Lab Report Sample: Vitamin C Content of Commercial Fruit Juices

Model Answer

Abstract

This report describes a comparative analysis of vitamin C (ascorbic acid) content in four commercial fruit juices – fresh orange juice, shop-bought (UHT, long-life) orange juice, apple juice and cranberry juice – using the DCPIP (2,6-dichlorophenolindophenol) titration method. Following standardisation against a known ascorbic acid solution, triplicate titrations were performed on each juice. Fresh orange juice contained the highest vitamin C content (50.0 mg/100 mL), followed by shop-bought orange juice (38.0 mg/100 mL), cranberry juice (12.0 mg/100 mL) and apple juice (3.0 mg/100 mL). These results are consistent with known patterns of ascorbic acid degradation during processing and storage, and with the naturally low vitamin C content of apples. The findings illustrate both the practical utility and the specificity limitations of the DCPIP titration method for classroom food analysis.

Introduction

Vitamin C (ascorbic acid) is an essential water-soluble micronutrient required for collagen synthesis, immune function and antioxidant protection, and cannot be synthesised by the human body, making dietary intake, largely from fruit and vegetables, essential (Damodaran, Parkin and Fennema, 2017). In the United Kingdom, the reference nutrient intake (RNI) for vitamin C in adults is 40 mg per day. Fruit juices are a major dietary source of vitamin C for many consumers, but ascorbic acid is notably unstable, readily oxidised by exposure to heat, light, oxygen and prolonged storage, meaning that the vitamin C content of processed and stored juices can differ substantially from that of freshly squeezed fruit (Johnston and Bowling, 2002).

The DCPIP titration method is a long-established, simple and inexpensive technique for estimating vitamin C content in food samples, based on the ability of ascorbic acid to reduce the blue dye 2,6-dichlorophenolindophenol (DCPIP) to a colourless form; the volume of DCPIP decolourised by a sample is therefore proportional to its ascorbic acid content, once calibrated against a known standard (AOAC, 2000; Ranganna, 1986). While the method is not fully specific to ascorbic acid, since other reducing substances present in some foods can also decolourise the dye, it remains widely used in teaching and quality-control settings because of its speed, low cost and minimal equipment requirements.

More sophisticated analytical alternatives exist, including spectrophotometric assays and high-performance liquid chromatography (HPLC), both of which offer greater specificity for ascorbic acid and lower detection limits than titration, but require instrumentation, calibration standards and analysis time that are rarely available in an undergraduate teaching laboratory (Klimczak and Gliszczyńska-Świgło, 2015). DCPIP titration therefore remains the standard introductory method for comparative food analysis teaching, offering students hands-on experience of volumetric technique and calculation alongside a genuinely informative result.

The present study aimed to use DCPIP titration to compare the vitamin C content of four commonly consumed commercial fruit juices, representing a fresh, minimally processed product (fresh orange juice), a heat-processed and stored product (shop-bought UHT orange juice), a fruit naturally low in vitamin C (apple), and a fruit associated with a moderate vitamin C content (cranberry), and to evaluate the results against the UK reference nutrient intake for vitamin C. It was hypothesised that fresh orange juice would show the highest vitamin C content, followed by shop-bought orange juice, reflecting processing-related losses, with apple juice showing the lowest content of the four samples.

Method

Apparatus and materials. Materials used were: a 1 mg/mL ascorbic acid standard solution prepared in 1% metaphosphoric acid (to stabilise the ascorbic acid against oxidation); a 0.1% DCPIP indicator solution; a 50 mL burette; 100 mL conical flasks; graduated pipettes; and four commercial juice samples (fresh orange juice, shop-bought UHT orange juice, apple juice and cranberry juice), each obtained from a single retail batch and stored refrigerated until use on the day of testing. Triplicate titrations were performed for every sample, including the standard, so that a mean titre and an indication of measurement consistency could be reported for each juice rather than relying on a single, potentially unrepresentative reading.

Standardisation. A 1 mL aliquot of the 1 mg/mL ascorbic acid standard was titrated against DCPIP from the burette, with the endpoint taken as the first persistent faint pink colour lasting at least 15 seconds. This was repeated in triplicate (titres of 0.98, 1.02 and 1.00 mL; mean = 1.00 mL), giving a dye factor of 1.00 mg ascorbic acid per mL of DCPIP.

Sample titration. For each juice, a 10 mL aliquot was pipetted into a conical flask and titrated against DCPIP in the same manner, in triplicate, with the mean titre used for calculation. Because cranberry juice is strongly coloured, the endpoint was judged with particular care against a white background, titrating slowly near the expected endpoint to avoid over-titration.

Calculation. Vitamin C content per 10 mL sample was calculated as mean titre (mL) × dye factor (mg/mL), and then scaled to a standard 100 mL serving by multiplying by 10, following AOAC (2000) titrimetric procedure.

Results

Table 1 shows the mean titration volumes and calculated vitamin C content for each juice sample, alongside the percentage of the UK adult reference nutrient intake (RNI = 40 mg/day) that a 100 mL serving would provide.

Table 1. Mean titre and calculated vitamin C content by juice sample.

A worked example for fresh orange juice illustrates the calculation. The mean titre of three replicates (5.1, 4.9 and 5.0 mL) was 5.0 mL. Multiplying by the dye factor gives 5.0 mL × 1.00 mg/mL = 5.0 mg ascorbic acid in the 10 mL sample. Scaling to a 100 mL serving: 5.0 mg × (100/10) = 50.0 mg/100 mL. Expressed as a percentage of the UK adult RNI: (50.0/40) × 100 = 125%. The same calculation applied to apple juice, with a mean titre of only 0.3 mL, gives 0.3 mg in 10 mL, or 3.0 mg/100 mL – only 7.5% of the RNI, and notably close to the lower limit of what can be measured precisely using this titration volume and burette graduation.

Fresh orange juice provided more than a full day’s RNI in a single 100 mL serving, while shop-bought orange juice provided slightly less, consistent with a modest reduction attributable to heat processing and storage. Cranberry juice provided roughly a third of the RNI, and apple juice provided only a small fraction, reflecting the naturally low ascorbic acid content of apples relative to citrus fruit. Across all four samples, the standard deviation of the three replicate titres was small relative to the mean, generally within 5 to 8 per cent, indicating that the titration technique produced consistent, repeatable readings within each sample and that the differences observed between juices reflect genuine differences in vitamin C content rather than measurement noise.

Discussion

The results support the study hypothesis: fresh orange juice contained the highest vitamin C content of the four samples, shop-bought orange juice was somewhat lower, and apple juice was lowest by a wide margin, with cranberry juice falling between the two orange juice samples and apple juice. The 24% reduction observed between fresh and shop-bought orange juice (50.0 versus 38.0 mg/100 mL) is broadly consistent with previous findings that pasteurisation, oxygen exposure during processing, and storage time can each contribute to measurable ascorbic acid losses in commercial orange juice, even when packaging is designed to limit light and oxygen exposure (Johnston and Bowling, 2002). The very low vitamin C content of apple juice reflects the underlying composition of apples themselves, which are a comparatively poor source of vitamin C relative to citrus fruit and berries, regardless of processing (Lee and Kader, 2000).

The DCPIP titration method used here has well-documented limitations that should be considered when interpreting these results. The reaction is not fully specific to ascorbic acid; other reducing compounds present in juice, including certain sugars, tannins and polyphenols, particularly abundant in strongly coloured juices such as cranberry, can also reduce DCPIP and may have led to a modest overestimation of true ascorbic acid content in that sample (Ranganna, 1986). More precise, specificity-controlled alternatives such as high-performance liquid chromatography (HPLC) are available and are generally preferred for research-grade quantification, though at substantially higher cost and complexity, making DCPIP titration more appropriate for comparative, teaching-oriented analysis of this kind (Klimczak and Gliszczyńska-Świgło, 2015).

A further limitation concerns measurement precision at low vitamin C concentrations: the apple juice titre of only 0.3 mL is close to the smallest volume that can be read reliably from a standard burette, meaning the resulting estimate carries proportionally greater uncertainty than the titres obtained for the orange juice samples; a larger sample aliquot or a more dilute DCPIP solution would improve precision for low-vitamin-C juices in future work. Each juice was also tested from a single retail batch, so these results cannot be generalised to other brands, harvest seasons or storage durations without further sampling, and no assessment was made of vitamin C loss over the shelf life of the opened juices themselves.

Future analyses would benefit from testing multiple brands and batches of each juice type to assess typical variability, from monitoring vitamin C loss over a period of refrigerated storage after opening, and from cross-validating a subset of samples using a more specific method such as HPLC, to quantify the extent of any overestimation introduced by non-ascorbic-acid reducing substances in the DCPIP method.

These results also carry a practical, dietary-relevant message: a single 100 mL serving of fresh orange juice comfortably met the full UK adult reference nutrient intake for vitamin C in this analysis, whereas an equivalent serving of apple juice provided only a small fraction of that intake, illustrating that not all fruit juices are nutritionally interchangeable as vitamin C sources, and that consumers seeking to meet vitamin C intake through juice specifically should be aware that fruit type, and processing history, materially affect the nutrient content of the product they are drinking (Rickman, Barrett and Bruhn, 2007).

Conclusion

This titration-based comparison found that fresh orange juice provided the highest vitamin C content of the four juices tested, at more than the full UK adult reference nutrient intake per 100 mL serving, followed by shop-bought orange juice, cranberry juice, and finally apple juice, which provided only a small fraction of the RNI. These findings are consistent with known patterns of ascorbic acid degradation during commercial processing and storage, and with the naturally low vitamin C content of apples relative to citrus fruit. The DCPIP titration method proved a practical, low-cost and instructive technique for comparative classroom food analysis, though its lack of full specificity to ascorbic acid, and reduced precision at low analyte concentrations, mean that results for weakly coloured, low-vitamin-C samples should be interpreted with appropriate caution.

References

• Association of Official Analytical Chemists (AOAC) (2000) Official Methods of Analysis. 17th edn. Gaithersburg, MD: AOAC International.
• Damodaran, S., Parkin, K.L. and Fennema, O.R. (2017) Fennema’s Food Chemistry. 5th edn. Boca Raton: CRC Press.
• Johnston, C.S. and Bowling, D.L. (2002) ‘Stability of ascorbic acid in commercially available orange juices’, Journal of the American Dietetic Association, 102(4), pp. 525–529.
• Klimczak, I. and Gliszczyńska-Świgło, A. (2015) ‘Comparison of UPLC and HPLC methods for determination of vitamin C’, Food Chemistry, 175, pp. 100–105.
• Lee, S.K. and Kader, A.A. (2000) ‘Preharvest and postharvest factors influencing vitamin C content of horticultural crops’, Postharvest Biology and Technology, 20(3), pp. 207–220.
• Ranganna, S. (1986) Handbook of Analysis and Quality Control for Fruit and Vegetable Products. 2nd edn. New Delhi: Tata McGraw-Hill.
• Rickman, J.C., Barrett, D.M. and Bruhn, C.M. (2007) ‘Nutritional comparison of fresh, frozen and canned fruits and vegetables’, Journal of the Science of Food and Agriculture, 87(6), pp. 930–944.
• Scientific Advisory Committee on Nutrition (SACN) (2020) Statement on Nutrient Reference Values for Vitamin C. London: Public Health England.