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Subjects:
Food Science & Technology
The incorporation of probiotics in non-dairy matrices is challenging, and probiotics tend to have a low survival rate in these matrices and subsequently perform poorly in the gastrointestinal system. Encapsulation of probiotics with a physical barrier could preserve the survivability of probiotics and subsequently improve delivery efficiency to the host.
- encapsulation
- non-dairy
- probiotics
- stability
- storage
1. Introduction
The growing awareness among consumers regarding healthy lifestyles has increased the demand for food that could provide additional specific health benefits beyond nutrition. Functional food is one of the leading trends in today’s food industry. The term “functional food” refers to foods containing (either present naturally or added by manufacturers) ingredients or bioactive compounds that provide extra health benefits over its adequate nutritional effects, which can beneficially affect one or more physiological mechanisms in the body, resulting in an enhancement in health and reduction in risk for disease, in the amount consumed in a diet [1]. For example, probiotics are one of the dominant groups of functional foods [2].
Probiotics, from the Greek word, “for life”, are defined as “live microorganisms that, when administered in adequate amounts, confer a health benefit to the host” by a joint United Nations Food and Agricultural Organization/World Health Organization working group in 2001 and The International Scientific Association for Probiotics and Prebiotics (ISAPP). Probiotics have also been considered functional foods due to their health-promoting abilities [3]. Among probiotic strains in use today, strains from genera of Lactobacillus and Bifidobacterium are the most frequently used. In addition, other non-pathogenic microorganisms that occur within the host gut or tissues have also been developed as probiotics. These include strains from genera Propionibacterium, Pediococcus, Bacteroides, Bacillus, Streptococcus, Escherichia, Enterococcus, and Saccharomyces. Lately, Faecalibacterium prausnitzii, Akkermansia muciniphila, and Eubacterium hallii have also been identified as potential next-generation probiotics with promising health-promoting functionalities [1,4].
By regulating the natural balance of gut bacteria in the human gastrointestinal tract, probiotics have been shown to promote a wide range of health benefits such as improving intestinal health, improving lactose digestion, enhancing the host’s immune response, reducing serum cholesterol, diarrhea diseases, and inflammatory bowel disease, counteracting allergies, and lowering the risk of certain cancers [5]. For a potential probiotic strain to exert therapeutic effects on the host, the viability of probiotics in food should be at least 6 to 7 log CFU/mL (or CFU/g) when reaching the small intestine and colon. In this regard, the viability of at least 8 to 9 log CFU/mL (or CFU/g) of probiotics in food before ingestion is necessary [3,6].
Probiotics must be stable throughout the digestive tract and able to adhere to human epithelial cells when they reach the intestine. However, the survival of probiotics is greatly affected by the harsh conditions of the gastrointestinal tract, including the acidic pH of the gastric environment and bile acids (a loss of around 2 log CFU/mL or CFU/g during digestion) [7]. Several intrinsic (e.g., pH, water activity, molecular oxygen, the composition of the food, food additives added, and oxidation-reduction potential) and extrinsic factors (e.g., temperature, relative humidity, and gas composition) have also been observed to negatively affect the viability and stability of probiotics during food preparation and food processing, as well as over a prolonged storage period [5,7,8].
Traditionally, dairy products have been recognized as the best carriers of probiotics. Current probiotics have been formulated into numerous dairy products, such as fermented milk, yogurt, cheese, and ice cream. However, consumers’ preferences today lie more with non-dairy-based probiotic products because of the ongoing trend of vegetarianism and awareness of drawbacks associated with the intake of dairy products, such as lactose intolerance, high cholesterol content, and milk protein allergy [2,9]. In recent years, non-dairy matrices, such as fruits [10,11,12], fruit and vegetable juices [7,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26], fermented rice beverages [27], tea [28,29], jelly-like desserts [30], bakery products [31,32,33], cereal bars [34], sauces [35], gum products [36], and powdered functional drink [37] have been explored as vehicles to deliver probiotics. Although non-dairy food matrices are more versatile (absent of lactose, dairy allergens, and cholesterol) than dairy food matrices, the delivery of probiotics using non-dairy food matrices is more challenging. As an example of a dairy food matrix, milk, which is rich in proteins and fats, could effectively act as a protective matrix to protect the probiotics throughout the digestive tract [38]. In contrast, non-dairy food matrices, such as fruit and vegetable juices, have considerable amounts of organic acids, dissolved oxygen, and inherently low pH values that could negatively affect the viability of inoculated probiotics [9]. Dairy food matrices are usually stored at refrigerated temperature (4 °C), and therefore, the viability of probiotics can be well-maintained throughout the product’s shelf life. In contrast to dairy food matrices, non-dairy food matrices are often stored at ambient temperature, which could adversely affect the viability of probiotics [2]. The sensory qualities of non-dairy food matrices could also be enhanced or deteriorated by the metabolic compounds produced through the interaction between the probiotics and food matrices [2,9].
2. Encapsulation
To date, encapsulation is one of the most promising techniques in protecting active compounds against adverse environments. Encapsulation technology has been widely used in the pharmaceutical, medicine, nutritional, food science, biological, agriculture, toiletries, and cosmetics industries for over 50 years. The goal of encapsulation is to protect the encapsulated active compound (core material) against unfavorable or adverse environments (such as light, moisture, temperature, and oxygen). In food industries, a broad range of products (including probiotics, antioxidants, antimicrobials, flavors, enzymes, and nucleic acids) are encapsulated to (a) prevent the core material from degradation, (b) slow down the evaporation rate of volatile core material, (c) separate the components that would otherwise react with each other, (d) modify the nature of the core material for easier handling, (e) increase the stability, (f) to mask undesired tastes, colors, and odors, (g) enable sustained and controlled release (release slowly over time at a constant rate), (h) control oxidative reactions, (i) use with bacteriophages to control foodborne pathogens, and (j) extend the shelf life. Indeed, encapsulation is one of the new and effective methods to protect probiotics from the harsh conditions they encounter throughout food processing, shelf storage, and gastrointestinal transit [1,40,41,42].
3. Probiotic Encapsulation Techniques
Numerous encapsulation technologies have been developed and adopted to protect probiotics. All the techniques aim to protect the viability and stability of probiotics. However, their concepts, operation methods, and properties of produced capsules are different. Each technique also has its own strengths and drawbacks. Figure 1 illustrates different types of probiotics encapsulation techniques and the morphologies of corresponding microcapsules obtained. Various aspects must be taken into consideration before the selection of encapsulation techniques. Selecting a suitable encapsulation technique depends on several parameters, such as the nature of the probiotics, the operational conditions of the encapsulation technique, the properties of the biomaterials used, the particle size needed to deliver the adequate probiotics load without affecting the sensory properties, the release mechanism and release rate, the composition of the target food application, the storage conditions of the food products before consumption, and lastly, the cost limitation of production [43,44].
Figure 1. Different types of probiotics encapsulation techniques and the morphologies of corresponding microcapsules obtained: (a) ionic gelation (emulsion, extrusion); (b) coacervation; (c) fluidized bed coating; (d) freeze-drying; (e) spray-drying; (f) spray chilling; (g) layer-by-layer method; (h) co-encapsulation.
Table 1 shows the main properties, advantages, and disadvantages of encapsulation techniques that can be applied in multilayer and co-encapsulation techniques of probiotics.
Table 1. Overview of common probiotic encapsulation techniques.
4. Biomaterials Utilized for Probiotics Encapsulation
To be an effective encapsulation material (core or wall material), the biomaterial used must be able to protect the encapsulated probiotics along the gastrointestinal tract until reaching the target site (small intestine/large intestine), where they can exert their health-promoting effects. The encapsulation material should only release the encapsulated probiotics when it is exposed and triggered by certain environmental conditions (such as temperature, pH, and enzyme activity). In other words, the capsules containing probiotics should remain protected inside the encapsulation material during the passage through the stomach and only decompose after reaching the target site to release the probiotics. The commonly used biomaterials in probiotic encapsulation include carbohydrates, proteins, and lipids, which will be discussed in detail in the coming subsections. Their specific advantages and limitations in probiotic encapsulation are also summarized in Table 2.
Table 2. Common biomaterials for encapsulating probiotics.
5. Application of Probiotics Encapsulation in Non-Dairy-Based Food and Beverage Products
The growing demand for non-dairy probiotic food products has encouraged scientists and researchers to explore more new non-dairy food matrices (Table 3). Recent studies have proved that non-dairy food matrices (known to be free of lactose, dairy allergens, and cholesterol and rich in nutrients) are promising vehicles for probiotic delivery. Furthermore, the probiotics were also observed to adapt well to encapsulation using non-dairy food matrices owing to their richness in nutrients. However, researchers still face some challenges, such as the maintenance of probiotic viability and sensory properties of probiotic food products [2,9]. For instance, the composition, pH value, and storage condition of the non-dairy food substrate could negatively affect the viability of inoculated probiotics. Under certain conditions, the metabolic compounds produced through the interaction between the probiotics and food matrices could negatively affect the sensory qualities of non-dairy food products. While probiotics do not usually replicate in non-dairy matrices, it is necessary to keep the viability of probiotics at an adequate level. In addition, components such as carbohydrates, proteins, and flavoring agents in the food matrix could also negatively affect the viability of probiotics. Encapsulated probiotics with bigger particle sizes were also reported to be adverse to the mouthfeel sensation.
Table 3. Examples of recent application of probiotics encapsulation in non-dairy-based products.
This entry is adapted from the peer-reviewed paper 10.3390/app121910005