Preparation of allicin-whey protein isolate conjugates: Allicin extraction by water, conjugates’ ultrasound-assisted binding and its stability, solubility and emulsibility analysis

Highlights

• Allicin and whey protein conjugates were prepared to enhance allicin stability.

• The binding ability of whey protein to allicin was improved following sonication.

• Solubility and emulsibility attributes of conjugates were significantly increased.

Abstract

The instability of allicin makes it easily decomposed into various organic sulfur compounds, resulting in significant decrease in biological activity. In this study, allicin was firstly extracted with water, then bound with whey protein isolates (WPI) which were pretreated by ultrasound to form conjugates, and the stability, water solubility and emulsibility of conjugates were as well investigated. The research results showed that there were no significant differences in the extraction yields of allicin from water, 40% and 80% ethanol. Appropriate frequency (20/40 kHz), power (50 W/L) and time (20 min) of ultrasonic pretreatments significantly increased (P < 0.05) the sulfhydryl groups content of WPI by 35.05% over control, causing improvement in binding ability of protein to allicin. The binding process of allicin-WPI displayed good fit with Elovich kinetic model (R² = 0.9781). The mass retention rate of the conjugates (in 60% combination rate) with ultrasonic pretreating kept at 95.97% after 14 days of storage at 25 °C, whereas allicin’s mass retention rate was only 61.79% at same storage condition. The water solubility of the prepared conjugates was significantly higher than allicin. And with optimal condition ultrasonic pretreatment of WPI, the conjugates showed the highest emulsifying capacity and emulsion stability (49.56 m²/g, 10.06 min). In conclusion, the ultrasonically pretreated allicin-WPI conjugates exhibited better stability, water solubility and emulsifying properties compared to allicin, this expands the application field of allicin.

Introduction

Allicin, a dially lthiosulfinate, accounting for 70% in thiosulfinate of garlic [1], [2]. Allicin can not only inhibit and kill numerous Gram-negative or positive bacteria but also act on fungi and viruses. Allicin bioactivity, especially antioxidant activity, is much stronger than other chemical components in garlic [3], [4], [5]. Allicin inhibits lipid peroxidation caused by nicotine and other activities, and scavenges free radicals such as 1,1-diphenyl-2-picrylhydrazyl (DPPH) [6], [7], [8]. Although the biological activity of allicin is outstanding, it has not been well applied in practice because of the presence of thiosulfinic acid and allyl groups in the allicin structure, making it sensitive to temperature and pH, and as well easily decomposed into various organic sulfur compounds [9], [10]. Allicin is almost completely decomposed at 80 °C, the process takes only 30 min [11]. In the cut fresh garlic, allicin has a half-life of only 2.5 d in room temperature [12]. Besides unstability, allicin’s solubility in water is low as merely 0.8% [13], in spite that it is easily dissolved in ethanol, chloroform or ether. These property deficiencies limit the utilization of allicin in food.

In order to improve the stability and water solubility of allicin, many scholars did research on allicin microcapsules or liposomes. Wang et al. [14] prepared allicin microcapsules by spray drying technique using β-cyclodextrin and porous starch as wall materials. Pinilla et al. [15] co-encapsulated nisin and garlic extract into phosphatidylcholine nanoliposomes, but these methods are with some limitations such as high wall material requirements, high cost and complicated process, and different liposome particle sizes, etc. The current research involves some studys on combinations of allicin and sulfhydryl-based substances (mainly cysteine or glutathione) general focusing on the physiological activity of allicin without stability evaluation. Lee et al. [16] indicated the biotransformed garlic derivative S-allylmercapto-L-cysteine (SAMC) could inhibition of proliferation of human gastric cancer cells. Liang et al. [17] showed that S-allylmercaptocysteine effectively inhibited the proliferation of colorectal cancer cells under in vitro and in vivo conditions. However, cysteine or glutathione have limitation of additive amounts in food. For example, the maximum use in fermented noodles is low as 0.06 g/kg. An excessive use of cysteine or glutathione in adding to allicin products might cause quite a risk in food.

Whey protein isolate (WPI) is a high-quality natural protein [18], and is considered to be a relatively good protein resource due to its easy digestion/absorption, low price and a variety of essential amino acids [19]. The sulfhydryl groups in whey protein isolates can replace other sulfhydryl compounds, exchange sulfhydryl disulfide bonds with the sulfonyl groups in allicin, and stabilize the conjugates through the binding of disulfide bonds. The sulfhydryl groups (SH) content in natural WPI is relatively low than those in cysteine or glutathione, requiring pretreatment to improve its content. A number of studies showed that ultrasound irradiation could increase the sulfhydryl groups content in proteins. As a green technology, ultrasounication was often used effectively in liquid systems due to its cavitation phenomena (nucleation, bubble growth/rupture) [20]. Xiong et al. [21] found significant increase in SH content after ultrasonic treatment of pea protein extract, which may be linked to cavitation destroying disulfide linkage of the protein and forming sulfhydryl groups. At same time, it proved that pea protein isolate structure has partially expanded. Jin et al. [22] reported the SH groups of glutelin increased (P < 0.05) following ultrasound pretreatment, the change in disulfide linkage was inverse to the change in sulfhydryl groups. This finding may be due to the cavitation impact which destroyed disulfide bond, leading to the increase in sulfhydryl groups. These sulfhydryl groups promotion of protein by ultrasonic irradiation could greatly improve the binding ability of proteins with allicin.

In this study, the extraction rate, stability of allicin by water and different concentrations of ethanol solution were compared, and ultrasound-assisted preparation of allicin-whey protein isolate conjugate was explored for the purpose of improving the storage stability of allicin. The water solubility and emulsifying attributes of conjugates were also investigated.