Acidogenic potential of soy and bovine milk beverages

Abstract

Soy beverages are water extracts of whole soybeans and are often promoted as a healthy alternative to bovine milk. Little analysis has been carried out to determine the effects of soy beverages on oral health, especially their potential acidogenicity.

Objectives

The aim of this study was to determine the potential acidogenicity of a range of soy and bovine milk beverages.

Methods

In vitro acid production by Streptococcus mutans was measured in soy and milk beverages at a constant pH of 6.5 or 5.5, as was the fall in pH over a 10 min period. The acid buffering capacity and calcium and phosphate concentrations (total and soluble) of the beverages were also determined.

Results

The rate of acid production by S. mutans in the milk beverages was five to six times lower at pH 6.5 than in the soy beverages and three to five times lower at pH 5.5. Whilst the pH fall in the presence of S. mutans over 10 min was negligible in the milk beverages there was a significant decrease in pH in the soy beverages. This was also reflected in the lower buffering capacity of the soy beverages. The levels of soluble calcium in the soy beverages were lower than those in the milk beverages although total calcium contents were similar.

Conclusions

Soy beverages have a higher potential acidogenicity than bovine milk beverages.

Clinical significance statement

Patients consider soy beverages to be a healthy, low cariogenic alternative to other beverages, including bovine milk. This study shows that soy beverages have a higher potential acidogenicity than bovine milk and therefore may have a greater potential cariogenicity.

Introduction

Soy beverages, which are also known simply as soy, or incorrectly as soy milk, are water extracts of whole soybeans. The composition of soybeans varies somewhat according to variety and growth conditions, which then affects the composition of the extract.1, 2 Soybeans typically contain around 3.5% protein and 2.2% oil, up to 3.5% simple carbohydrates (sugar) and about 0.5% mineral salts.3, 4 The principle soluble carbohydrates of mature soybeans are the disaccharide sucrose, the trisaccharide raffinose, and the tetrasaccharide stachyose. Insoluble carbohydrates in soybeans include the complex polysaccharides cellulose, hemicellulose and pectin. Soy beverages are a stable emulsion of water, oil, protein and mineral salts with an off-white appearance similar to mammalian milk. Sucrose and/or glucose are usually added to the soy beverages for taste. Soy beverages are often promoted as a healthy alternative to bovine milk as they are a source of lecithin, isoflavones (non-steroidal oestrogens) and vitamin E, contain no lactose and have less saturated fat than milk. Soy has approximately the same protein content as cow's milk, although not the same amino acid profile. However natural soy beverages contain little bioavailable calcium and to address this many manufacturers enrich their products with calcium salts.⁵ In addition soy products contain relatively high concentrations of phytate which chelates important minerals such as zinc, iron, calcium and magnesium ions, reducing their bioavailability.⁶ Phytate (inositolhexaphosphate), is the principal storage form of phosphorus in many plants, especially legumes and cereal grains. Phytate is not digestible by humans who lack the phytase-containing microbiota of ruminants.

Dental caries is one of the most prevalent chronic diseases of humans and affects the vast majority of individuals.⁷ Dental caries is a dynamic process that is initiated in the bacterial biofilm (dental plaque) on the tooth surface resulting in the disturbance of the equilibrium between tooth mineral and the surrounding plaque fluid so that over time there is a net loss of mineral from the tooth, producing a subsurface lesion. Caries is a multifactorial process that requires a shift in plaque ecology to favour acidogenic and aciduric microbial species that is usually driven by frequent consumption of simple carbohydrates and is modified by host factors such as saliva.⁸ The main bacterial species traditionally associated with caries initiation are Streptococcus mutans and Streptococcus sobrinus although other bacteria, and more recently fungal species, have also been implicated in disease initiation and/or progression.9, 10, 11, 12 These species are part of the normal oral microbiota and are regarded as opportunistic pathogens. The two major virulence factors of known cariogenic species are: (1) acidogenicity, the ability to rapidly catabolise simple carbohydrates, including sucrose, lactose, glucose and fructose, generating organic acids as the major end-product when these sugars are present in excess; (2) acidurance, the ability to metabolise and grow at low environmental pH.10, 13, 14

The aim of this study was to determine the potential acidogenicity of soy and milk beverages by S. mutans fermentation and to determine the acid buffering capacity and fluoride, calcium and phosphate contents of the beverages.