Mochi is a popular traditional Japanese rice cake made from the short-grain Japonica rice cultivar. Due to the low-calorie, low-fat, and high protein content of Japonica rice flour, mochi is eaten alone, as a major component of main meals and/or snacks, and as an ingredient in other prepared foods [1]. Agemochi is a popular fried snack food made from the leftovers of dried, cut rice cake (Kiri-mochi). It is deep-fried in edible oil without breading or butter (in Japanese, “Age” and “Kiri” mean deep-fry and cut, respectively) (Fig. 1). The crunchy and crispy tastes of Agemochi are enjoyed by Japanese children and adults alike as a common homemade snack.

Fig. 1.

Photographs of Kirimochi cut into approximately 1.5 cm cubes (Inset) (a), sun-dried Kirimochi pieces (b), Agemochi during deep frying (c), and the final product (d).

Recent concerns regarding the edible oils used in home cooking have focused on the relationship between the nutritional habits and overall health of consumers. Medium-chain triglycerides (MCT) consist of mixed fatty acids containing predominantly free caprylic acid (C8) and capric acid (C10) extracted from natural products such as coconut oil and palm kernel oil. Growing evidence suggests that edible oils containing MCT have beneficial health effects, including enhancement of muscle mass, suppression of body and visceral fat, and improved cognition in older adults [2, 3, 4, 5, 6]. However, MCT oils are unsuitable for deep-frying because of adverse physicochemical characteristics during cooking at high temperatures (160–180 °C), including a lower smoking point (approximately 140 °C) and their tendency to foam [7].

In the setting of home-cooking, it is important to assess the quality of frying oils due to their absorption into fried food and hence our diet, particularly at high temperatures. An edible oil containing MCT (medium- and long-chain triglycerides, MLCT) has been manufactured through the process of lipase-catalyzed enzymatic transesterification. MCLT has an improved smoking point (approximately 200 °C) and foaming properties that are comparable to standard cooking oils consisting of long-chain triglycerides (LCT) [7, 8]. This product was released in March 2023 and is now available in stores.

Considering its thermal tolerance and MCT-based health and medical benefits, MLCT oil appears to be an ideal choice for home cooking, including deep-frying. However, only a few studies have focused on the value and safety of MLCT oil and its product in real-world settings. The purpose of this study was to assess the quality of MLCT oil during domestic use for deep-frying of the most popular Japanese traditional snack food, Agemochi, compared to standard cooking oil. Non-volatile substances, collectively known as total polar material (TPM), were analyzed because they affect the quality of fried food, reduce the oil quality, and are also damaging to human health if consumed over a long period. Since the percentage of TPM in edible oil is almost identical to that present in oil absorbed by food [9], we evaluated TPM in frying oil as a measure of the TPM content in fried food.

Changes in TPM values after heating MLCT and control oils are shown in Supplementary Table 2. In all samples, MLCT showed higher initial TPM values than the control oil (p 0.001).

Continuous and intermittent heating of the control oil increased the TPM (Fig. 2a) value in relation to the exposure time (p 0.001, within-subject time effect). This value almost reached the limit for rejection (25%) after intermittent heating at 180 °C and 8 h of total frying time (Fig. 2b). In contrast, MLCT tended to maintain the baseline value of TPM (p = 0.062, within-subject factor), with none of the conditions exceeding the TPM limit for rejection, irrespective of the heating method.

Fig. 2.

Changes in total polar material (TPM) values during continuous (Cnt) or intermittent (Int) frying with medium- and long-chain triglyceride (MLCT) or control oils heated at 160 °C (a) or 180 °C (b). The dotted line indicates the regulatory limit (25%) for discard. ^*p 0.05 vs. initial values (Pre); ^†p 0.05 vs. MLCT.

Changes in the TPM values of each oil after deep-frying Agemochi are shown in Supplementary Table 3. Although the limit of rejection was not reached with either oil type, the TPM values for MLCT were significantly lower than those of control oil after the fourth day of cooking (p 0.001) (Fig. 3a). The mean absorption rate was 5.24 $\pm$ 0.70% for MLCT oil, and 5.22 $\pm$0.45% for test oil (p = 0.96). Regarding sensory evaluation of the final product, the score for aftertaste tended to decline in both groups after the sixth frying cycle (Supplementary Table 4). However, all samples showed high scores overall, as well as for the other attributes. No significant differences were observed between MLCT and control oils (Fig. 3b and Supplementary Fig. 1).

Fig. 3.

Changes in TPM values for each frying cycle (a), and overall sensory scores for Agemochi (b) at different time points using MLCT and control oils. The dotted line indicates the rejection limit (25%). ^*p 0.05 vs. initial values (Pre); ^†p 0.05 vs. MLCT.

The main finding of this study was that MLCT oil displayed better frying stability for home cooking of deep-fried Agemochi than non-MCT vegetable oil during repeated frying operations (up to two daily 1 h sessions for 8 cycles). To the best of our knowledge, this is the first study to evaluate the performance of MLCT in healthy Japanese traditional food.

The measurement of polar components is recognized as one of the most reliable assessments of thermo-oxidative degradation of frying oils, since most non-volatile by-products accumulate during the major reactions that occur during frying. The formation of polar compounds has been shown to increase with the degree of oil unsaturation, both during heating and repeated frying of oils [13]. While column chromatography is the standard method for analysis of TPM, the Testo 270 device offers a practical alternative for rapid, on-site monitoring of oil quality by sensing the dielectric constant of the oil solution. The results from this method correlate well with the more time-consuming chromatography technique [14, 15]. In the present study, higher initial values of TPM were detected in MLCT oil throughout the assessment period, possibly due to the higher diglyceride content and polar structure of medium-chain fatty acids (MCFAs) [16, 17]. Coconut oil consists of $>$65% MCFAs and is more polar than oils containing long-chain fatty acids, which account for $>$35% of TPM [18], thus supporting the current observations.

In general, the TPM levels in deep-fried oils and oils extracted from fried food are similar, regardless of the type of oil. Depending on the type of frying medium and food, the uptake of absorbed oil in deep-fried food ranges from 4 to 14% of the total weight [19]. In addition, the frying process involves dehydration and moisture loss from the food products, which could also play a role in promoting oil absorption [20]. However, to provide crispness, dried rice cake for Agemochi already has less water content than other types of rice cake. The effect of moisture loss on the total product weight used to estimate oil absorption may therefore be minimal. Given the oil content of Agemochi in the present study was found to be approximately 5%, the estimated quantity of ingested MLCT oil (15 g per day) is close to the amount of dietary supplementation reported to enhance the suppression of body fat accumulation in healthy participants [21]. Such health benefits of MLCT oil may be attributable to the different kinetics of low-level MCFA components in the degradation of ingested fatty acids by activating $\beta$-oxidation in liver mitochondria of overweight individuals with a body mass index between 25 and 30 [10].

Our results suggest that MLCT and vegetable salad oils have acceptable thermal and sensory stabilities for repeated deep-frying of Agemochi, as demonstrated by a sustained level of polar compounds below the regulatory limit for rejection of 25% required in most countries [22]. Notably, MLCT oil showed smaller increases in the TPM percentage, irrespective of the cooking cycle (intermittent or continuous), whereas intermittent heating of the control oil resulted in the TPM rising close to the discard level. This could be advantageous for MLCT oil in the deep-frying of Japanese Agemochi, since intermittent frying may cause a greater degradation of quality than continuous frying [13]. Although MLCT and control oils showed similar results for the final product, only a limited number of reports have been published on the sensory properties of deep-fried Agemochi. Further studies on this topic should provide more accurate and reliable information on the thermal degradation of MLCT oil. In addition to measuring TPM in the oil used as the frying medium and analyzing the polar component composition of the breakdown products (e.g., oxidized triacylglycerols, diacylglycerols, and free fatty acids) [23, 24], it would also be useful to extract the oil adsorbed by the cake to evaluate any hydrolysis and oxidation products. Indeed, the sensory properties of frying oils, such as flavor and aroma, are significantly influenced by the types and amounts of these polar compounds [25].

The present data confirm the superior thermal stability of MLCT oil compared to control oil for domestic deep-fried cooking, as represented by the Japanese low-calorie, low-fat rice cake, Agemochi. We anticipate that similar studies on other foods will confirm MLCT as a more attractive choice of cooking oil for domestic deep-fat cooking.

The datasets generated and analyzed during the present study are available on reasonable request. The data are not publicly available due to privacy and ethical restrictions.

YM designed and performed the research study. KS and RT provided help and advice throughout the study. YM analyzed the data. YTat and TM performed data curation. YM prepared and wrote original draft of the manuscript. YTak and HT supervised the research and contributed to critical revision of the manuscript. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

This study was approved by the Ethics Committee for Agricultural Food-Related Research of the Sendai Animal Care and Research Center (approval number: SACRC25-01). All participants or their families/legal guardians were informed about the study’s purpose and their right to participate voluntarily, from all study participants verbal informed consent was obtained with ethical approval.

This research received no external funding.

The authors declare no conflict of interest.

Supplementary material associated with this article can be found, in the online version, at https://doi.org/10.31083/JFSFQ44132.