Quality Aspects of Citrus Juices

By Renée Goodrich and Charles Sims

In this era of huanglongbing (HLB), almost every scientific and popular press article alludes to HLB’s impact on fruit and juice quality. The citrus industry uses the term “high-quality” to denote a fruit, product or byproduct that has an array of positive, expected and measurable attributes. Consumers also use terms referring to quality, but seldom in the same specific and objective manner. Processors, marketers and market researchers spend time, effort and money in the quest to understand how consumers think about citrus products, and to deliver products that meet those expectations. Consumer research will always be a fruitful area for exploration and understanding.

This article, however, will review some aspects of citrus juice quality and quality measures from an industry perspective, with a focus on orange juice. Understanding some of the traditional quality determinants is important as the industry makes strategic production and processing decisions moving forward.

Almost every specific food industry (dairy, meat, etc.) has their own specific quality measurements, and citrus is no exception. Key quality control (QC) parameters for citrus juices include Brix, acidity, Brix/acid ratio (BAR), the Scott oil analysis, color measurement and flavor assessment. These parameters are routinely and regularly measured by citrus processors who must consider U.S. regulations (as exemplified by the current discussion of the U.S. Food and Drug Administration standard of identity for orange juice), quality grading programs [through the U.S Department of Agriculture (USDA) Agricultural Marketing Service, for example], and internal and customer product specifications.

SOLUBLE SOLIDS
The soluble solids in orange, grapefruit and mandarin juices consist primarily of carbohydrates (sugars) and organic acids. The sugars (sucrose, glucose and fructose) represent about 80% of those solids for orange. The soluble solids value is an indication of the maturity of the fruit as well as the sugar content of the product — once the measurement is corrected for temperature and acidity. The acidity correction is utilized in the citrus industry due to the significant contribution of citric acid in the product. This provides a better representation of the sugar content of the juice, which is the primary driver of sweetness. The soluble solids content can be measured by hydrometer (based on density) or refractometer (based on light refraction) and is normally reported as ⁰Brix or ⁰B after the appropriate calculations.

ACIDITY
The acidity, or titratable acidity, is a common measurement of the citric acid content of the juice and is accomplished by titration of a known amount of juice with an aqueous alkali solution to end point. Acidity is a key quality attribute of citrus juice and is responsible for the characteristic tartness or sourness of these products. Measurement of acidity provides an indication of fruit maturity, is used in the correction of the soluble solids measurement and is used in the BAR calculation. It is normally reported as % citric acid.

BRIX/ACID RATIO
The BAR is a derived value and is calculated by dividing the acid- and temperature-corrected ⁰B value by the % by weight citric acid value. This ratio is one of the key quality indicators of fruit maturity as well as citrus juice quality. BAR represents the sweet/tart balance that is so very characteristic in many citrus juices, especially orange. A product with an excessively low BAR can be perceived as too sour. Conversely, citrus juice products with high BAR have been described as “insipid” or “cloying.” The BAR is often used to develop juice blending algorithms that ensure consistency in final, packaged products.

VOLATILE COMPONENTS
Sweetness and tartness contribute to the overall taste of the juice, but the distinctive flavor of a citrus juice is heavily dependent on the volatile components of the juice. Many of these desirable compounds are present in juice at very low levels (parts per million) and are not common QC analyses.

The exception is d-limonene, the major hydrocarbon in citrus oils. While d–limonene is not a major contributor to citrus juice flavor per se, excessive levels in the juice can contribute to “oil burn,” an unpleasant sensation in the mouth and on the tongue. The Scott oil method is used to monitor levels of d-limonene in the juice that might vary due to preharvest conditions or processing practices and lead to lower-quality product when excessive. The Scott oil method involves distillation and titration steps with the results reported in % oil by volume. Most orange juices contain 0.015% to 0.025% oil, which is a level that will not contribute to significant oil burn and that is well-below USDA grade A and B maximums.

COLOR
The color of citrus juices is an important and measurable quality attribute. The main carotenoids responsible for orange and tangerine juices include α- and β-carotene. Lycopene is responsible for the pink or red color of some grapefruit juices. However, these specific compounds are not measured in QC labs due to the specialized instrumentation required. Instead, the industry and the USDA utilize standard grading tubes (rarely) and tristimulus testing via a colorimeter (e.g., a Gretag-MacBeth colorimeter) to determine the so-called color score for orange juices. Orange juices can be blended with up to 10% tangerine juice to enhance their color if needed. The vibrant color of a typical Valencia orange is one reason for its desirability as a processing orange.

FLAVOR
Of the common quality attributes, flavor is perhaps the most difficult to objectively measure in the QC laboratory. Flavor is assessed for grading as very good, good or poor and is usually based on training rubrics and experience. In general, very good flavor indicates the particular juice is highly characteristic of the sound, mature fruit from which is derived. Sub-optimal flavor can be a result of many factors, including preharvest considerations (fruit under- or over-maturity, effect of HLB or variety) or postharvest and postprocessing conditions including those related to plant practices such as fruit grading, extraction/finishing, thermal processing and cleaning/sanitation.

OTHER FACTORS
There are other quality factors assessed during the commercial production of citrus juice in addition to those described above. These include the assessment of residual pectin methylesterase enzyme activity and visual assessments for embryonic seeds, hesperidin crystals and other potential juice defects.

CONCLUSION
The goal of juice processing is to preserve the character of the incoming materials (i.e., the fruit) through careful handling, processing and quality monitoring while concurrently operating in a cost-effective manner. Adhering to best practices in plant cleaning and sanitizing, as well as temperature control of the product(s), are important. Growers, processors, and packagers all have important roles in producing consumer-accepted citrus juices. The careful consideration of quality factors, including those described above, are part of planning strategies to maintain a successful citrus juice industry.

Renée Goodrich and Charles Sims are professors at the University of Florida in Gainesville.

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