Received 18 January 2016; accepted 19 April 2016; published 22 April 2016
Juice drinks are an important commercialization alternative for lychee, a tropical and subtropical fruit with high consumer acceptance and high nutritional value  . Although the lychee juice content is important in the assessment of the quality of a juice drink, there are no published methods to determine it. The RSK-values system was officially established by the Federal Republic of Germany in 1980 to measure the juice content in drinks. Numerous methods have been developed to detect juice content in drinks based on measuring characteristic components found in the juice. Beatriz et al. published phenol profiles of 58 citrus varieties in Spain in a study aimed at developing a database to be used in the identification of adulterated drinks but were not appropriate for the routine determination of juice content  .
Ehling and Cole used the pomegranate juice organic acids spectrum to identify adulteration  . With the same objective, Obón et al. identified the types and amounts of anthocyanins in red fruit juice  . In Slovenia and Cyprus, Ogrinc et al. used nuclear magnetic resonance (NMR) spectroscopy to determine the
As there is a correlation between lychee juice content and buffering capacity, when a given amount of citric acid is added into a lychee juice drink, its pH will change more if the lychee juice content is lower. This observation was used to develop a simple and rapid method to detect lychee juice content in juice drinks.
2. Materials and Methods
2.1. Lychee Juice
Fresh lychee juice (Litchi chinensis Sonn.) was obtained from CongHua Shunchangyuan Green Food Co. Ltd., Guangzhou, China, was produced by washing, peeling, pitting and pressing the fruit and finally by filtration through a 60-mesh sieve. Ten lots of commercial juice were diluted with distilled water to prepare in a 250 mL beaker, 50 mL lychee juice drinks containing 100%, 90%, 80%, 70%, 60%, 50, 40%, 30%, 20% and 10% pure lychee juice.
2.2. Determination of the Buffering Capacity of Juice Drinks
Under constant agitation of the 10% - 100% lychee juice drinks using a magnetic stirring bar, up to 1.6 mL of
Figure 1. Effect on the molar H+ concentration when adding at 0.2 mL intervals up to 1.6 mL of monohydrate citric acid (
Table 1. Determination of the drink buffering capacity (BC) as the slope of the linear correlation between the molar H+ concentration values (Y) measured after adding up to 1.6 mL of monohydrate citric acid (
Figure 2. Effect of the lychee juice content (LC) on the buffering coefficient (BC) of the juice drink defining a method to identify the juice content in commercial products.
monohydrate citric acid (25 g/100mL water) were added into each dilution. After the addition of every 0.2 mL citric acid, the pH of the lychee juice solutions was measured (Type PB-20, Sartorius, Germany) and recorded. The monohydrate citric acid used was of analytical reagent quality.
The molar H+ concentration (=10-pH) was linearly correlated to the amount of citric acid (g/L drink) added to each of the 10% - 100% lychee juice drinks. The slope for each drink was defined as its buffering coefficient (BC) and correlated to its lychee juice content. Origin 8.6 was used for statistical analysis and graphics software
3. Results and Discussion
The effect on the molar H+ concentration when adding up to 1.6 mL of monohydrate citric acid (
A linear correlation between the buffering coefficient and the juice content in lychee juice drinks was determined experimentally to develop a simple and rapid method of analysis. The juice content in lychee juice drink can be estimated by measuring the buffering coefficient. However, the buffering capacity of lychee juice may be influenced by several factors, such as proteins, phosphate, citrate, lactate, carbonate, acetate and propionate ions which may reflect not only lychee juice content but other factors not included in this study.
This work was funded by Guangdong Project (2015A030312001), the Project of the National Key Technology Research and Development Program for the 12th Five-year Plan (2012BAD31B03), Guangzhou project (2014Y2- 00099), and Guangdong Maoming Project (2012A012012001).
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