## 4 Discussion  
[PERSON] and [PERSON] conducted a study on vitamin stability under room temperature storage and they reported that 56-57% of vitamin A was lost after one month of storage [11]. The BASF company (BASF 1994, cited by [PERSON]) reported that the retention of vitamin A in VTM premix was 85% and 58% after one month and six months of storage, respectively [12]. [PERSON] and [PERSON] investigated the stability of vitamin premixes under the conditions of 20 \({}^{\circ}\)C and 50% humidity [13], and they observed that after six months of storage, the retention of vitamin A was 76%. The loss activity of vitamin A during storage reported in previous studies is higher than the results from the present experiment. The high loss of vitamin A after stability testing reported in the previous study may be easily explained by a limitation in production processes decades ago that affected vitamin A stability. In the present study, it was observed that the loss of vitamin A in the premixes did not exceed 10% after 12 months of storage, which indicates that the commercially available vitamin A products have good stability. [PERSON] et al. [14] reported that vitamin A activity in vitamin and VTM premixes decreased with prolonged storage time. Additionally, the vitamin A stability in vitamin premix was 86% after 120 days of storage, while vitamin A activity in VTM premix decreased to 64.12% [14]. In the current study, the retention of vitamin A in vitamins and VTM premixes is higher than the results of [PERSON]'s study [14]. This may be caused by the vitamin A production process, but on the other hand, these differences may be due to the composition of the premix in the study by [PERSON], as well as the use of vitamin A in the free alcohol form. The current study indicated that extended storage time reduces the stability of vitamin A in the VTM premix, which is in agreement with the previous study [8; 9]. The pure product of retinol has poor stability and cannot be used directly in feed production [15]. Feed-grade vitamin A mostly uses vitamin A acetate with better stability, but oily vitamin A acetate is usually processed into dry powder for feed processing. The commercially available vitamin A (dry powder) in the Chinese feed additive market is 500,000 IU/g. The dry powdered vitamin A sources vary in their susceptibility to degradation by chemical and physical factors, such as light, oxygen, temperature, and humidity [6]. The degradation of vitamin A is accelerated by trace minerals in the presence of a premix [6; 9; 12]. However, the present study observed that there was no significant effect of vitamin source on the stability of vitamin A in vitamins and VTM premixes and the retention of vitamin A was over 90% after one year of storage. This may be due to commercially available vitamin A products providing stability far superior to the raw vitamin product. On the other hand, this may be that different manufacturers are homogeneous in their technological development, reducing the difference in vitamin A stability. Coatings (e.g., carbohydrate, protein, ethyl cellulose) or encapsulation gives vitamins such as vitamin A greater protection against moisture, heat, and pressure during storage and processing [16]. Spray drying is a technique where a liquid product is atomized in a hot gas which instantaneously leads to the formation of powders. With the advantages of low cost, high productivity, good continuity, and rapid processing, this spray drying method is often used to prepare microencapsulation to protect vitamin A. Modified starch and gelatin are usually used as coating materials [17]. Currently, Xinhecheng, Zhejiang medicine, and Kingdomway have applied a spray drying approach for vitamin A production. DSM uses cross-linking coating technology to create a matrix of a cross-linked beadlet generally composed of gelatin, sugar, gum, starch or some similar type of hydrocolloid.