Changes in the physicochemical properties of ready-to-feed liquid infant formula (LIF) stored at different temperatures (10, 20, 30, and 40°C) for 6 mon, focusing on 5-hydroxymethylfurfural (HMF) content, color, pH, fat globule size distribution, and rheological properties were determined. The HMF content increased with storage time, and LIF stored at 40°C had a higher HMF content than that of LIF stored at 10°C. The lightness (L*) decreased while redness (a*) and yellowness (b*) increased with increasing HMF content. The fat globule size and pH of LIF stored at 10°C did not change. However, in the case of LIF stored at 30°C and 40°C, the fat globule size increased and the pH decreased during storage for 6 mon. LIF stored at 40°C had a higher apparent viscosity (ηa,10) than that of LIF stored at 10°C, and the shear-thinning behavior of LIF stored at higher temperature was stronger than that of LIF stored at low temperature. The physicochemical changes of LIF during storage were accelerated by Maillard reaction (MR) at higher storage temperatures. Therefore, even if LIF is aseptically manufactured, we recommend that sterilized LIF should be stored at low temperature in order to minimize quality changes during storage.

Melatonin is a hormone produced by the pineal gland in dark conditions. It plays a major role in the regulation of the sleep-wake cycle. Melatonin synthesis is known to be suppressed by environmental light. Cortisol is a steroid hormone that is a major indicator of physiological alterations due to stressful stimuli. It also displays a circadian rhythm, like melatonin. The highest levels are encountered during early morning and the lowest levels are observed at around midnight. In the present study, the effects of milking time on the melatonin and cortisol concentrations of raw milk and milk powder at the summer and winter solstices were examined. The melatonin concentration in milk increased significantly if cows were milked in the dark at night (p<0.05). The melatonin concentration in milk powder showed the same pattern with respect to the milking time (p<0.05). However, no significant difference in the cortisol concentration was observed between day-and night-time milk. Although the time of day did not affect the level of milk cortisol, seasonal factors affected the release of cortisol in milk (p<0.05). In conclusion, night-time milk is rich in endogenous melatonin. In this respect, it has potential applications for the development of melatonin rich-dairy products, which serve as natural sources of melatonin. 서 론 일주기 리듬(circadian rhythm)은 24시간을 주기로 반복되 는 생체현상으로 체온과 호르몬 변화를 통해 조절되며, 체내 에서 일정한 패턴의 생체리듬을 나타나게 한다. 빛은 생체리 듬에 영향을 주는 중요한 요인 중 하나이며, 이에 민감하게 반응하는 호르몬들을 매개로 하여 생리적 기능을 조절한다. 멜라토닌(melatonin)은 뇌의 송과선(pineal gland)에서 arylalkylamine-N-acetyltransferase(AANAT) 및 hydroxyindole-O-methyl transferase(HIOMT) 효소의 작용을 받아 세로토닌(serotonin) 으로부터 합성된다. 이 두 효소는 빛에 의해 활성이 저해되므 로 체내 멜라토닌 농도는 빛의 유무에 따라 영향을 받는다. 즉, 멜라토닌의 합성과 분비는 암기에 촉진되고, 명기에 억제 된다(Wurtman et al., 1963;Kim et al., 2013). 멜라토닌의 대 표적인 생리적 기능으로는 신체를 이완시키고, 수면을 유도 하는 것이다. 노화가 진행되면 멜라토닌 농도는 낮아지게 되고, 이로 인해 노인에게서 노화에 의한 불면증(age-related insomnia)이 빈번하게 유발된다. 노인을 대상으로 한 선행 연 구에 따르면, 외인성 멜라토닌 투여를 통해 혈청 멜라토닌 농 도를 정상적인 암기의 혈청 멜라토닌 수준으로 높였을 때, 불 면증이 개선됨을 확인하였다 (Zhdanova et al., 2001).

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