An in vitro investigation of the initial ph and titratable acidity of a selection of fruit smoothies

Material and methods

Four commonly available fruit smoothies were investigated in this study:

Tropicana® Smooth Orange Juice was used as a positive control. Measurements of the pH of each drink were made using a calibrated pH electrode (Microelectrodes Inc MI-410, NH, USA) connected to an Orion EA940 IonAnalyser (Orion Research, Cambridge, UK). Calibration of the pH electrode was carried out at the start of each laboratory session using standard buffers of pH 4.0 and pH 7.0.

Statistical analysis

The average initial pH levels of the five drinks and the total volume of NaOH required to reach (i) pH 7 and (ii) pH 10 for each of the drinks were compared using one-way analysis of variance with Bonferroni-corrected pair-wise follow-up comparisons. These analyses were undertaken after checking that the data were approximately normally distributed using the Kolmogorov-Smirnov normality test (all p-values>0.05).

Results

Four Innocent® fruit smoothies and Tropicana® Smooth Orange Juice (positive control) were investigated for baseline acidity and titratable acidity.

Titratable acidity

Figure 1 illustrates the titration curves for each of the four smoothie drinks, along with the orange juice control. For a drink to be considered ‘safe’ as far as dental erosion is concerned, the initial pH should be near neutral (which none of the drinks has in this test) and the titratable acidity should be ‘low’ – that is, even if the drink has a low initial pH value, the rise in pH, with the addition of alkali, should be rapid.

All of the drinks, including the orange juice, start off with a pH between 3.30 and 3.80. With the addition of increments of the alkali (NaOH), all five drinks then demonstrate an extremely slow rise in pH, with the Strawberry and Banana being the earliest to demonstrate a quickened rise in pH (at the addition of approximately 11.0 mL of alkali) and the Mango and Passion Fruit being the slowest (at the addition of approximately 15.5 mL of alkali) with the orange juice control demonstrating a more rapid rise in pH at the addition of 12.5 mL of NaOH. Of all the fruit smoothies, the Mango and Passion Fruit drink required the largest volume of 1M sodium hydroxide (mean of 19.5 mL) to raise the pH to a value of 10. The Cranberry and Raspberry smoothie drink exhibited the same tendency as the Mango and Passion Fruit smoothie, starting with a low baseline pH and demonstrating a high titratable acidity, with 17.7 mL of 1 M NaOH being necessary to raise the pH to a value of 10. The Strawberry and Banana smoothie drink required the least volume of 1M NaOH (14.0 mL) to raise the pH to a value of 10. The orange juice control, despite having the highest baseline pH of all the drinks investigated, required a mean volume of 16.5 mL to raise the pH to 10 (Figure 1, Table 2). For comparability with previous work, the mean volume of NaOH necessary to raise the pH to a value of 7 for each drink is also listed in Table 2.

Table 2. The mean (standard deviation) volume of 1 M NaOH required to raise the pH of all the drinks investigated to (i) pH of 7 and (ii) pH of 10.

	pH 7 (mL of NaOH)		pH 10 (mL of NaOH)
Drink	Mean	SD	Mean	SD
Orange juice	13.0	0.00	16.5	0.00
Strawberry/Banana	11.0	0.00	14.0	0.00
Cranberry/Raspberry	14.2	0.29	17.7	0.29
Blueberry/Blackberry	12.0	0.00	15.3	0.29
Mango/Passion Fruit	16.7	0.29	19.5	0.50

Analysis of variance confirmed that there were statistically significant differences between the drinks in terms of the total volume of NaOH needed to reach both pH 7 and pH 10 (both p<0.001). The follow-up pair-wise comparisons showed that there were significant differences between all pairs of drinks, for both pH 7 and pH 10 (all pair-wise p-values<0.01; Table 3).

Table 3. Bonferonni-adjusted pair-wise p-values comparing the volume of 1 M NaOH required to raise the pH up to (i) 7 and (ii) 10.

	Orange Juice	Strawberry/Banana	Cranberry/Raspberry	Blueberry/Blackberry
(i) pH 7
Strawberry/Banana	< 0.0001
Cranberry/Raspberry	0.0001	< 0.0001
Blueberry/Blackberry	0.0005	0.0005	< 0.0001
Mango/Passion Fruit	< 0.0001	< 0.0001	< 0.0001	< 0.0001
(ii) pH 10
Strawberry/Banana	< 0.0001
Cranberry/Raspberry	0.0058	< 0.0001
Blueberry/Blackberry	0.0058	0.0021	< 0.0001
Mango/Passion Fruit	< 0.0001	< 0.0001	0.0002	< 0.0001

Discussion

In 2010, 51 million litres of fruit smoothies were consumed in the United Kingdom.2 This yields an average of 0.8 litres per person and an 8.5% increase on the previous year.2 In 2007, fruit juice consumption in children is known to have accounted for 20% of all UK children's choice of soft drink.4 The additional contribution to this figure from smoothies could, therefore, be substantial. A fruit smoothie is a ‘blend of crushed and freshly squeezed fruit’. It should be 100% pure fruit with no added sugar or water and, very importantly, be made with fresh rather than concentrated juices.16 The actual current level of fruit smoothie consumption by children is unknown.

The success of fruit smoothies seems, at least in part, to be attributed to the public's perception of these drinks being considered ‘healthy’, ‘natural’ and convenient.17 The public is currently being encouraged to eat more healthily by consuming more fruit and vegetables and less saturated fat. The consumption of fruit smoothies is viewed by some as a means of fulfilling this requirement.3,17 Various ‘boosters’, such as vitamin C (ascorbic acid), aloe vera, ginseng, ginger, wheatgrass juice and bee pollen, are also offered in many smoothie bars as additional ‘shots’.1 Some smoothies are being made with so-called ‘superfruits’, such as Goji and Açai berries which, the manufacturers claim, have superior anti-oxidant properties compared to other fruit.

Erosion appears to be the major cause of TSL in childhood and adolescents.11 Some of the fruit smoothies on the market are aimed specifically for consumption by children, with ‘child friendly’ recipes and lunchbox-sized cartons of fruit smoothies. These are readily available from supermarkets, newsagents and are even sold from vending machines in some schools in the United Kingdom. Drinks consumed during school hours are either brought in lunchboxes (71%) or purchased at school (26%). Recent research would suggest that the frequency of acidic exposure may be an important factor in dental erosion.18 It is reassuring, however, to note in this regard that 77% of fruit smoothies are reported to be consumed at mealtimes.4

The investigation of the baseline pH of the fruit smoothies showed that all the drinks tested demonstrated a pH value of between 3.0 and 4.0. The differences in the mean baseline pH and titratable acidity of the various fruit smoothies could be due to the different types of acids and their concentrations in the fruit from which the fruit smoothies are prepared. The main organic acids in Mango and Passion Fruit and Raspberry and Cranberry fruit smoothies are ascorbic acid, and ascorbic, gallic, quinic acids, respectively. The term pKa is a measure of the amount of dissociation (or strength) of an acid solution. An acid molecule is made up of the conjugate base and a single proton, or hydrogen ion. When there is only one single proton available per molecule, these are known as monoprotic acids. An acid with a low pKa indicates a relatively high amount of dissociation and indicates, therefore, a relatively ‘strong’ acid. In contrast, an acid with a high pKa value indicates low dissociation and, therefore, a relatively ‘weak’ acid. Ascorbic and citric acids, however, have two (diprotic) and three (triproic) protons, respectively, available per single molecule, each of which will be released at differing pKa values. L-Ascorbic acid has a pKa1 and pKa2 of 4.04 and 11.7, respectively, whereas citric acid has a pKa1, pKa2, pKa3 of 3.13, 4.76 and 6.40, respectively.19 Gallic acid has a pKa of 4.40.20 Quinic acid has a pKa of 3.572 +/- 0.006 at 25°C.21

One reason for the high titratable acidity in the Mango and Passion Fruit and Raspberry and Cranberry smoothies could be the different ‘acid systems’, the concentrations and combinations of the fruit acids and how they compete with each other when titrated with sodium hydroxide. The way an acid behaves depends on the pH of the solution. For example, Hughes et al found that lactic acid (which is usually viewed as a ‘weak’ acid) at low pH was more erosive than either citric or malic acids, while malic acid was less erosive than either citric or lactic acids at a high pH.22 In comparison with other fruit-based drinks, eg fruit teas, flavoured mineral waters, ciders and alcopops/designer drinks, the mean baseline pH and titratable acidity of fruit smoothies were comparable.23 The fruit-based smoothies tested here had both a low pH and a high titratable acidity, which makes their erosive potential greater than orange juice. Edwards et al demonstrated that, whilst the orange juice had the highest titratable acidity of the drinks they tested, the pH of that drink was the highest, but was still deemed to be the most potentially erosive drink in that study.24 In complete contrast, the orange juice in the study reported here (which was employed as the control) demonstrated the highest pH and lowest intractable acidity when compared with all of the fruit smoothies. Thus, quite surprisingly, the fruit-based smoothies may be estimated as demonstrating far greater potential for damage to the mineralized tissues of the tooth than the orange juice.

Another factor which may have a bearing on the quantity of potential erosion that fruit smoothies could produce is the temperature at which they are consumed. Eisenburger and Addy found that, in an in vitro study, tooth erosion depth increased significantly with increasing temperature.25 The fruit smoothies investigated in this study were investigated at room temperature (approximately 25°C). Amaechi et al found, also, that the erosiveness of orange juice was less pronounced at a lower temperature although, interestingly, they found no statistical difference in pH over different temperatures.26 In any case, the temperature employed here probably simulates the highest temperature that a fruit smoothie would be consumed at and would, therefore, simulate the maximum erosiveness of the fruit smoothie.

Drinks made with fruits containing high concentrations of citric acid and vitamin C (ascorbic acid) showed, in general, a higher titratable acidity and have been linked with greater levels of dental erosion.23 Investigations of the effect of different acids on enamel have shown that more erosion was found when there was a low pH and a high acid concentration than at a high pH and low acid concentration.22 Since the fruit smoothies investigated had a high titratable acidity, it seems reasonable to assume such drinks could be potentially erosive to the human dentition.

Patients and parents should be made aware of the potential risk of dental erosion in consuming ‘healthy’ fast food such as fruit smoothies. Prevention of dental erosion is by far the preferred choice rather than restorative intervention. Treatment can be complicated and prolonged and varies from monitoring the dentition, placement of simple composite restorations, to restoration with complex extra-coronal restorations, in the case of more advanced conditions, which can be both time-consuming and expensive.^27,28

Conclusions

Further research is required to determine the effect of fruit smoothie drinks on the human dentition. From the data presented here, such drinks exhibit both low baseline pH values and high titratable acidity and, therefore, may pose a potential risk of dental erosion in susceptible individuals.

Fruit consumption should still be encouraged, since it is part of a healthy diet.5 Consumption of fruit smoothie drinks, as with many other acidic beverages, should be limited to mealtimes and consumed through a straw.29,30