequence was employed to measure transverse relaxation time (T2). The typical pulse

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parameters were as follows: sampling frequency = 250 KHz, repetition time = 2000 ms, echo count

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= 5000, echo time = 1 ms, repeat scan times = 4. The volume of all the samples (both the French

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fries and palm oil) for T2 relaxation analysis was 2.5 mL. From the analysis, the intrinsic T2 values

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(which was defined as the start time of peak), peak area (A) and percentage relative contribution

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(RC) of each peak were determined, as well as the monoexponential fitting result, i.e., the single

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component relaxation time (T2W), to provide an average estimation of all T2 values with regard to

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their contents in the sample. The samples were measured in triplicate and each reported value is the

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average of a minimum of nine measurements.

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MRI analysis. MesoMR23-040V-1 (Niumag Electric Corporation, Shanghai, China) was used

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for T2-weighted imaging. The permanent magnetic field strength was 0.5 T, corresponding to a

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proton resonance frequency of 21.16 MHz at 32 ± 0.01 °C. At least ten strips of each batch were 6


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selected for MRI analysis. Each sample was placed in a glass tube with diameter of 18 mm, and T2-

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weighted images were acquired using a spin-echo (SE) imaging sequence. The field of view (FOV)

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was 100 mm × 100 mm, slice width and slice gap were 3 mm and 1 mm, respectively. The echo

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time (TE) was 50 ms, and the repetition time (TR) was 1600 ms. Signal intensity was measured and

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analyzed by the pseudo-color images.

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Total polar compounds (TPC). The total polar compounds in the palm oil during the frying

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process were measured by Testo 270 (Testo Inc., USA), which can provide stable results for the

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determination of oil quality in deep-frying operations20. Before measurement, the equipment was

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calibrated with the calibration oil sample supplied by the manufacturer. Oil samples were measured

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directly by inserting the sensor into the previously heated (180 ± 5 °C) oil. Then, the temperature

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and percentage of TPC content in frying palm oil displayed on the screen after stagnating about 10

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s. The measurement of the indicator was performed every half hour until the percentage of TPC

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content exceeded 27%, which is the standard limit of polar compounds in frying oils in China

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GB2716-201821. It was cleaned with deionized water and dried thoroughly between measurements.

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Statistical data analysis. All the tests were performed in triplicates. Data was processed as the

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mean ± SD in Microsoft Excel 2016. Then Origin 8.5 (OriginLab Corporation, Northampton,

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England) was used to draw the figures. SPSS 20.0 (SPSS Inc., Chicago, USA) was used to perform

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multiple regression analysis and significant statistics, and the differences were analyzed by ANOVA

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at a confident level of 95%.

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RESULTS AND DISCUSSION

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Identification of water and oil signals in French fries by LF-NMR. The T2 decay curves of

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French fries with different frying time are shown in Figure 1(a). Although the similar decay 7



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tendency was exhibited, it can be seen from the partial enlargement that the attenuation curvature

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appeared much difference. The time point of maximum curvature indicates the time required for the

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decay curve to have the maximum curvature value, and can be used to characterize the attenuation

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curvature of decay curve. The smaller the time point of maximum curvature, the bigger the curvature,

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indicating a shorter echo decay process. The unfried French fries was chosen as a control and the

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time point of maximum curvature was 132.78 ms. In comparison, the time point of maximum

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curvature decreased to 87.89 ms and 47.89 ms, respectively, within the first two minutes, showing

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a shorter decay process. This may due to the rapid evaporation of water under frying temperature.

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The water content as well as the mobility of hydrogen protons in water molecules decreased

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accordingly, and the decay process would become faster. However, with the extension of the frying

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time, the turning point occurred at 3 min. After that, the time point of maximum curvature increased

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to 210.89 ms and 256.56 ms, respectively, which corresponded to a longer decay process. As the

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frying process prolonged, the uptake of oil in the starch structure gradually increased, and in

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comparison, the oil has a higher mobility than the trapped water, thus, the relaxation behavior of the

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protons in oil slowed down the decay process, causing the time point of maximum curvature to

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increase again. This result was consistent with the research reported by Wu et al. 22, who use LF-

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NMR to determine the oil and water contents of soybean, and found the higher the oil content, the

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longer the decay time.

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Figure 1(b) shows the single component relaxation spectrum (T2W) of French fries, which is used

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to reflect the overall relaxation behavior of hydrogen protons in frying. It can be seen that the signal

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amplitude in the frying process decreased overall, reflecting the weakening of the hydrogen protons

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density in the sample. At the beginning of fry, the T2W for the control was 52.76 ms, then it dropped 8


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from 46.07 to 32.81 ms in the first two minutes of frying. With the extension of frying, T2W increased

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substantially, which were 84.20 ms and 114.89 ms for 3 and 4min, respectively, and it was much

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bigger than that of the control. This change could be explained by the dynamic balance of oil and

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water23. Within the first 2 minutes, although the diffusion and vaporization of water reduced the

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proportion of water molecules trapped inside the structure, the water content of the sample was still

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higher than 35.85%, which was identified as water-dominated system. Due to the interaction

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between water and macromolecules, here, the starch molecules of potato, their mobility were

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restricted, thus, a shorter T2W was observed 24,25, making the overall relaxation time shift to the left.

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After frying for 3 minutes, the water-oil balance was broken caused by the intrusion of oil (which

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also reflected on the multi-component diagram discussed later), where the oil content was more than

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41%, forming an oil-dominated system. Enhancement of oil uptaking inside the French fries led to

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an increase in entire relaxation time of the hydrogen proton, and this was consistent with the changes

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of their decay curves.

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As seen in Figure 1(c), it is the distributions of the transverse relaxation time (T2) spectrum of

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French fries at different frying time. There are three peaks labeled T21 (1.57 - 5.19 ms), T22 (7.31 -

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18.47 ms) and T23 (38.77 - 199.39 ms) for samples from different treatment groups. As the frying

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progress went on, the signal amplitudes of T21 and T23 increased from 74.46 to 208.76 a.u, and 28.56

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to 138.65 a.u, respectively, while T22 was found decreased gradually from 193.28 to 7.05 a.u and

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almost disappeared at the end. With the extension of the frying time, the relaxation times of all

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components were reduced, and the entire T2 relaxation spectrum was continuously shifted to the

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left.

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Considering these samples contain two immiscible liquids, water and oil, the multi-exponential 9



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relaxation behavior was expected reasonably. In order to distinguish the proton signal between water

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and oil of the samples, the oven drying method was used to remove the moisture. Figure 1(d) was

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the distributions of the transverse relaxation time (T2) spectrum of the samples after drying. After

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desiccation, the relaxation signals of water protons almost disappeared, and the existed signals can

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be attributed to the protons in the oil molecule. It can be seen that only two peaks, including a small

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characteristic peak T21’ (9.95 - 13.42 ms) and an obvious peak T22’ (16.64 - 39.26 ms), stably

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appeared on the spectrum. The overall distribution of the spectrum was similar to that of a reported

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fried palm oil16.

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For the control, the peak at 5.19 ms shown in Figure 1(c) completely disappeared after

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desiccation, and the signal amplitude of the T22’ at 16.64 ms reduced greatly. Interestingly, the shape

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of the T22’ of the control was very similar to that of T2 relaxation spectrum of vegetable oil, which

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could be explained by the fact that the purchased sample was pre-fried. As for the fried sample, the

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peak area of T22’ was much bigger than that of control, and it increased continually as the frying

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time prolonged. This was in accordance with the research on deep fried potato chip17, and it was

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found that the short T2 was associated to water phases while two longer T2 components (which

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ranged from 54 to 58.6ms, and 200 to 216 ms, respectively) were ascribed to the oil content.

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Concerning to the high signal amplitudes peaks with T2 values ranged from 1 to 30 ms, which are

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named as T21 and T22 in Figure 1(c), the signals were almost disappeared in Figure 1(d), indicating

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that it was the peaks produced by the signal of water molecule. This result was consistent with

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previous research results18. In particularly, it can be seen in Figure 1(d), a small peak appeared

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ranged from 9.95 to 13.42 ms, which may be identified to the characteristic peak due to frying26.

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Summarizing above, it was reasonably attributing T23 to the peak generated by the oil signal, and 10


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T21, T22 to the peak generated by the water signal, which are named as Toil and Twater, respectively

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(Figure 1(c)).

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T2 relaxation characteristic of French fries. Table 1 shows the water and oil content as well as

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the T2 relaxation characteristics, which extracted from Figure 1(c). As the frying prolonged, the

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shortening of Twater and Toil were observed, which ranged from 5.19 to 1.57 ms and 199.39 to 38.77

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ms, respectively. It was similar to the phenomenon which has been observed on T2 spectrum of

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French fries19. Furthermore, it can be found that a sharp decline of Toil occurred in the early stage

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of the frying process, which dropped down to 59.37 ms at 2 min. As the frying reached 3 to 4 min,

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it tended to stabilize around 40 ms, which was similar to the characteristic signal peak of palm oil

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on T2 spectrum (which will be discussed in Section 3.4).

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As shown in Table 1, there are significant differences in oil content and water content with

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different frying time. With the proceeding of frying, the water contents of French fries decreased

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from 0.682 to 0.228 g/g. Conversely, the oil contents increased from 0.1278 to 0.4333 g/g, which

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was associated with the oil infiltration. Usually, the peak area and percentage relative contribution

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of a certain range of T2 are used to assess the number of hydrogen protons in a certain population27.

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It can be clearly seen that Awater decreased from 3844.47 to 1188.69, while the Aoil increased from

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70.84 to 1507.95. This result was corresponded to the tendency of RC. In the first two minutes, the

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RCwater was more than 77%, indicating that the water predominated throughout the system. When

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the frying time extended to 4 minutes, the proportion of oil exceeded that of water, with the RCoil

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as high as 57.35%. Thus, the changes for the A and RC values are consistent with water and oil

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content in the French fries.

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The relationship between LF-NMR result and water and oil contents in French fries. In 11



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order to predict the water and oil content of French fries by LF-NMR, the relationship between the

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T2 characteristics and the actual water and oil content of French fries has been studied. Table 2

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presents the regression result, and it can be noted that the T2 parameters were highly correlated with

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water and oil content. Obviously, among the three T2 parameters, the Awater possessed the highest

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correlation with water content, with R2 of 0.992, and the RCoil possessed the highest correlation with

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oil content, with R2 of 0.960. The significant correlations indicated that the LF-NMR might be a

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complementary technique in monitoring the water and oil content of the French fries during frying

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process. This result is similar to the research done by Chen et al.18. Besides, based on their research,

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our advantage lies in the real simulation of commercial frying, simpler experimental processing and

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more practical results. The results mentioned above stated clearly that LF-NMR method had

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conspicuous advantages over Soxhlet extraction in the determination of oil content in fried samples.

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MRI of French fries. MRI can provide visualized information on the state and spatial distribution

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of water and oil in a non-destructive way, and was considered as a complementary method to LF-

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NMR. The pseudo-color images of French fries at different frying time were shown in Figure 2.

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And the deeper the red color, the higher density the hydrogen proton was, while the blue color

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corresponds to the lower proton density28. With the extension of frying time, it could be found that

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the signal intensity and spatial distribution have changed significantly. In comparison, the unfried

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sample showed a deeper and relatively evenly distributed red color, and a non-uniform density

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distribution tendency can be observed for the fried ones. For example, as for 2 minutes of frying,

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the red color was still deep for the inner core portion of the French fries, while the crust tended to

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blue and a “gap” appeared between the core and the crust. When the frying time reached 4 minutes,

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the red color in the core almost disappeared, further indicated that the proton density decreased with 12


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extension of frying time.

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Meanwhile, the T2-weighted images showed dramatic changes from red to blue color, suggesting

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a significant structural change associated with the loss of water and uptake of oil during the frying

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process. Frying was a complicated process, which not only involves heat transfer from oil to the

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inner core of French fries, but also the simultaneous mass transfer of water vapor from the sample

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to the oil. With the extension of the frying, the internal structure becomes loose from compaction,

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and the pores inside gradually become larger. Due to the higher temperature of the frying, the crispy

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crust of the French fries was formed and gradually separated from the inside23. The oil infiltration

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was in accordance with the order from monolayer to multilayer. It was distributed along the contours

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of the cells and filling cell interstitial29, and the oil content in the small area of edge layer of the

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potato chips was higher than other areas, which seems to explain the higher proton density along

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the crust. Moreover, according to research of Dhital et al.30, the starch is gelatinized at high

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temperatures but subsequently become dehydrated due to the loss of water, and maybe it was the

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main reason for the disappearance of the hydrogen proton signal in the inner core of the French

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fries.

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Relationship between T2 relaxation characteristics and TPC measurements. As one of the

278

most reliable indicators, TPC is often used to evaluating the quality of frying oil12, and previous

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research by Li et al.1 showed that the TPC content was no significant difference between French

280

fries and deep-fried oils. Hence, it is reasonable to use the content of TPC to reflect the quality of

281

the oil in the French Fries.

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Table 3 show the change of TPC content during frying process. The TPC content of the initial

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frying palm oil sample was only 3%, which was lower than the results measured by Li et al.1 in the 13



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unfried oil and qualified for frying. With the prolonged frying time, it can be found that the content

285

of TPC increased linearly, and after frying for 28 hours, the TPC content in the oil sample had

286

exceeded 27% and reached 30%. The changes could be expressed by the linear regression equation:

287

288

y = 0.954 x + 4.244

R2 = 0.996

(1)

Where y represents the TPC content, and x represents the frying time.

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Research has shown that with the high frying temperature, the rate of a series of chemical

290

reactions, such as hydrolysis, cracking, polymerization rises quickly, and more polar substances

291

such as triglyceride monomer, dimer and polymer are produced, thereby increasing the content of

292

TPC12, 31.

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Figure 3(a) shows the distributions of the transverse relaxation time (T2) spectrum for the oil

294

samples of different frying intervals. With similar shape and signal amplitudes, the two peaks ranged

295

from 5.68 to 9.16 ms for T21, and 24.84 to 42.78 ms for T22, respectively. Moreover, the transverse

296

relaxation time (T2) spectrum shifted to the left gradually as the frying time increased.

297

In general, the horizontal axis of two peaks commonly represent the different components of the

298

oil samples, and the peak areas are proportional to the number of stable hydrogen protons27. In order

299

to better analyze the state of the components and the change on the T2 relaxation spectrum, the

300

specific values are shown in Table 3. With the extending of frying process, the T21 fluctuated within

301

a certain range, i.e., 5.68 - 9.16 ms, but the corresponding A21 value increased from 22.30 to 162.86.

302

As for T22, it can be seen from Figure 3(a) that it decreased from 42.76 to 24.84 ms, while the A22

303

values remain essentially constant with increasing frying time at about 7010 ms. Therefore,

304

considering the total area as a whole, the proportion of RC1 increased from 0.33% to 2.34%, which 14


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corresponds to a downward tendency in RC2. Based on the research reported by Zhang et al.26, the

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polarity and molecular weight of the cracking products and polymers formed during deep-frying

307

were different from that of triglyceride, which have lower mobility and higher viscosity, thus the

308

hydrogen protons showed faster relaxation, and peak T21 could be the characteristic peak of

309

constituent with higher polarity. As the extension of the frying process, the peak area of T21 increases

310

significantly, which corresponded with the tendency of TPC mentioned above. It was observed that

311

the degree of polymerization in the oxidized substances formed in the oil gradually increased with

312

the frying prolongation32, and the strong interaction between the molecules also has effect on the

313

whole relaxation of the samples, which caused the increase of the viscosity and the decrease in

314

mobility of the oil protons. This may be the main reason for the shortened relaxation time of T22.

315

Figure 3(b) shows the single component relaxation spectra (T2W) of the oil sample. It should be

316

noted that the T2W of the oil sample also shifted to the left gradually, which varied from 138.26 to

317

95.10 ms (Table 3). Generally speaking, after heating at a high temperature, many physical and

318

chemical reactions occur in the oil, resulting in an increase in the viscosity and TPC content of the

319

oil33. Higher viscosity results in a shorter T2 relaxation time for the frying oil34. This can explain the

320

shortening of T2W in single component relaxation analysis.

321

In order to reflect the quality of frying oil more clearly, multiple regression analysis was used to

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assess the correlation model between LF-NMR relaxation characteristics and TPC of palm oil during

323

frying. The TPC was selected for the dependent variable, and the T2 characteristics results were

324

selected for the independent variables (T21, T22, A21, A22, RC1, RC2, T2W). The backward elimination

325

variable method is used to exclude the inconspicuous independent variables, and only the significant

326

variables and the optimal combination between the variables could be contained in the model. A 15



327 328

Page 16 of 33

multiple linear regression equation was established by: TPC (%) = – 0.705 T22 – 6.559 RC2 + 688.066

R2 = 0.989

(2)

329

Which suggested good prediction of the quality of frying oil with LF-NMR parameters. Therefore,

330

the experiment above proved that LF-NMR has good potential for a rapid, accurate and non-

331

polluting way of predicting the quality of frying oil.

332

In conclusions. Low-field nuclear magnetic resonance has been proposed as a rapid and non-

333

destructive technology to measure the water and fat contents in French fries as well as

334

simultaneously evaluate the quality of frying oil deterioration during deep-frying. Multi-exponential

335

relaxation behavior was observed for French fries. Through the analysis of transverse relaxation

336

behaviors of French fries at different frying times and the comparison of T2 relaxation spectrum

337

before and after desiccation, the proton signals corresponding to water and oil in the fried French

338

fries were clearly distinguished. During frying, the relaxation time of water and oil (Twater and Toil),

339

the peak area and percentage relative contribution of water (Awater and RCwater) all decreased, while

340

Aoil and RCoil increased. Good linear relationships between Awater and water content, RCoil and oil

341

content, have been found, which demonstrates great reliability of LF-NMR measurement, and

342

proved that this technique is suitable for rapid monitoring of oil and water content during frying.

343

Moreover, MRI results could visually show the changes of signal intensity and spatial distribution,

344

as well as the internal structural changes during frying. Meanwhile, the transverse relaxation

345

behavior of palm oil during frying was also studied, and a good correlation model between LF-

346

NMR relaxation characteristics and TPC has been built using multiple regression analysis. Thus,

347

LF-NMR could be further used as a supplementary method to predict the quality of frying oil. In

348

summary, this approach could be used as a rapid and easy choice for the analysis of water and oil 16


Page 17 of 33


349

content of French fries, besides the monitoring of frying oil quality. In addition, we have illustrated

350

the potential application of LF-NMR and MRI for the optimization of frying parameters and quality

351

assurance during commercial frying.

352 353

ACKNOWLEDGEMENTS

354

This work was supported by the National Natural Science Foundation of China (NSFC China

355

81773482, 31201365); and the development of major scientific instruments and equipment of the

356

state (2013YQ17046303).

357 358

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448

Figure captions

449

Figure 1. The transverse relaxation properties of French fries at different frying times. Relaxation

450

decay curves (a); Single component relaxation spectra (b); Distribution of the transverse relaxation

451

time (T2) spectra (c); Distribution of the transverse relaxation time (T2) spectra after drying (d).

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Figure 2. MRI of French fries at different frying times (Change from 0 min to 4 min).

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Figure 3. The transverse relaxation properties of palm oil during frying. Distribution of the

454

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22


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Figure 1




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Figure 2



Figure 3


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TOC


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Table 1 T2 relaxation characteristics, water content and oil content of the samples. Frying time

T2 relaxation characteristics Water content (g/g)

Oil content (g/g)

(min)

Twater (ms)

Toil (ms)

Awater

Aoil

RCwater (%)

RCoil (%)

0

0.6828 ± 0.001a

0.1278 ± 0.002a

5.19 ± 0.49a

199.39 ± 27.88c

3844.47 ± 37.78e

70.84 ± 9.59a

98.19 ± 0.62e

1.81 ± 0.25a

1

0.4587 ± 0.001b

0.2861 ± 0.010b

3.98 ± 0.18b

143.59 ± 19.94b

3238.02 ± 99.63d

488.98 ± 29.89b

86.73 ± 1.45d

13.07 ± 0.98b

2

0.3585 ± 0.003c

0.3682 ± 0.000c

2.86 ± 0.23c

59.37 ± 4.29a

2399.66 ± 169.64c

709.89 ± 49.23c

77.12 ± 4.24c

22.88 ± 1.39c

3

0.2742 ± 0.023d

0.4171 ± 0.007d

2.16 ± 0.17d

40.47 ± 3.09a

1461.65 ± 77.65b

1119.97 ± 47.19d

56.54 ± 1.51b

43.46 ± 1.27d

4

0.2280 ± 0.001e

0.4333 ± 0.004e

1.57 ± 0.15e

38.77 ± 8.58a

1188.69 ± 36.29a

1507.95 ± 62.73e

42.65 ± 0.82a

57.35 ± 1.04e

Twater and Toil: the start time of characteristic peak; Awater and Aoil: The absolute value of the peak area; RCwater and RCoil: the percentage relative contribution. Different letters in a column indicate significant differences (p < 0.05).



Page 32 of 33

Table 2 The correlation between water / oil contents and LF-NMR parameters Parameters

Factor

Regression equation

R2

p

Water content

Twater

y = 0.079 x - 0.017

0.974

0.002

Awater

y = - 1.304 x2 + 179.869 x - 2342.588

0.992

0.008

RCwater

y = - 0.035 x2 + 4.365 x - 37.315

0.991

0.009

Toil

y= - 5.624 x + 279.934

0.957

0.004

Aoil

y = 1.593 x2 - 47.687 x + 439.224

0.951

0.049

RCoil

y = 0.085 x2 - 3.142 x + 28.791

0.960

0.040

Oil content

p

Rapid Assessment of Deep Frying Oil Quality as Well as Water and Fat

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