How to Prepare Microcapsule Lycopene Powder?

Lycopene is a type of carotenoid and one of the main carotenoids in the human diet. Lycopene is found in high concentrations in vegetables and fruits such as tomatoes, apricots, guavas, melons, papayas and pink grapefruit, and gives them their bright red color [1]. Lycopene contains 13 double bonds, 11 of which are conjugated double bonds. This structure enables lycopene to effectively scavenge reactive oxygen species and quench singlet oxygen [2]. Lycopene is one of the most effective antioxidant carotenoids. Studies have shown that lycopene has a variety of physiological activities.

The intake of lycopene in daily life or the level of lycopene in the blood is negatively correlated with prostate cancer, stroke, cardiovascular disease, metabolic syndrome and other diseases [3−5]. Many epidemiological studies have shown that lycopene has in vitro antioxidant capacity (such as singlet oxygen quenching and hydrogen peroxide radical scavenging) [6−7], and that higher plasma lycopene concentrations can reduce the risk of cardiovascular disease in women [8]. However, due to the large number of conjugated double bonds in the structure of lycopene, free lycopene is easily oxidized or isomerized during processing and storage under the action of oxygen, light, temperature, and chemical factors [9].

In recent years, microencapsulation technology has gradually developed, which can protect bioactive compounds from adverse external conditions [10]. Microcapsules are tiny particles or droplets coated in a coating or embedded in a homogeneous or heterogeneous matrix, giving them many useful properties. Microencapsulation technology can also improve stability and reduce losses. Lycopene can be encapsulated using a variety of methods, including spray drying, freeze drying, coacervation, emulsification and ionic gelation [11–12]. The types of wall material used include sugars, proteins and combinations of sugars and proteins [13]. For carotenoids such as lycopene and β-carotene, microencapsulation technology can effectively solve problems such as their poor stability and improve their bioavailability. Therefore, microencapsulation technology has been widely used in the field of carotenoid preparations.

This paper reviews the preparation methods, stability and bioavailability of lycopene microcapsules, with a view to providing a theoretical basis for further research and application of lycopene microcapsules.

1 Preparation methods for lycopene microcapsules

The preparation of lycopene microcapsules involves the encapsulation of lycopene. Methods include spray drying, coacervation, freeze drying, entrapment and ionic gelation [11−12].

1.1 Spray drying method

Spray drying technology is widely used in the food industry and is often used to encapsulate enzymes, spices, antioxidants, preservatives and bioactive substances [14-15]. For spray-dried functional ingredients, the bioactive compounds are first dispersed in a solution of an encapsulating matrix, and then quickly evaporated to form a shell that encapsulates the bioactive compounds. The effect of the encapsulating wall material on spray drying is affected in different ways.

Commonly used wall materials for spray drying include maltodextrin, β-cyclodextrin and soy polysaccharides [16]. Athanasia et al. [17] used maltodextrin as a wall material to prepare lycopene microcapsules by spray drying. The results showed that when the ratio of lycopene to maltodextrin was 1:3.3, the feed temperature was 52 °C, and the inlet air temperature was 147 °C, the microcapsule encapsulation rate could reach 93%. Compared with maltodextrin, cyclodextrin has a hydrophobic center that can interact with the physicochemical properties of carotenoid pigments to form stable inclusion complexes. Itaciara et al. [18] prepared lycopene microcapsules by spray drying with β-cyclodextrin as the wall material.

The results showed that the encapsulation rate of the microcapsules could reach 94%–96% when the core-wall ratio was 1:4. In addition to the commonly used wall materials for spray drying such as maltodextrin and β-cyclodextrin, soy polysaccharides can also be used to prepare lycopene microcapsules. Qiu Weifen et al. [19] demonstrated the feasibility of preparing lycopene microcapsules by spray drying using water-soluble soy polysaccharides as the wall material and optimized the production process. The mass concentration of the wall material was set to 0.28 g/mL, the core material: wall material was 1:7, the mass fraction of the emulsifier was 2%, the inlet temperature was 160 °C, the outlet temperature was 88 °C, and the encapsulation rate of the resulting microcapsules could reach 91.8%. There have also been studies on the preparation of lycopene microcapsules using a combination of two or more wall materials.

Shu et al. [20] prepared lycopene powder microcapsules by spray drying with gelatin and sucrose as wall materials, and studied the effect of process parameters on the preparation of microcapsules. When the ratio of gelatin to sucrose was 3:7, the ratio of core to wall material was 1:4, the feed temperature was 55 °C, the inlet temperature was 190 °C, and the homogenization pressure was 40 MPa, the purity of lycopene in the resulting microcapsules is not less than 52%. Shu et al. [21] used the above equivalent conditions to microencapsulate lycopene extract, and the encapsulation rate of the resulting microcapsules reached 44.33%.

Spray drying is a low-cost method that is easy to use and simple to operate [22]. The particles produced by spray drying are matrix-based, i.e. the core is trapped in a continuous network of a polymeric matrix. The main advantage is that they are easy to reconstitute, which is important for applications with liquid and pasty foods or instant powders. The advantage of spray drying over other preparation methods is that it can be used for continuous production of large or small batches [23]. Products prepared using spray drying have good dispersibility and solubility. However, the high temperature in the spray tower and the exposure of the microcapsules to air can easily deactivate the active substances. This disadvantage can be avoided at low temperatures.

1.2 Coacervation method

Coacervation microencapsulation involves the separation of one or more hydrocolloids from an initial solution, followed by the deposition of the newly formed coacervate phase around the active ingredient suspended or emulsified in the same reaction medium. Microcapsules prepared by coacervation are insoluble in water and have excellent controlled release and heat resistance.

The coacervation method commonly used is the composite coacervation method, which is to compound the raw materials and then adjust the pH or lower the temperature of the system to cause the materials to settle and form microcapsules [24−25]. Dima et al. [26] used the composite coacervation method to microencapsulate lycopene from tomato peels using whey protein and gum Arabic as the wall materials.

To promote coacervation, the reaction mixture was freeze-dried and the powder was collected. The resulting powder was fine, orange in color, and had an encapsulation rate of (83.6% ± 0.20%). Rocha et al. [27] also used complex coacervation to microencapsulate lycopene, and the resulting microcapsules had an encapsulation rate of greater than 93.08%. In addition, the coacervation effect of the wall material also varies under different pH conditions. Silva et al. [28] used gelatin and pectin as wall materials and analyzed the interaction between gelatin and pectin under different pH conditions (3.0–4.5). It was found that the composite coacervation was most effective at a final pH of 3.0, with an encapsulation rate of 89.50%.

Coacervation is one of the most promising encapsulation techniques, with high carrying capacity (more than 99%) and easy control of the release of the contents by mechanical stress, temperature or pH changes [28]. This method requires a mild temperature for encapsulation, which can reduce the oxidative degradation of lycopene during preparation. The agglomeration process is costly and complex compared to other techniques, but it is suitable for the encapsulation of the hydrophobic compound lycopene as the core material [29].

1.3 Freeze-drying method

Freeze-drying is carried out below ambient temperature without air to prevent the product from deteriorating due to oxidation or chemical modification. It can minimize the damage caused by decomposition or changes in structure, texture, appearance and flavor due to the high drying temperature during spray drying [30−31].

Pang Zhihua et al. [32] prepared lycopene microcapsule powder by freeze-drying with walnut isolated protein as the wall material. Based on a core-wall ratio of 1:2 and a monoglyceride addition of 0.5%, the highest encapsulation yield was 80.60% when the homogenization shear rate was 9000 r/min, the embedding temperature was 50 °C, and the embedding time was 50 min. The use of a combination of various wall materials can greatly improve the encapsulation rate of lycopene microcapsules. Long Haitao et al. [33] used a variety of raw materials such as esterified microporous starch, maltodextrin, gelatin, sucrose and VC as composite wall materials to prepare lycopene microcapsules by freeze-drying. Encapsulation was carried out at a core-to-wall material ratio of 10:90 and a wall material ratio of 1:0.67:0.56:0.22:0.44, with a temperature of 50 °C. At this time, the encapsulation rate of the microcapsules was as high as 91.78%. However, lyophilization may cause lycopene loss. Chiu et al. [34] used gelatin and poly γ-glutamic acid as wall materials to prepare lycopene microcapsules with lycopene extracted from tomato juice waste as the emulsion, which was lyophilized. The lycopene content in the microcapsules reached 76.5%, and the results showed that lycopene was lost by 23.5% during the drying process, which was probably due to oxidative degradation.

The methods used to prepare microcapsules mainly include spray drying, freeze drying, and coacervation. Among these methods, freeze drying is widely used in the food and pharmaceutical industries and is mainly used to dry heat-sensitive substances [35−36]. Lycopene microcapsules are mostly prepared using spray drying, and there are few reports on the preparation of lycopene microcapsules using freeze drying.

1.4 Other

In addition to spray drying, agglomeration, and freeze drying, which are commonly used methods for preparing lycopene microcapsules, lycopene microcapsules can also be prepared using entrapment and ionic gelation methods. The ionic gelation method is ideal for preparing stable lycopene-rich microcapsules. The polymers commonly used for encapsulation are alginate and pectin [36]. Sampaio et al. [37] encapsulated lycopene in ionogels with sodium alginate and pectin as polymers, and characterized the lycopene microcapsules before and after freeze-drying under different thermal (60 and 90 °C) and pH (2, 5 and 8) conditions. The results showed that lycopene was highly protected. When evaluating the storage stability at different temperatures, the lycopene retention rates of the microcapsules produced by alginate and pectin after 8 weeks of refrigeration were 29% and 21%, respectively, while the retention rate of the lycopene in the freeze-dried microcapsules was above 80% at 25 °C. Encapsulation is also known as molecular encapsulation, which uses polymers with a special molecular structure as wall materials. Common wall materials include β-cyclodextrin and its derivatives. Jin Xueyuan et al. [38] used β-cyclodextrin as a wall material to prepare lycopene complexes. The results showed that when the molar ratio of lycopene to β-cyclodextrin was 1:150, the encapsulation rate of lycopene reached a maximum of 73.6%, and the retention rate of encapsulated lycopene was 92.2% within 60 days. Sun Xinhu et al. [39] also conducted the same type of experiment, and the results showed that after encapsulation, the water solubility of lycopene can be significantly improved, and its stability can be improved.

2. Commonly used wall materials for lycopene microcapsules

2.1. Carbohydrate-based

Carbohydrate-based wall materials can form amorphous glassy solids, providing structural support for the wall materials of the delivery system. They are widely used as wall materials for encapsulating food ingredients and are the first choice for encapsulation materials [40]. Carbohydrate-based wall materials are sugar-based wall materials. Commonly used wall materials for encapsulation include cyclodextrins, dextrins, gum arabic, trehalose, etc. Patricia et al. [41] studied the stability of α, β and λ-cyclodextrins encapsulating all-trans lycopene from tomatoes, and optimized the encapsulation according to the different molar ratios of lycopene and cyclodextrins and the types of cyclodextrins. The results showed that the stability of lycopene was best when it was encapsulated in β-cyclodextrin, and the highest complexation rate was achieved when the molar ratio of cyclodextrin to lycopene was 1:0.0026. Cyclodextrins are cyclic oligosaccharides produced by the degradation of starch and are a viable encapsulation technique. β-Cyclodextrin is easily soluble in water, and in aqueous solution it can combine with both hydrophilic and hydrophobic substances. It is not easily absorbed by moisture in the air and is chemically stable, making it more suitable for encapsulating lycopene [42−43].

In addition to using a single wall material, the yield can be increased by using a combination of various carbohydrates. Tatiana et al. [44] used alginate, trehalose and galactomannan as wall materials to prepare lycopene microcapsules. The retention of lycopene, isomerization stability and release were analyzed, and the results showed that the addition of trehalose can better retain lycopene and minimize isomerization. Sun Chuanqing et al. [45] used gum arabic and dextrin as wall materials to microencapsulate lycopene. The results showed that when the ratio of gum arabic to dextrin was 1:1, the lycopene content was 20%, and the appropriate ratio of core material to wall material was 1:6, high-pressure homogenization could effectively improve the microencapsulation efficiency and yield of natural lycopene. Both gum arabic and dextrin have a positive effect on the extract. Gum arabic has a series of beneficial properties, such as film-forming ability, water solubility, low viscosity, good retention of volatile components, and emulsifying ability [46]. After acid or enzymatic hydrolysis of the starch, small molecules called dextrins are obtained, which can improve the solubility of the microcapsules in water. This allows for a high solid-to-liquid ratio with low viscosity, but poor film-forming ability and easy drying. The synergistic effect of the two improves the encapsulation rate of the microcapsules.

Carbohydrate wall materials are highly water-soluble, low-cost and come in many varieties. However, they have poor emulsifying properties because they lack surface activity. Carbohydrates should be used in combination with other ingredients that have good emulsifying properties, such as gum arabic and whey protein, or they can be chemically modified with hydrophobic groups.

2.2 Protein and carbohydrate combinations

Proteins and isolates such as whey protein, soy protein, casein protein and gelatin have excellent emulsifying properties. Protein substances have strong self-binding properties, which is beneficial for the dissolution and film formation of hydrophobic active ingredients, and are therefore often used as matrix materials [41−42]. However, these substances are expensive and have poor solubility in cold water. Because carbohydrates have poor surface activity and no emulsifying ability, they are often used in combination with proteins or protein-containing gels for microencapsulation. The proteins mainly play an emulsifying and film-forming role [12].

Hou Yuanyuan et al. [47] compared the effects of soy protein isolate grafting products with konjac gum, carrageenan and gum arabic grafting products on the encapsulation of lycopene in experiments on the encapsulation of lycopene by soy protein isolate grafting products. The results showed that the best encapsulation effect was achieved with the lycopene microcapsules prepared with the soy protein isolate and gum arabic as the wall material, with a yield and efficiency of 74.27% and 71.60%, respectively. Jia et al. [2] used a Maillard reaction to prepare a whey protein isolate-xylooligosaccharide conjugate to microencapsulate lycopene. The Maillard reaction was used to improve the functional properties of the protein using oligosaccharides. Glycosylated whey protein isolate significantly improved the emulsifying properties of the lycopene microcapsules, and the protection of lycopene was better than that of whey protein isolate alone. The co-use of soy or whey protein and sucrose is common in spray-dried microcapsules. Wang Shikuan et al. [48] prepared lycopene microcapsules with soy protein isolate and sucrose as the wall material. The wall material was made of soy protein isolate and sucrose (in a ratio of 4:6), and the lycopene content was 40%. The efficiency of the prepared microcapsules can reach more than 90%. Zhan Hui et al. [49] used gelatin and sucrose as wall materials to encapsulate lycopene. The wall materials were mixed in a mass ratio of 3:7, and 0.4% sucrose ester was added. The content of solid raw materials was 40%, and the efficiency and yield of the obtained microcapsules were the highest, reaching 91.26% and 89.35%.

The combination of proteins and carbohydrates as the wall material for lycopene microencapsulation can not only reduce costs, but also compensate for the poor solubility of proteins and the poor emulsifying ability of carbohydrates. The synergistic effect of proteins and carbohydrates on the physical properties of microcapsules is of great help, and to a large extent, it can improve the stability of lycopene microcapsules.

3 Stability of lycopene microcapsules

Lycopene is a carotenoid. Due to the high degree of unsaturation of carotenoids, isomerization and oxidation are likely to occur during processing and storage. The main cause is enzymatic or non-enzymatic oxidation, which limits its application in the food industry. Lycopene microcapsules can improve the stability of lycopene and increase its solubility, which is of great significance for the application of lycopene in the food and pharmaceutical industries. Aguiar et al. [50] and other researchers have also conducted experiments to study the stability of lycopene microcapsules, and the results show that the stability of lycopene microcapsules is better than that of free lycopene, and that lycopene microcapsules can uniformly release pigments and coloring.

The storage stability of lycopene microcapsules is affected by the capsule wall material, storage temperature, and the number of coating times. The degree of stability can be reflected by the retention rate of lycopene in the microcapsules. Microencapsulation can prevent the degradation of lycopene and avoid oxygen-mediated autoxidation, thereby improving the stability of lycopene [50]. The different microcapsule wall materials have a significant effect on the storage stability of lycopene microcapsules. Zuo Airen et al. [51] prepared lycopene microcapsules by spray drying using various formulations as wall materials, such as gelatin and sucrose, and studied the effect of adding antioxidants during the preparation of microcapsules on the retention rate of lycopene. The results showed that after the addition of antioxidants such as salad oil and ethyl acetate, the retention rate of lycopene microcapsules under natural light exposure at room temperature was as high as 100% in the first week. After being stored for 3 weeks, the retention rate was still above 70%. Lin Weiting et al. [52] used whey protein isolate and xylo-oligosaccharides as wall materials to prepare lycopene microcapsules by homogenization and spray drying after the Maillard reaction. The results showed that under the optimal conditions, the lycopene retention rate of the obtained microcapsules could reach 47.91% when stored at room temperature in the dark for 24 d, and as high as 78.25% when stored in the dark at 4 ℃ for 24 d. The loss of free lycopene under these conditions was significant, indicating that the microcapsules can largely protect lycopene from the adverse effects of the external environment and improve its stability.

The storage temperature of the microcapsules has an impact on the retention rate of lycopene. Jia et al. [2] found that the degradation of lycopene in microcapsules increased with increasing storage temperature. The results showed that the retention rate of lycopene in microcapsules stored at 4 °C for 36 days was 79%, while the retention rates at 25 and 40 °C for 36 days were 46% and 40%, respectively. Aguiar et al. [50] evaluated the temperature stability of three lycopene microcapsules with lycopene contents of 5%, 10%, and 15%, respectively. The results showed that the fewer the core materials, the better the performance. When the lycopene content was 5%, the retention rates at 10 and 25 °C were 82.53% and 67.11%, respectively. Rocha et al. [27] also conducted the same type of experiment, and the results all showed that as the storage temperature of the microcapsules increased, the retention rate of lycopene in the microcapsules decreased, but it was higher than that of free lycopene.

In addition, the number of coating times has a significant effect on the stability of lycopene microcapsules. Fan Shaoli et al. [53] measured the stability of lycopene microcapsules with one coating and two coatings. The results of the stability experiment showed that the stability of lycopene was greatly increased after microencapsulation. After 90 days of storage, the retention rate of lycopene in single-encapsulated microcapsules remained at 78.6%, while the retention rate of lycopene in double-encapsulated microcapsules reached 92.60%. Microencapsulation of lycopene can effectively prevent the degradation of lycopene and reduce the damage caused by oxygen.

4 Bioavailability of lycopene microcapsules

The proportion of a pharmaceutical formulation that reaches the site of action via the body's circulation under normal physiological conditions is known as its bioavailability. Lycopene microcapsules have a significantly higher bioavailability. The factors that affect the bioavailability of lycopene microcapsules mainly involve the encapsulation method and the choice of wall material.

4.1 Encapsulation method

The intestinal release rate of lycopene microcapsules is affected by the encapsulation method. Long Haitao et al. [33] conducted an in vitro sustained-release experiment on lycopene microcapsules prepared with esterified microporous starch, maltodextrin, gelatin, sucrose and VC as composite wall materials. The results showed that the release rate of lycopene microcapsules in the gastrointestinal tract was significantly higher in intestinal fluid than in gastric juice.  After 14 h of sustained release, the cumulative release rate of lyophilized microcapsules in gastric juice was 38%, while the cumulative release rate of lyophilized microcapsules in intestinal juice was as high as 82%, indicating that lycopene microcapsules are mainly released in the intestine. In addition to the above-mentioned single coating technology, the use of double coating technology results in a higher sustained release rate of microcapsules in the intestine. In a study on the effect of double-encapsulation technology on the bioavailability of lycopene, Jing Siqun et al. [54–55] compared the in vitro release of lycopene soft capsules, lycopene oil resins, single-encapsulated and double-encapsulated lycopene microcapsules by simulating the in vitro gastrointestinal environment. The double-coated lycopene microcapsules in artificial intestinal fluid have good sustained-release properties and a high release rate of 92%, which can effectively improve the bioavailability of lycopene. Lycopene microencapsulation can effectively protect it from being released in the stomach, allowing it to have a high release rate in the intestines and improving its bioavailability in the human body.

4.2 Encapsulating wall material

The choice of wall material is also an important factor affecting the bioavailability of lycopene microcapsules. Lycopene microcapsulation with a protein-based wall material can effectively improve its absorption in the intestine and its bioavailability in the body. Xue et al. [56] used corn protein powder as a raw material to prepare a zein-based lycopene microcapsule. The use of protein to neutralize the pH in the stomach provides some protection for lycopene.

The results showed that when the corn protein powder first entered the buffer solution, the particles formed aggregates, and after 2 hours, less than 30% of the lycopene was released. The corn zein particles can protect most of the lycopene from being released in the stomach, and then be released in the large and small intestines, thereby improving its bioavailability in the human body. In addition to zein, lycopene microcapsules made with a wall material of glycosylated whey protein isolate (WPI) can also improve its bioavailability. Long Haitao et al. [57] conducted a simulated release experiment on lycopene microcapsules prepared with a starch-based composite wall material. The results showed that the simulated release of the prepared lycopene microcapsules in intestinal fluid conformed to the Higuchi diffusion model and belonged to the skeleton erosion mechanism. Jia et al. [2] evaluated the bioavailability of lycopene microcapsules prepared with a whey protein isolate-xylooligosaccharide conjugate as a wall material. The bioavailability of free lycopene was (16% ± 3%), while the bioavailability of the microcapsules was (60% ± 4%), higher than the bioavailability of lycopene (42%±3%), which may be due to the increased solubility of lycopene after microencapsulation. The whey protein isolated from the wall material has been glycosylated, and the resulting microcapsules are more stable than those with whey protein isolate as the wall material during simulated gastric digestion. This is consistent with the results of Feng et al. [58] who used ovalbumin-dextran nanogels prepared by the Maillard reaction to improve the bioavailability of curcumin.

5 Application and prospects of lycopene microencapsulation

At present, lycopene microencapsulation technology is widely used in food production and can also be applied in the pharmaceutical field. In the food industry, lycopene microcapsules are used in cake processing, and compared with free lycopene, microcapsules can uniformly release the pigment and color the cake [50]. Microencapsulated lycopene can also be used in extrusion coloring research. Lycopene microcapsules were used for extrusion of rice flour. Under all extrusion conditions, the color retention of microencapsulated lycopene in the extrudate was better than that of free lycopene. Compared with free lycopene, the storage stability of microencapsulated lycopene was twice as high within 96 h. Adding lycopene microcapsule powder to a sunflower seed oil and soya milk-based seasoning formula can improve the antioxidant activity of the seasoning [26]. Sampaio et al. [37] used ionic gelation to encapsulate lycopene concentrate to obtain stable particles of a natural additive in food. In the pharmaceutical field, lycopene microcapsule powder can be used to inhibit amylase to prevent excessive increases in blood glucose levels and effectively prevent diabetes, especially non-insulin-dependent type II. The inhibition effect of the microcapsule powder on α-amylase is lower than that on α-glucosidase. Therefore, the powder is more effective against α-amylase than against α-glucosidase [26].

At present, microencapsulation technology can protect unstable substances such as lycopene, improve their antioxidant activity and increase their bioavailability to the body. With further research, lycopene microcapsules will be used in more fields such as health food or medicine, and have good development prospects.

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