Oral Processing and Consumer Perception

Biophysics, Food Microstructures and Health

Bettina Wolf, Serafim Bakalis, Jianshe Chen, Bettina Wolf, Serafim Bakalis, Jianshe Chen

• Publisher: Royal Society of Chemistry
• Year: 2022
• ISBN:
  9781839160639
• Edition: 1
• Topic: Médecine
• Subtopic: Nutrition, diététique et bariatrie

This is the first book for some years that provides a comprehensive overview of food oral processing. It includes fundamental chapters at the beginning of each section to aid the understanding of the later more specific oral processing chapters. The field is rapidly developing, and the systems researched in the context of food oral processing become increasingly complex and therefore the fundamental sections include information on how to build complex food systems.

The main coverage includes the biomechanics of swallowing, the biophysics of mouthfeel and texture as well as the biochemistry of flavours and how food microstructures can be manipulated. It contains up-to-date research findings, looking at consumer preferences and the response to these preferences by food process technologists and those developing new foods.

The book will be of interest to postgraduate students and researchers in academia and industry who may be from very diverse backgrounds ranging from food process engineers to functional food developers and professionals concerned with swallowing and taste disorders.

Part A

Fundamental Principles of Food Oral Processing

CHAPTER 1

Saliva: Properties and Functions in Food Oral Processing

Donghao Zhang ^a , Xinmiao Wang^*a and Jianshe Chen ^a

^a School of Food Science and Biotechnology, Zhejiang Gongshang University, Hangzhou, Zhejiang, China,
^*E-mail: [email protected]

The role of saliva in oral health and the diagnosis of disease has been widely recognized, but the interactions between saliva and diet have received less attention. We review the properties and functions of saliva, as well as the role of saliva in the perception of taste and texture. The salivary secretion mechanism and the physical and chemical characteristics of saliva and its main components are described in detail. We also introduce how saliva participates in the formation of a food bolus and its influence on the perception of food texture in the oral cavity. The interactions between saliva and food components as they affect taste are also discussed.

1.1 Introduction

Saliva has an essential role in food digestion, bolus formation and sensory perception during food oral processing. Saliva is a complex heterogeneous clear fluid mainly secreted by the parotid, sublingual and submandibular glands. It consists of roughly 98% water and 2% organic and inorganic substances. ¹ The presence of proteins and other large molecules means that saliva is a unique colloidal fluid with distinct rheological and lubricating properties. The origin and composition of human saliva have been studied extensively by scientists from biological, physiological and dental research backgrounds. ^2,4

Saliva is the first body fluid that contacts or interacts with ingested food, whether it is a solid or liquid. The mixture of saliva and food, which soon becomes an integrated body, ready, in most cases, to be safely swallowed, is referred to as a bolus. There has been a growth in research about saliva and food oral processing in recent years and this chapter focuses on salivary properties and functions, as well as saliva–food interactions, to present a full picture of recent research about the properties of saliva and how saliva participates in food oral processing.

1.1.1 Properties and Functions of Saliva

The secretion of saliva differs from person to person and varies within the same individual at different times of day (a circadian rhythm). When resting with no external stimulation, the average flow rate of saliva secretion is about 0.25–0.35 mL min⁻¹. However, the secretion of saliva is greatly increased (typically 4–10 times higher) when stimulated (chemically, mechanically or aromatically), with >50% attributed to saliva secreted from the parotid gland. ⁵ The inorganic components in saliva include ions and ionic groups such as Na⁺, K⁺, Cl⁻, Ca²⁺, Mg²⁺, HPO₃ ²⁻ and HCO₃ ⁻. The organic components in saliva (see Table 1.1) consist of body secretion products (urea, uric acid and creatinine), putrefaction products (putrescine and cadaverine), lipids (cholesterol and fatty acids) and >400 different types of protein. In terms of these proteins, much attention has been paid to proteins that are glandular in origin (e.g. amylase, histatins, cystatins, lactoferrins, lysozymes, mucins and proline-rich proteins) and plasma derivatives (e.g. albumin, secretory immunoglobulin A and transferrin). ⁶

Table 1.1 Salivary proteins. Reproduced from ref. 22 with permission from Elsevier, Copyright 2007

	Origin	Function	Concentration
Total protein			0.9 ± 0.2 mg mL^−1 ^23
α-Amylase	Parotid glands	Starch digestion	476 ± 191 µg mL^−1 ^24
Albumin	Plasma	Diagnostic markers of oral health	0.2 ± 0.1 mg mL^−1 ^23
Mucin	Mucous glands	Lubrication	1.92 ± 0.09 µg mL^−1 ^25
Lysozyme	Submandibular and sublingual glands	Antimicrobial	3.5–92.0 µg mL^−1 ^26

1.1.2 Salivary pH

The pH of saliva is a major indicator of oral health in dentistry due to its close relation with dental caries across all age groups. ^7,9 The natural pH of saliva is in the neutral range, between 5.6 and 7.6 for healthy individuals, with an average of 6.75. ¹ However, this is particularly dependent on salivary calcium and phosphate concentrations, which vary between individuals. ¹⁰ It has been reported that the pH of saliva has a circadian rhythm –the average intra-oral pH is about 6.7 during sleep, but this increases to 7.2 when awake. ¹¹ A high salivary pH has been found to lead to better oral health and a lower incidence of dental caries. ¹² A lower pH, such as caused by tobacco consumption (containing pyridine alkaloids and aromatic hydrocarbons), may damage the oral mucosa and, furthermore, affect taste perception. ¹³

Saliva has a buffering capacity due to the presence of bicarbonate/carbonate ions and, to a lesser extent, phosphate ions and proteins, and can neutralize acids in the oral cavity, maintaining a stable pH environment. ¹⁴ Factors such as oral health, the demineralization–remineralization balance, dilution and antimicrobial activity are all important factors influencing the buffering capacity of saliva. The time it takes for the pH of saliva to return to the resting state after stimulation (e.g. drinking an acidic sparkling beverage) can be used to evaluate the buffering capacity of saliva. ¹⁵ Bicarbonate is believed to be the principal buffer of saliva. Its concentration varies dramatically from about 5 mmol L⁻¹ in unstimulated human whole saliva, when it is produced at a flow rate of 0.3 mL min⁻¹, up to 24 mmol L⁻¹ in stimulated whole saliva at a flow rate >2 mL min⁻¹. ¹⁴ Carbonic anhydrase VI helps to maintain a high bicarbonate level in saliva with the reversible reaction between CO₂ and HCO₃ ⁻. Although the optimal buffering pH for the phosphate and carbonate systems at 25 °C occurs at pH 7.2 and 6.3, respectively (the pH of the HCO₃ ⁻/H₂CO₃ ⁻ buffer system ranges from 5.1 to 8), buffering below pH 5 relies more on protein buffering. ¹⁴ Protein buffer systems are mostly determined by their amino acid composition. Proteins at concentrations such as those found in human saliva (amyloglucosidase with lysozyme/α-amylase) exhibit a measurable buffering capacity. ¹⁵ One study ¹⁶ looked at the human salivary α-amylase subproteome and found that 67 amylase spots most frequently matched a range of isoelectric points from pH 3.5 to 7.6 with a molecular weight range of 18–59 kDa. These α-amylase variants may function like zwitterionic buffers, a buffer system operational between pH 3.5 and 5, with auxiliary buffering through anionic and cationic sites present as non-interacting carboxylate and ammonium side-chains between pH 5 and 8. ¹⁷

1.1.3 Major Salivary Proteins

More than 1000 proteins have been reported in saliva. These have several biological and antimicrobial functions that influence numerous aspects of oral health, food digestion and taste perception. ¹⁸ In dentistry, saliva is an effective medium for the monitoring and diagnosis of disease. Salivary proteins and biomarkers (immunoglobulins, lysozyme, lactoferrin, cystatins and histatins) show good diagnostic potential in monitoring and detecting periodontal disease, oral cancers and dental caries. ^19,22 Table 1.1 lists the proteins widely studied in food oral processing.

As the most abundant enzyme in saliva (40% of total salivary proteins), salivary α-amylase has long been used clinically as a non-invasive biomarker for sympathetic activity. ^27,30 It is a calcium-containing metalloenzyme that hydrolyses the α-1,4 linkages of starch to glucose and maltose. ³¹ Adults are reported to have higher salivary α-amylase activity than children. ³² In the oral cavity, salivary α-amylase is mainly involved in the initial digestion of starch. Salivary α-amylase activity shows a distinct diurnal profile with a pronounced decrease within 60 min after awakening and a steady increase in activity during the course of the day. ³³ Several studies have shown that smoking, caffeine intake and psychological stress will affect salivary α-amylase activities. ^34,35

Salivary α-amylase is also associated with obesity. Mennella et al. ³⁶ compared people aged 19–54 years and found that the overweight participants had higher salivary α-amylase activity and salivary lipolysis than the normal weight participants. There may also be a genetic link between carbohydrate metabolism and obesity. A decrease in the copy number of the salivary amylase gene (AMY1) leads to a decrease in salivary α-amylase levels and a higher risk of obesity. ³⁷ When the proportion of dietary carbohydrates is higher, the AMY1 copy number and salivary α-amylase activity are higher than in a low-starch diet. Mejia-Bentez et al. ³⁸ evaluated the number of AMY1 copies in 597 Mexican children (293 obese and 304 normal weight) and found that the average AMY1 copy number in obese children was lower than that in normal weight children. The children with >10 copies of AMY1 were all of normal weight.

Dietary habits might also have an impact on salivary α-amylase activity. ³⁹ The secretion of saliva was assessed in the two major ethnic groups of China: Chinese Han and Chinese Mongolian. People from the Chinese Han ethn...