1 Introduction

Dysphagia, derived from Greek, where dys means “disorder” or “illness”, phago means “eat” or “swallow”, normally related with the damage to the structure and/or function of the jaw, lips, tongue, soft palate, throat, esophagus and other organs. As dysphagia is widely associated with age-related congenital or acquired underlying diseases, it has been a global health problem affecting about 8% of the world’s population, especially the elderly and infants [1]. For example, it is estimated that 16 million American elderly, 30 million European elderly and 10 million Japanese elderly suffer from oropharyngeal dysphagia at the beginning of the twenty-first century [2]. In China, the prevalence of dysphagia in the elderly in the community was 25.82%, and that in the elderly aged 80 and above was as high as 48.67% [2], while that in Parkinson’s disease patients in China significantly increased to 87.1% [3]. With the development of population aging, the number of patients with dysphagia will continue to increase.

Because the bite force of patients with dysphagia was significantly reduced, the diet of dysphagic people should be changed to avoid pain or risk of choking [4]. Without compromising nutritional quality, changing food texture and fluid thickness is one of the most popular dysphagia intervention techniques. The texture required by patients with dysphagia can be provided by different processing methods and thickeners. For example, high-pressure processing can reduce the hardness of vegetables, and the soft texture is more conducive to the consumption of dysphagic patients. Thickeners are essential to achieve food with a consistent texture, which can increase the time it takes for food to travel from the mouth to the esophagus [5]. Different thickeners can even produce similar viscosity to each other and their synergistic effect can be an ideal choice to meet the needs of dysphagia foods.

Meanwhile, the classification of texture modified foods is very important for consistent communication among health professionals, care providers, and industry processors. Although International Dysphagia Diet Standardization Initiative (IDDSI) was established in 2016, providing global standardized terms and definitions that apply to patients with dysphagia of all ages, all care settings, and all cultural backgrounds, the existing food studies applicable to patients with dysphagia have their own reference standards. The dysphagia diet terms, labels, and levels of food texture and thickened beverages vary within and across countries, leading to more confusion and increasing the risk to patient safety.

Consequently, based on a comprehensive analysis of the classification standards pertaining to texture-modified foods across various countries, the methods of delivering texture-improved foods to individuals with dysphagia, encompassing traditional food softening, biopolymers, and emerging structural technologies, were discussed in this study, hoping to provide a reference for the production of dysphagia foods.

2 Texture modified food: standards

The adjustment of food traits, that is, food texture modification and fluid thickening, is a preferentially recommended compensatory methods for the treatment of dysphagia. And to improve the patient safety and nursing, standards for texture-modified foods and thickening fluids have been established in many countries, such as the UK, the US, Australia, Japan, and Ireland in recent years [1, 6,7,8], which are shown in Tables 1 and 2. Numerical or alphabetical order was generally used in most standards, but the classification and description of texture-modified foods are not the same. For example, the texture-modified foods are divided into five levels in UK, while only three levels were applied in China and Denmark [8, 9], and some other countries have four classification levels. Meanwhile, although IDDSI lists regular food as level 7, which was not included in the ranking of the scale by Australia, Ireland, and the UK. In Table 2, the fluid thickness of the thickened liquid also has 3 to 5 levels of classification. Most standards use a thickness description, such as light, medium or extra thick. Half of these standards use a digital and descriptive dual labelling system. The higher the number, the greater the thickness. On the other side, similar food levels were observed for the standards of Australia and Ireland, which can be explained by that the description about food and fluid consistency of Irish Code was adapted from the “Australian Standardised Definitions and Terminology for Texture Modified Foods and Fluids”. Similarly, Chinese dysphagia diet was classified by reference to Japanese Dysphagia Diet 2013 (JDD2013) and IDDSI in combination with Chinese dietary habits.

Table 1 Relevant standards for texture-modified foods in different countries
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Table 2 Relevant standards for thickened liquid in different countries
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Even at the same grade of food according to the description of food texture, the particle size may be varied in different countries. For example, the IDDSI Level 5, that is, Minced & Moist, means that small lumps can be visible within the food (pediatric 2–4 mm; adult 4 mm) [1]; while in National Dysphagia Diet (NDD) of the US, it stipulates that the particle size of food is within 0.6 cm, and Australia and Ireland recommended the particle size of 0.2–0.5 cm for infants and children (based on tracheal size), and 0.5 cm for children and adults over 5, respectively. Similar difference is also reflected in the thickening fluid [6]. The viscosity measurement is not included in IDDSI, because the committee believes that viscosity measurement has practical and scientific limitations. For example, IDDSI Level 4 (Extremely Thick) refers to the fork drip irrigation test, however, this level was described as over 1750cP in NDD, 670–1040 mPa·s in Australia, and 300–500 mPa·s in Japan, respectively.

Furthermore, different terms were used to classify texture-modified foods at the regional and national levels. Some national standards tried to use familiar terms to reflect the real fluids, such as “nectar” or “honey” in US, “chocolate milk” in Denmark, while the utensils (such as spoons) needed to eat was used as a thickness indicator. However, the name, definition and rheological value of “nectar thickening” in North America may be different from that used in other parts of the world. When the viscosity of a product does not match the expected range implied by its label, people with dysphagia may confuse and misuse the product [10]. Although the effectiveness of liquids thickened by the thickener labelled as nectar (300 mPa·s) or honey (3000 mPa·s) on the 3-month cumulative incidence of pneumonia in patients with dementia or Parkinson disease have been proved, the honey-thickening fluids actually fell in the pudding-thick range according to the standard of NDD [11].

Similarly, even at the same concentration of thickener, the grading results are not always consistent with the label when it was used to thicken different liquids. Here, the thickening effect of several commercial thickeners on water, orange juice and milk at the recommended concentration were evaluated according to IDDSI and rheology in our lab. The result was shown in Table 3. The IDDSI grades and the viscosity at the shear rate of 50 s−1 were significantly different. Most obviously, the IDDSI levels of water, orange juice and milk were 1, 0 and 2, respectively, when the concentration of thickener B (Germany) was 3%. Furthermore, in addition, although the main components of thickeners A (China) and C (Japan) from different countries were similar, at a concentration of 2.5%, the IDDSI grades of thickened milk were 4 and 3, and the apparent viscosities were 1301.9 and 929.4 mPa·s, respectively.

Table 3 IDDSI flow test values, IDDSI level, and apparent viscosity of thickened fluids at different thickener types and thickness concentration
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Therefore, the implementation of standardized terms and definitions will be helpful for the texture modification of food for patients with dysphagia. The International Dysphagia Diet Standardization Initiative (IDDSI) framework provides new global guidelines for texture modification and standardization. The adoption and implementation of IDDSI are taking place globally. Information from the IDDSI website (https://iddsi.org/) was used to understand the current implementation of IDDSI. As shown in Fig. 1, seven countries have either formally implemented IDDSI or transitioned from their national standard terminology to IDDSI. The implementation of IDDSI is in progress in 57.89% of countries, some of which have translated IDDSI files into their languages, and some are partially implementing the IDDSI framework but have not yet reached a nationwide implementation consensus.

Fig. 1
figure 1

Implementation of IDDSI around the world (Date from IDDSI website (https://iddsi.org/))

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3 Texture modified food: processing methods

The recommended food texture for treating dysphagia must be soft, moist, elastic, and easy to swallow [12]. At present, several technologies have been employed to realize the soft texture to different degrees while maintaining the colour and flavour of food, which were summarized in Table 4. The dietary softening approaches, such as traditional heat treatment, freezing and thawing, enzymatic impregnation, as well as the emerging technologies including high-pressure treatment, pulsed electric fields and ultrasound treatment, were mostly employed in developing foods for swallowing disorders. Meanwhile, the use of thickeners or emerging structural technology (3D printing) also provided important approaches to modify the rheological or textural properties of foods.

Table 4 Texture-modified food processing method for dysphagia
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3.1 Traditional dietary softening approaches

Dietary softening was helpful for easy chewing and oral handling. The use of hot water is the simplest form of heat treatment that effectively converts hard food into soft food, often used in homes and industry. However, excessive heat treatment of food products is usually detrimental to the flavour, texture and colour of the final product and leads to a loss of nutrients [13]. Therefore, several emerging technologies were introduced to achieve dietary softening. For example, high-pressure processing (HPP) was used to modify the texture of meat and meat products, rice, starch and carbohydrate-based products [14, 15], as well as fruits and vegetable [16]. By applying appropriate level of pressure, HPP can alter the conformational structure of protein and improve the gelation behaviour of protein [17]. For the production of meat-type dysphagia food, it was suggested that the pressure of HPP should be at least 300 MPa [17]. Yoshioka et al. [12] found that the low molecular weight protein of the fish meat gel was easier to release, more glossy, juicy, moderate elasticity, and smoother after applied a 400 MPa high pressure treatment, it was suitable for individuals with dysphagia.

Meanwhile, as one of the most economical alternatives available for heat treatment, ohmic heating is widely used in pasteurization, drying, concentration, extraction and nutritional preservation [18]. It is characterized with better nutrient retention and less change of sensory properties by directly generating heat inside the food through an electric current. However, there are few studies on the application of ohmic heating to texture-modified foods for dysphagia. Olaru et al. [19] studied the effects of two different natural colloids (apple pectin and citrus pectin) on melon puree, and found that ohmic heating treatment can retain the bioactive compounds without changing the pseudoplastic behaviour, which was easier to swallow. Similar to ohmic heating, microwave heating also provides high heating rate and better sensory and nutritional quality [20]. Pure et al. [21] found that the texture of the pea protein gel was within the recommended range after microwave treatment.

Enzyme treatment is also a traditional way of dietary softening, which can decompose cell wall components and/or structural tissues, and thereby changes the food texture and reduces hardness [22]. Cho et al. [23] reported that the yellowfin sole modified by proteolytic enzyme showed lower hardness, higher acceptance and nutritional value, which was more suitable for the dysphagia elderly.

Furthermore, dietary softening effects of pulsed electric fields treatment and ultrasound treatment were also observed on meat products [24, 25], rice [26], and fruit and vegetable [27, 28], or the texture change of starch and carbohydrate product [29, 30]. For example, the softening effect of blueberry texture after the pulsed electric field treatment was observed by Jin et al. [27], however, they pay more attention to the microbial inactivation effect of the treatment. Pattra et al. [31] have shown that ultrasound can effectively soften the sticky rice, making it more suitable for people with reduced chewing ability. Nowak et al. [32] reported that the combination of freeze-thawing and ultrasonic treatment can significantly lower the hardness and chewiness of blueberries. Meanwhile, the softening effect of ultrasonic on the food texture mainly depends on the ultrasonic conditions. Hao et al. [33] found that the hardness and chewiness of the eel products would be increased when the ultrasound treatment was above 400 W. And Huang et al. [34] stated that the rheological properties of pectin-rich mango nectar treated by high-intensity ultrasound showed complex changes, all of which were related to the ultrasound time. As a result, these techniques have not yet been applied to the preparation of food for patients with dysphagia, and it is still necessary to study the effects of various processing parameters on the key attributes of dysphagia diet.

3.2 Using biopolymers

Biopolymer particles and microgels can be used to change the rheology of the fluid, such as increasing viscosity, water retention, hardness and smoothening, therefore modifying the food structure and making it easier to swallow. As a regulator of food injection flow, it can reduce the pharyngeal injection flow rate and increase the time from the mouth to the esophagus to minimize the risk of asphyxia [5]. The thickeners used in dysphagia diets can be divided into two categories, that is, the starch-based thickeners and gum-based thickeners. As the most common hydrophilic colloids, the starch-based thickeners are cheap and easy to obtain [35]. However, starch-based thickeners are generally not well accepted by patients with dysphagia due to that starch-based thickeners may increase the post-volatilization residues, which would increase the risk of post-volatilization aspiration [36].

For gum-based thickeners, it increases the viscosity of the fluid by creating entanglements in which water becomes trapped, maintains a stable viscosity over time, and is not easily digested by salivary amylases, resulting in less oropharyngeal residue [37, 38]. As the most studied hydrophilic colloid in dysphagia diet, the transparent, weak gel-like xanthan gum liquids has a series of ideal properties for food formulations with dysphagia, including high viscosity at low shear rates and low viscosity at high shear rates, as well as stable viscosity over a wide range of pH, temperature and ionic strength [39]. In addition to xanthan gum, cellulose derivatives, such as methyl cellulose and carboxymethyl cellulose (CMC) [40], pectin [41], guar gum [42], gellan gum [43], κ-carrageenan [44], flaxseed gum [45] have also been studied to treat dysphagia. Some of these are already commercially available for dysphagia diets in different food matrices. For example, Akapong et al. [46]evaluated several commercial gel-based thickeners for dysphagia management, and found that xanthan gum, guar gum and tara gum were the main ingredients in several commercial thickeners.

Meanwhile, gellan gum, a new member of food thickeners for dysphagia, has good lubricating properties and stable apparent viscosity. It is worth noting that gellan gum (0.15–0.3 wt%) can achieve similar performance at significantly lower concentrations than xanthan gum (0.5–1.0 wt%), demonstrating its potential as a tough-to-swallow thickener [47]. Guar gum, a galactomannan polysaccharide extracted from the guar bean, has a very high viscosity even at very low concentrations [48]. And modified-texture cooked beef pastes using guar gum as a thickener has been developed by 3D printing [49, 50]. Meanwhile, flaxseed gum, a soluble polysaccharide extracted from Linum usitatisum, is considered to be a good alternative thickener due to its high water solubility and structural interaction with other hydrophilic colloids, not mention its health benefits [45, 51]. It was believed that the FG-based beverages may have an advantage over modified starch-based beverages in the rehabilitation of patients with dysphagia due to obesity and/or diabetes, showing a lower glucose content [52].

Meanwhile, carboxymethylated polysaccharides can also be used in the development of dysphagia foods. For example, the Carboxymethylated Konjac Glucomannan (CMKGM) can be used as a beverage thickener, whose shear viscosity increased with the increase of CMKGM concentration and decreased with the increase of pH value [53]. Wei et al. [54] developed a novel carboxymethylated polysaccharide thickener, that is, carboxymethylated coagulation (CMCD), for dysphagia management. The model nutrition emulsion prepared by CMCD has a high viscosity, strong shear thinning property and proper viscoelasticity, showing similarly rheological properties to xanthan gum. While CMC prepared from pineapple leaves showed comparable properties and behaviour to commercial food thickeners, which can sufficiently increase the viscosity of tested liquid foods (water, orange juice, milk and mushroom cream soup), resulting in the behaviour change of thin liquid to nectary-like consistency [55].

Mixed use of multiple colloids seems to be more promising in the development of dysphagia foods. Using konjac glucomannan as the main raw material and guar gum (GG), xanthan gum (XG), locust bean gum (LBG) and carrageenan (KC) as additives, three thickening effects were obtained that are suitable for people with different chewing disorders [56]. On the other hand, thickeners based on xanthan gum and apple pectin is considered to have promising biomedical/pharmaceutical applications for different levels of dysphagia [57]. To obtain food inks for 3D printing of tasty dysphagia foods, Pant et al. [58] modified the texture of a representative vegetable food with different water contents by adding different colloidal solution, such as, xanthan gum (XG), kappa carrageenan (KC), and locust bean gum (LBG).

3.3 Emerging structural technologies

As an emerging structural technology based on digitally controlled material deposition and layer-by-layer stacking, 3D printing has been employed in the development of dysphagia foods due to its advantages of less processing, better internal structure, more visual appeal and automated food preparation [59,60,61]. In addition, one of the most significant benefits of 3D food printing is the consistency of the product, which almost always provides the right texture for the patient, making it easy to add certain nutrients or non-traditional high levels of certain nutrients to the food [62].

Furthermore, concerning the poor printability of vegetables and fruits due to excessive water content and low carbohydrate and protein content, functional materials, such as different types of hydrophilic colloids and protein components, were proposed to solve this problem [63, 64]. On the other side, the physical control technologies such as temperature, energy-carrying electromagnetic fields and ultrasound can also improve the printing performance of 3D food printing. Xu et al. [65] found that microwave heating and ultrasonic-microwave combined heating pretreatment can change the viscosity of wheat starch-papaya, making it more suitable for 3D printing. Xu et al. [66] found that after heat treatment, the egg yolk became a shear thinning and pseudo-plastic fluid, with suitable viscosity and sufficient to maintain its shape.

Besides 3D, 4D food printing, which is defined as the physical or chemical change of food composition over time, may increase visual attraction of food. For example, Ghazal et al. [67] changed the colour of a 3D printed anthocyanins-potato starch gel over time by taken anthocyanins as a smart pH changing ingredient, therefore achieved a four-dimensional food with health-promoting properties.

Meanwhile, supramolecular polymers, an interdisciplinary direction of polymer science and supramolecular science, have attracted increasing interest in the preparation of food for dysphagia due to their special structures and properties. Supramolecular polymers can form polymeric arrays of monomeric units which are held together by highly directional and reversible noncovalent interactions, therefore changing the polymeric properties in solution and bulk [68]. Dysphagia diets can be fabricated by adding trace amounts of hydrocolloids to form supramolecular polymers [69, 70]. For example, trace amounts (0.02%, w/v) of xanthan gum and konjac glucomannan were employed by Zhang et al. [69] to fabricate a supramolecular sample. This not only greatly reduced the use of additives, but also had significant advantages compared to the XG solution in reducing swallowed residues. However, there is still short of the study on such application of supramolecular polymers in dysphagia food.

4 Conclusion

This article focuses on the relevant standards and processing methods of texture modified food. For food interventions, the translation of recommendations into practical guidelines and the development of texture-modified food methods are important aspects to consider. At the same time, the patient's preference for texture-modified foods and/or thickening fluids must be taken into account. Therefore, the palatability and sensory properties of the ingredients should be considered during the development of food for patients with dysphagia. Furthermore, the integration of protein-rich or other functional components into foods to enhance patients’ nutrient intake should be examined.