ReviewFood oral processing—A review
Introduction
Human beings eat and drink for two reasons: to obtain energy and essential nutrients and to have the pleasure. These two reasons can be simply interpreted as we have to and we like to. It is not the purpose of this paper to discuss why we have to eat and drink. The essential need of eating and drinking is obvious to every single person and does not require much explanation. Main purpose of this review is to reveal the mechanisms and principles which underpin our enjoyment and pleasure of eating and drinking. We will discuss how a food is handled inside the mouth and how its rheological properties influence such processes. It is hoped that the review will help us to better understand physiology as well as rheology principles of food texture and sensory perception and to better interpret textural results from physical (instrumental) measurements. It is also hoped that the review will provide a useful knowledge to food researchers and manufacturers in producing high-quality foods (both nutritionally and texturally) to meet consumers’ need, in particular the needs of some specific consumer groups such as, junior and senior citizens, denture wearers, and people with swallowing dysfunction.
The appreciation of foods comes from a combined perception of multi-contributions, including the texture, the flavour and taste, and the visual appearance. However, this review of oral processing is written with the main attention to food texture, no coverage is given to food flavour (aroma) and taste. This is not at all to suggest that food flavour and taste is less relevant to oral processing. The importance of oral processing to food flavour and taste has already been clearly evidenced by many observations of positive correlations between mastication and flavour release of the food (Neyraud, Peyron, Vieira, & Dransfield, 2005; Neyraud, Prinz, & Dransfield, 2003). The exclusion of food flavour and taste in this review is simply based on the consideration that food texture and food flavour and taste are commonly viewed by food scientists as two different disciplines. For progresses on food flavour and taste studies, readers are referred to various articles and textbooks (Fisher & Scott, 1997; Taylor & Linforth, 1996; Taylor & Roberts, 2004). The surface texture and surface appearance also play a significant role in influencing consumers’ perception and preference of a food product. For progresses in this area, a recent review article on food surface texture (Chen, 2007) is recommended.
This review is organized as follows. Section 1 gives a brief description of the major achievements on food texture studies in the last half-century. Section 2 describes the physiology characters of humans’ oral device, including oral cavity, teeth and biting, tongue, and the composition and production of saliva. Section 3 explains various operations involved in food oral processing, explaining the pathway of a food, from grip and first bite, to chewing, to bolus formation, to until swallowing. Section 4 focuses on the rheology aspects of oral processing, examining what happens to a food material inside the mouth. This will be followed by detailed analysis of swallowing process, including bolus formation, triggering mechanisms, swallowing criteria, and the deformation of food bolus. The review is ended with a brief summary.
Section snippets
A brief history of food texture studies
No one knows exactly how far we can trace back the history of human's appreciation of food texture. Probably as far back as human's evolution began. However, the term of texture for food description was first seen only at around the middle of last century (Matz, 1962), defined, rather less accurately to today's knowledge, as “the mingled experience deriving from the sensations of the skin in the mouth after ingestion of a food or beverage, as it relates to density, viscosity, surface tension
Oral cavity
Mouth is human being's ultimate device for food consumption and appreciation. Although we use our oral device all the time throughout the whole life, most of us appreciate little of its complexcity. Fig. 4 shows an anatomic diagram of human's oral organ. While the general features and functionalities of mouth would be same for all human beings, the oral individuality should never be underestimated. Sex, age, race, health status, etc., will all make a difference and it is this variation that
Strategy of food oral management
Food management during oral processing, starting from the first bite till after swallowing or clearing, is to ensure that food is transformed from its initial shape and size to a form comfortable to swallow (a bolus) and to ensure a full appreciation of texture and flavour during this process. Food oral processing involves a series of decision makings and oral operations and it is critically important that these procedures occur in right sequence and are well coordinated. A number of models
Oral processing and food rheology
Oral processing is both a physiological process controlled by central nerve system and a physical process modulated by mechanical and geometrical properties of the food. Scientists have the habit of grouping influencing factors into intrinsic and extrinsic. Readers are reminded to be cautious in using these two terms. Physiologists prefer to treat physiological factors as the intrinsic ones and food properties as the extrinsic, while food scientists may think in the opposite way. In order to
The three phases of swallowing
Swallowing could be seen as the last stage of oral processing. Using videofluroscopy technique, Okada et al. (2007) concluded that human beings needed at least two swallows, even with one bit of (solid and semi-solid) foods. They suggested that a complete feeding sequence involves interposed swallows (preceded and succeeded by chewing cycles) and an isolated terminal swallow (to clear the food from the oral cavity and pharynx). The duration length of the swallowing may depend on the bolus
Summary
Food texture is a sensory perception derived from the structure of food (at molecular, microstructure, and macroscopic levels). The appreciation of food texture could involve one or many stimuli, including visual, audio, touch, and kinaesthetic, working in combination. While seeing and touching could provide useful information, oral processing is above all the most important stage for textural perception and appreciation.
Food oral processing involves a series of complex operations, including
Acknowledgements
The author thanks Prof. E. Dickinson and Prof. M. Povey for their advices in writing this review article. Thanks also go to Dr. B. Murray and Dr. R. Ettelaie for their valuable comments and discussion.
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