Application of wool keratin: an anti-ultraviolet wall material in spray drying

Abstract

Low-molecular-weight keratin (LMWK) obtained from wool was employed as a wall material for the spray drying encapsulation of fish oil. Microcapsules with different LMWK contents were prepared, and their anti-ultraviolet performance and other features were studied. The results showed that LMWK was able to improve the encapsulation efficiency of fish oil because of its good emulsifying properties. When the LMWK content was increased from 0 to 10, 30 and 50%, the shelf life of the microcapsules under ultraviolet irradiation increased from 48 to 96 h, 144 h and 168 h, respectively. The strongest absorption efficiency of LMWK is shown in the UVc band. The chemical structure of LMWK did not change during an ultraviolet accelerating ageing test.

Electronic supplementary material

The online version of this article (10.1007/s13197-020-04897-2) contains supplementary material, which is available to authorized users.

Introduction

In recent decades, spray drying encapsulation (SDE) has become increasingly popular in food and pharmaceuticals due to its cost-effectiveness and high efficiency (Assadpour and Jafari 2019; Yeşilsu and Özyurt 2019). During the SDE process, a core material (volatiles, oils, emulsions, probiotics, and sugar-rich compounds) is embedded in a wall material (protein, octenyl succinic acid modified starch, amyloin and Arabic gum), making the product easy to use and transport (Sarabandi et al. 2019; Wang et al. 2019). In addition, the wall material can also protect the core material against damage caused by variations in temperature, humidity, oxygen level, pH value, and light exposure (Assadpour and Jafari 2019).

Selecting an appropriate combination of wall materials to obtain higher encapsulation efficiency or other functionality is current research topic of interest (Assadpour and Jafari 2019). While previous research has examined most wall material properties, the anti-ultraviolet performance has been largely ignored. It is well known that ultraviolet irradiation can promote the oxidation of fish oil, vitamin C and other materials that are not resistant to photocatalytic oxidation (Lee et al. 2016; Sweet et al. 2018; Thiele et al. 1999). Whether on the factory floor or in the hand of the customer, foods and drugs (especially cake, mixing drinks and liquid medicines) containing SDE products are not always kept in an ideal dark place. In addition, ultraviolet radiation is commonly used as a method of sterilization in hospitals and food processing plants (Walker et al. 2018), and goods are at higher risk of exposure to ultraviolet radiation and deterioration. Therefore, to prevent ultraviolet radiation from accelerating the deterioration of SDE products and extend the shelf life of such products, it is necessary to improve the anti-ultraviolet radiation performance of wall materials.

The addition of anti-ultraviolet agents to decrease the photocatalytic oxidation rate is an effective method in the textile industry, food industry and pharmaceutical industry (Hu et al. 2017; Meng et al. 2001; Nhiem et al. 2018). However, most anti-ultraviolet agents are metallic compounds or other materials, neither of which is appropriate for food and pharmaceutical manufacturing due to their biological toxicity.

Low-molecular-weight keratin (LMWK) is a kind of natural resource derivative prepared by the degradation of feathers, wool and other materials (Bhavsar et al. 2016b; Rajabinejad et al. 2017). Due to its good biodegradability and biocompatibility, LMWK has been utilized in agricultural and textiles application (Arivithamani et al. 2014; Bhavsar et al. 2016a). Researchers also reported that LMWK has valuable antioxidant potential for human health due to its resistance to ultraviolet radiation (Lee et al. 2016; Tanrikulu-Kucuk and Ademoglu 2012; Thiele et al. 1999). Recent literature has also reported the application of LMWK in cosmetics and medicine, and good results have been achieved (Carvalho et al. 2019; Esparza et al. 2018). Unfortunately, there are very few applications of LMWK in the food and pharmaceutical industries, and there is no literature reporting its application in SDE. Considering this background, we believe that it is meaningful to study the anti-ultraviolet performance of LMWK in SDE products.

In the present work, LMWK was used as a wall material for fish oil encapsulation in a spray drying process. A series of samples were prepared, and their anti-ultraviolet performance and other properties were studied.

Materials and methods

Materials

Defatted wool was purchased from Shengrong Wool Products, Hebei, China. Fish oil (with EPA and DHA ratio of 3:1) and octenyl succinic acid modified starch (OSA-starch, Mn = 10,000 and DS = 0.02) were obtained from Prius Bioengineering, Xi’an, China. Hexane, isopropanol, acetic acid, trichloromethane, sodium dodecyl sulfate, soluble starch, potassium iodide and sodium thiosulfate were purchased from Jinshan Chemical Reagents, and all reagents were of analytical grade.

Preparation of LMWK

The preparation process of LMWK was previously described by Bhavsar (Bhavsar et al. 2016a, 2016b). Briefly, defatted wool and distilled water were placed in a hydrothermal digestion tank with a mass ratio of 1:5. Then the loaded tank was heated to 170 °C and maintained for two hours. The keratin degradation liquid was filtered through slow filter paper immediately after it cooled to room temperature, and the filtrate was collected. The filtrate was dried by a vacuum freeze drier at –50 °C and the obtained LMWK powder was stored in a desiccator at room temperature.

Preparation of emulsion

The formulations of the emulsions are shown in Table 1. The preparation process was as follows. First, OSA-starch (Mn = 10,000 and DS = 0.02) and LMWK were dissolved in distilled water with a certain mass ratio at 40 °C Second, the solution was cooled to 10 °C using a cold water bath with ice. Third, a certain amount of fish oil was added and the mixture was emulsified using a FJ200-SH homogenizer (Shanghai Specimen and Model factory, Shanghai, China) at 15,000 rpm for 5 min. Finally, the emulsion was further homogenized using a CGJB-60 laboratory high-pressure homogenizer (Shanghai Specimen and Model factory, Shanghai, China) at 20 MPa.

Table 1.

Emulsifying properties of wall material

	S-K0	S-K10	S-K30	S-K50	LMWK
EAI (m2/g)	42.30 ± 0.56c	49.17 ± 0.50d	53.63 ± 0.55e	56.50 ± 0.36a	67.20 ± 0.89b
ESI (min)	39.6 ± 0.8c	45.7 ± 0.6d	49.4 ± 0.8e	57.3 ± 0.6a	81.9 ± 0.8b

Values within a row with different superscript letters indicate significant differences (p < 0.05)

Spray drying encapsulation

The spray drying proceeded immediately after the emulsification of the fish oil emulsion. A pilot-plant spray dryer (JT-8000Y, Jtone Instruments & Apparatuses, China) equipped with a two-fluid nozzle atomizer was used to convert the emulsions into encapsulated powder. The diameter of the feed nozzle was 0.75 mm and the air pressure was 0.6 bar. The inlet and outlet temperatures were 175 and 80 °C, respectively. The obtained fish oil microcapsules were stored in a desiccator and placed in a dark place to avoid light exposure.

Determination of emulsifying properties

The emulsifying activity index (EAI) and the emulsion stability index (ESI) were measured using the turbidimetric method (Padial-Dominguez et al. 2020; Pearce and Kinsella 1978) with modification. Two aliquots of the emulsion (50 µL) were pipetted at 0 and 10 min and diluted with 5 mL of 0.1% sodium dodecyl sulfate (SDS) solution. The absorbance of the solution was determined at a wavelength of 500 nm with a UV-3100 ultraviolet and visible spectrophotometer (Mapada Instruments, China), and the optical path of the plate was 1 cm. The EAI and ESI were calculated by Eqs. (1, 2), respectively:

$$EAI\left(m^2/g\right)=\frac{2\times 2.303\times A_0}{\mathrm{\varnothing }\times l\times C}$$ 1

$$ESI\left(min\right)=\frac{A_0\times \mathrm{\Delta }t}{A_0-A_{10}}$$ 2

where A₀ and A₁₀ are the absorbance measured at the initial time and after Δt = 10 min, respectively. The variables Ф, l (m) and C (g/m³) represent the volume fraction of oil, the path length of the cuvette and the mass concentration of the emulsifier, respectively.

Determination of microcapsule particle size

The particle size of microcapsules was measured using a ZEN 3600 laser scattering particle size meter (Malvern Panalytical Co., UK). Isopropanol was used as the solvent in the test. The mass concentration of each sample was 0.5% and the path length of the cuvette was 1 cm. The Z-average size (ZS) and polydispersity index (PDI) were used to represent the average size and particle size distribution of the particles, respectively.

Determination of moisture content and hygroscopicity

The moisture content (C_W) was measured by the drying method. Briefly, by placing a certain amount of sample into a weighing bottle and drying it to constant weight at 105 °C, the moisture content was calculated.

The hygroscopicity (H_C) of the sample was measured using a temperature and humidity test chamber (YOUNG CHENN Instrument Industry, China). A certain amount of sample was placed into a glass culture dish with an even thickness (less than 2 mm). The dish was settled in the chamber at 25 °C of temperature and 65% relative humidity for 72 h, and the mass change was calculated.

Determination of water activity

The water activity (Aw) was determined using an HD-6 water activity meter (HuaKe Instruments, China) at 25 °C The result was recorded and testing was repeated three times.

Determination of encapsulation efficiency

The non-encapsulated fish oil content (C_N) of the microcapsules was determined using the washing method described by Aghbashlo (Aghbashlo et al. 2012a) and Anwar (Anwar and Kunz 2011) with modification. First, 5 g of microcapsule and 20 g of hexane were placed together in a glass beaker at room temperature. Second, the mixture was filtered after stirring at 120 rpm for 60 min, the residue was washed three times with hexane, and the filtrate was merged and transferred into a flask. Finally, the solvent was removed by continuous stream of dry nitrogen, the mass of fish oil was weighed and the non-encapsulated fish oil content was calculated.

The total fish oil content (C_T) of the microcapsules was determined using acid digestion method described by Yu (Yu et al. 2017) with modification. First, 1 g of microcapsule was added to a tube containing 5 mL of distilled water and 5 mL of hydrochloric acid. Then, the loaded tube was treated in a boiling water bath for 60 min (for complete digestion of the wall material). After that, the tube was cooled to room temperature, 10 mL of hexane was added and mixed for 5 min. Finally, the mixture was centrifuged at 2500 g, the upper layer was collected, any hexane in the collection was removed using the same approach described before, the mass of fish oil was weighed and the total fish oil content was calculated.

The surface fish oil rate (R_S), loading rate (R_L) and encapsulation rate (R_E) were calculated as follows:

$$R_S=\frac{C_{\text{N}}}{C_{\text{T}}}\times 100\%$$ 3

$$R_{\text{L}}=\frac{C_{\text{T}}}{m}\times 100\%$$ 4

$$R_{\text{E}}=\frac{C_{\text{T}}-C_{\text{N}}}{C_{\text{T}}}\times 100\%$$ 5

Microstructure observation

The particle morphology for sample was determined using field emission scanning electron microcopy (FEI Company, America). The samples were fixed onto double-sided adhesive carbon tabs mounted on SEM stubs, coated with a thin layer of gold in vacuum using a sputter coater. The testing was performed with 5 kV of voltage and 6000 × of magnification.

Determination of peroxide value

The peroxide (PO) value was determined according to GB/T 5538–2005(Xue et al. 2005). The test process was as follows: a certain amount of fish oil was added into an iodine flask. Then 30 mL of 1:1 (v/v) trichloromethane-acetic acid solution and 1 mL of saturated potassium iodine solution were injected into the flask. The flask Sealed with a grinding stopper and allowed to react for 2 min in the dark. The inside of the iodine flask was flushed with 30 mL of distilled water and injected with 1 mL of 1% starch solution. The mixture was titrated with by 0.1 mol/L sodium thiosulfate solution until the blue colour disappeared, and the volume of sodium thiosulfate solution consumed was recorded. The PO value was calculated as follows:

$$PO=\frac{1000\times V}{m}$$ 6

where V is the volume of sodium thiosulfate solution consumed, and m is the mass of fish oil.

Ultraviolet accelerated ageing test

An ultraviolet accelerated ageing test was performed according to GB/T 18,830–2009 (Xu et al. 2009). A WJ-LH-100 ultraviolet ageing chamber (Wujia Machinery Equipment, China) equipped with 6 UVa lamps (Philip TLK 40 W/10R) was used to carry out the experiment. The temperature was kept constant at 40 °C during the experiment. Samples used for testing were spread evenly in glass dishes with a thickness of less than 1 mm. The PO value of fish oil in each microcapsule was determined every 24 h for 168 h. The ultraviolet–visible spectrum and FTIR spectrum of LMWK and OSA-starch were obtained before and after the ageing test.

Ultraviolet–visible spectroscopy

The ultraviolet–visible (UV–Vis) spectra of wall materials were obtained using a UV-3100 ultraviolet and visible spectrophotometer with an optical length of 1 cm. The wavelength range of scanning was from 190 to 800 nm with a step size of 1 nm. The concentration of all testing samples was 0.1%, and distilled water was used as a control.

Fourier transform infrared spectroscopy

Fourier transform infrared (FTIR) spectra were obtained using a Nicolet iS10 spectrometer (Thermo Scientific, USA). All samples were previously dried at 105 °C for 8 h. The tested sample was mixed with potassium bromide and ground into a fine powder before preforming at 15 MPa. The test was carried out at 25 °C, and the wavenumber ranged from 500 to 4000 cm^–1.

Design of experiment and statistical analysis

The single-factor variable method was used in the experimental design. The variable factor was the keratin content in the wall material. In this paper, the proportion of keratin in the wall material was 0, 10, 30 and 50%. The solid content and wall-core mass ratio were 20% and 3:1, respectively, and remained unchanged. Samples were labelled as S-K0, S-K10, S-K30 and S-K50.

All experiments were performed in triplicate and the data obtained were analyzed using SPSS software version 25.0 (SPSS Inc., Chicago, USA) and expressed as the mean ± SD (standard deviation). The means were compared by Duncan’s multiple range test within the 95% confidence interval.

Results and discussion

Characterization of emulsifying properties

The emulsification characteristics of fish oil encapsulated by different wall materials are shown in Table 1. The EAI of OSA-starch was 42.3 ± 0.56 m²/g, which was lower than the 70.86 ± 0.16 m²/g reported by Xu, however, the ESI was 39.6 ± 0.8 min, much higher than the 3.003 ± 0.022 min reported by Xu (Xu et al., 2018). We suggest that the differences are due to the differences in OSA-starch molecular weight. Xu hydrolyzed starch in depth and the product had a lower molecular weight and viscosity. Therefore, their samples showed higher emulsifying ability and lower emulsification stability during the test.

Both EAI and ESI increase with increasing LMWK in the wall material. The results showed that LMWK could promote the formation and stability of fish oil emulsions. When the LMWK ratio was 100%, the EAI and ESI were 67.2 ± 0.89 m²/g and 81.9 ± 0.8 min, respectively, showing the best emulsification ability and emulsifying stability. We believe that this is due to the high content of hydrophobic amino acids (alanine, valine, leucine, isoleucine, phenylalanine and proline) in LMWK (over 30%) (Bhavsar et al. 2016b). Therefore, compared with natural proteins such as collagen and soy protein isolate, LMWK is more oleophilic, which means it exhibits a better emulsifying performance for fish oil (Pearce and Kinsella 1978; Vangsoe et al. 2018).

Characteristics of microcapsules

The characteristics of the microcapsules are shown in Table 2. When LMWK was not added to the wall material, the particle size of the fish oil microcapsules was 8.71 ± 0.33 µm and the PDI was 0.727 ± 0.025. The particle size was similar to that reported by Jafari and Pourashouri (Jafari et al. 2008; Pourashouri et al. 2014). The ZS of fish oil microcapsules decreased with increasing LMWK content. This indicates that the addition of LMWK promotes the dispersion of fish oil emulsion, which is consistent with the fact that the addition of LMWK improves the emulsifying ability of wall materials. The PDI decreased with increasing LMWK content, indicating that the particle size distribution uniformity of the microcapsules increased. This is because the fish oil emulsion with a higher LMWK content has increased stability. In the spray drying process, the more stable fish oil emulsion particles are less likely to accumulate, and the particle size uniformity is easier to maintain.

Table 2.

Physicochemical properties of microcapsules with different content of LMWK

	S-K0	S-K10	S-K30	S-K50
ZS (µm)	8.71 ± 0.33d	7.85 ± 0.35c	6.44 ± 0.31b	5.86 ± 0.24a
PDI	0.727 ± 0.025d	0.684 ± 0.021c	0.521 ± 0.021b	0.487 ± 0.023a
CW (%)	4.74 ± 0.08d	4.46 ± 0.12c	3.51 ± 0.13b	2.24 ± 0.12a
HC (%)	7.94 ± 0.41a	9.69 ± 0.42b	11.71 ± 0.22c	12.58 ± 0.44d
AW	0.277 ± 0.002d	0.242 ± 0.006c	0.204 ± 0.006b	0.152 ± 0.006a
RS (%)	11.38 ± 0.44d	7.80 ± 0.13c	5.39 ± 0.09b	4.58 ± 0.16a
RL (%)	23.64 ± 0.09a	24.30 ± 0.05b	24.73 ± 0.05c	24.94 ± 0.08d
RE (%)	88.62 ± 0.44a	92.20 ± 0.13b	94.61 ± 0.09c	95.08 ± 0.49c
PO	0.194 ± 0.007d	0.170 ± 0.006c	0.154 ± 0.004b	0.137 ± 0.003a

Values within a row with different superscript letters indicate significant differences (p < 0.05)

As seen from the table, with the increase in LMWK content, the water content and water activity of the microcapsule decreased. This indicates that the addition of LMWK can promote the drying process of fish oil microcapsules. We believe that the reason for this phenomenon is that the molecular weight of OSA-starch is much higher than that of LMWK, which exhibits better water retention performance (Lopez-Silva et al. 2019; Qian et al. 2019). The increase in OSA- starch content in the precursor emulsion results in a decrease in the water evaporation rate during drying and a difference in the water content among different samples. Obviously, the decrease in water content and water activity is beneficial to the storage of products, because water can promote the growth of bacteria. Unfortunately, the increase in LMWK content resulted in a small increase in the hygroscopicity of the product. We believe that this was due to the good hydrophilicity of LMWK.

As the LMWK ratio increased, the surface oil ratio decreased. Accordingly, the loading rate and encapsulation efficiency have also increased. We considered that LMWK was composed of different kinds of amino acid and equipped with good amphipathicity, which could improve the emulsification status of emulsions. When LMWK was added, a higher proportion of fish oil was emulsified into fine oil–water droplets than before (Zhou et al. 2014). As a consequence, after spray drying, microcapsules obtained from emulsion with better emulsifications status show lower surface oil contents and higher encapsulation rates and loading rates.

The peroxide value is a very important parameter that represents the antioxidation performance of fish oil. According to SC/T 3502–2016, the peroxide value of first-grade refined fish oil should not be higher than 5, and the value for second-grade oil should not be higher than 10 (Leng et al. 2016). Additionally, fish oil exhibiting a high peroxide value can be toxic to human health (Sweet et al. 2018). Thanks to the very short drying process, the peroxide value of fish oil in each sample was relatively low even when the drying temperature was higher than 80 °C. This result is consistent with other reports showing that spray drying is an effective preservation method for easily oxidized substances (Aghbashlo et al. 2012b; Wang et al. 2019; Yu et al. 2017).

Microstructure analysis

Figure 1 shows scanning electron microscopy (SEM) photographs of various kinds of microcapsules. All the observed samples show the shrinkage surface appearance of spray dried particles, similar to other researchers’ descriptions (Akbarbaglu et al. 2019; Medina-Torres et al. 2019). Theoretically, when the parameters of the spray drying process reach the optimal conditions, the surface of the microcapsules will be flat (Jafari et al. 2008; Pourashouri et al. 2014; Unnikrishnan et al. 2019). However, when the wall material formula changes, the corresponding optimal conditions will also change. The experiment in this paper is designed as a single factor variable method, and the selected variable is the content of LMWK in the wall material. To ensure the comparability of the experimental results, each spray drying parameter must be consistent. Therefore, the setting of spray drying parameters was not the optimal condition. We believe that the cause of microcapsule surface shrinkage may be the high temperature setting. Therefore, during the early drying stage, the evaporation rate of water in the droplet is higher than that in the wall material, the pressure inside the droplet increases and the volume expands. During the middle drying stage, after the water migrates outward, the outer surface of the wall material is dried and hardened, and a cavity is formed inside the particle. After drying and forming, the permeability of the wall material is poor and the temperature is high, so the internal pressure of the particle can be maintained. Thus, during this period, the particle remains a hollow sphere. During the later drying stage, the external environment temperature decreases, the internal pressure of the cavity decreases, and the surface of the particles shrinks, presenting the appearance on the SEM.

Fig. 1.

SEM photographs of S-K0 a, S-K10 b, S-K30 c and S-K50 (d)

Ultraviolet accelerate ageing analysis

As the peroxide value directly reflects the oxidized condition of fish oil, it is a suitable parameter to adjudge the oxidation status of the sample during the ultraviolet accelerated ageing test. As Fig. 2 shows, the peroxide value of fish oil without any treatment increased very fast. According to SC/T 3502–2016, the fish oil loweredd its product grade after an irradiation period from 48 to 72 h and lost its edible value after an irradiation period from 120 to 144 h. This result indicated that products containing fish oil need good protection to avoid damage caused by ultraviolet irradiation.

Fig. 2.

Variation of peroxide value during the accelerate aging process

Obviously, the antioxidant ability of fish oil was enhanced after microencapsulation, and the anti-oxidant ability increased with increasing LMWK content. For S-K0, the downgrading period and deterioration period increased to 96 h and more than 144 h, respectively. When LMWK was added, microcapsules did not lose their edible value during the entire testing period. For S-K50, the peroxide value of the fish oil never exceeded 5 after irradiation for 168 h. Before the accelerated ageing test, the peroxide values of the microcapsules were higher than the fish oil because of the thermal oxidation incurred during the spray drying process. In addition, the peroxide values of S-K30 and S-K50 for the first 24 h were low, and the difference was very small. We believe that the reason is that when the LMWK content is high, the encapsulation rate of fish oil is high, and the ultraviolet radiation is absorbed by the wall material, so it is difficult to produce a catalytic oxidation of fish oil inside the microcapsules over a short time. Therefore, during the first 24 h, the changes in the peroxide values of these two samples were mainly due to the oxidation of unencapsulated fish oil on the surface of the microcapsules. These results indicated that the addition of LMWK definitely improved the anti-ultraviolet performance of fish oil microcapsules.

Spectral analysis

It is well known that chemical structure stability plays a critical role in the preservation of food and medicine. Changes in chemical structure lead to changes in biochemical properties, which also lead to uncertainties in biological toxicity. At the same time, it is important to confirm the anti-ultraviolet mechanism of LMWK. In the present work, the FTIR spectrum and UV–Vis spectrum were used to determine the chemical structure change and anti-ultraviolet mechanisms of the wall material.

The mechanism of anti-ultraviolet irradiation can be divided into reflective and absorptive types (Hu et al. 2017). Until now, most anti-ultraviolet agents have been absorptive. Therefore, the UV absorption property of a material represents its anti-ultraviolet performance to some extent. The wavelength range of the ultraviolet band can be divided into UVa (320–400 nm), UVb (280–320 nm) and UVc (200–280 nm). In daily life, UVa is the most common kind of irradiation people contact and the wave band of most concern as overexposure may lead to skin injury (Little et al. 2019). The irradiance levels of UVb and UVc are much lower than that of UVa in solar radiation because of their very weak penetrating capability. However, UVc is widely employed in sterilization (Barrett et al. 2019; Walker et al. 2018).

As Fig. 3 shows, at the same mass concentration, no sample showed any absorption behaviour in the visible light band. In addition, OSA-starch did not exhibit any ultraviolet absorption either before or after accelerated ageing in the ultraviolet wave band, while LMWK and aged LMWK showed obvious absorption behaviour. In the wavelength range from 400 to 250 nm, the absorbance of LMWK and aged LMWK slowly increased with decreasing wavelength. From 250 to 210 nm, the absorbance rapidly increased and reached the highest absorbance at 210 nm. When the wavelength was shorter than 210 nm, the absorbance decreased sharply. We also found that, the UV–Vis absorbing capacity of OSA-starch and LMWK did not change after ageing treatment, which means that the anti-ultraviolet properties are long-acting.

Fig. 3.

UV–Vis absorption spectra of OSA-starch and LMWK before and after aging

According to other published reports, nearly all kinds of amino acids exhibit UV adsorption behaviour in the wavelength range from 200 to 300 nm(Ozaki et al. 2014; Preiss and Setlow 1956). The absorption phenomenon we observed in the wavelength range from 300 to 400 nm indicated that the difference in ultraviolet absorption behaviour still exists between amino acids and the polypeptide they compose. Unfortunately, little research on the ultraviolet spectrum properties of polypeptides has been published, hence, the principle remains uncertain. In our opinion, this result was mainly caused by the molecular space structure of the polypeptide (also named the secondary structure of protein).

The differences in ultraviolet absorption behavior among the wall materials explains the difference in peroxide values among microcapsules with different wall material formulas in the accelerated ageing experiment. The higher the proportion of LMWK used in the wall material was, the lower the proportion of ultraviolet irradiation the fish oil received, which resulted in a slower rate of increase in the peroxide value. The results shown above also indicated that LMWK is an absorptive anti-ultraviolet agent.

The FTIR spectra of the wall materials before and after accelerated ageing are illustrated in Fig. 4. For OSA-starch and aged OSA-starch, the band at approximately 3436 cm^–1 is attributed to the stretching vibration of O–H(Lopez-Silva et al. 2019). However, for the same band in the spectrum of LMWK and aged LWMK was formed by stretching vibrations of O–H and N–H(Shavandi et al. 2017). The weak band at approximately 3072 cm^–1 is related to the stretching vibration of C–H of the benzene ring (existing in tyrosine, tryptophan and phenylalanine). Peaks located at approximately 2971, 2925 and 2855 cm^–1 are attributed to the stretching vibration of C–H (Gaidau et al. 2019; Jiang et al. 2018). The peak at approximately 1645 cm^–1 is caused by the stretching vibration of C=O (also called amide I band)(Carvalho et al. 2019). The peak at approximately 1541 cm^–1 is caused by the bending vibration of N–H (also named amide II band). For OSA-starch and aged OSA-starch, peaks located at approximately 1452 and 1382 cm^–1 are attributed to the bending vibration of C–H(Qian et al. 2019). For LMWK and aged LMWK, the peak at approximately 1452 cm^–1 was composed of the stretching vibration of C–N and the bending vibration of C–H, hence, the strength of this peak was stronger than that illustrated in the spectrum of OSA-starch and aged OSA-starch (Jiang et al. 2018). The peak at approximately 1234 cm^–1 is the amide III band which is caused by the coupling between the bending vibration of N–H and the stretching vibration of C–N (Jiang et al. 2018). The peak at 1087 cm^–1 is attributed to the stretching vibration of C–O (Qian et al. 2019). In addition, peaks illustrated at 1168 and 1045 cm^–1 in the spectrum of OSA-starch and aged OSA-starch are related to the stretching vibration of C–O–C(Qian et al. 2019).

Fig. 4.

FTIR spectra obtained from OSA-starch and LMWK before and after aging

In Fig. 4 we can confirm that neither OSA-starch nor LMWK formed new bonds after ageing, which means that the chemical structure of both materials did not change. The results show that LMWK can be used as a reliable wall material.

Conclusion

In the present work, LMWK was successfully applied as part of the wall material during the spray drying process for fish oil encapsulation. Under the same drying conditions, microcapsules containing LWMK illustrated lower moisture content and higher encapsulation efficiency and anti-ultraviolet capability. The beneficial effects of LMWK were enhanced with increasing proportionality. According to spectral analysis, we confirmed that the anti-ultraviolet performance of LMWK was due to the absorption of ultraviolet irradiation, and the absorption behaviour was enhanced withdecreasing wavelength. The strongest absorption wave range was observed in the UVc band, which is of great value for the preservation of products in ultraviolet sterilization environments. The chemical structure of LMWK is stable under ultraviolet irradiation and its anti-ultraviolet performance is a long-term feature. Unfortunately, the mechanism of the anti-ultraviolet performance of LMWK has not been confirmed entirely.

Electronic supplementary material

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