Fruit Maturity Measures in Persian Lime, Rio Red Grapefruit & Sweet Orange

US Citrus - Fresh Citrus Fruit

Fruit Maturity Measures in Persian Lime, Rio Red Grapefruit and Sweet Orange – TSS, Brix, and Acid Determinations. 

Total Soluble Solids (TSS). Citrus fruit accumulates large amounts of edible materials in fats, starches, sugars, and acids. In general, citrus fruits, sugars, and acids constitute about 85% of the organic food reserves and vitamins, proteins, pigments, and minerals – together known as Soluble Solids. It represents 10-20% of the fruit fresh weight, and this weight increases as fruit mature to produce a less acidic but sweeter fruit. Therefore, a TSS/Acid ratio is an important measure. 

TSS is measurable, and it is an index of fruit maturity. The term Brix (sugar content) is often used with TSS in sweet oranges, grapefruit, and tangerines because sugars constitute 85% of TSS in these three citrus fruit types. 

TSS and Brix are not interchangeable in lime because sugars constitute 25% of TSS in lime, and the rest are citric and malic acid.  

Citrus juice has three predominant types of sugars – sucrose, glucose, and fructose. The proportions vary according to citrus variety, rootstock, geography and heat index, and cultural practices. The following table gives a good illustration of three different sugars and acid in three citrus types – grapefruit, sweet orange, and lime.  

  Citrus type







Total Sugars (%)



Sweet orange


















The citrus industry has minimum standards for Total Soluble Solids (TSS), Total Acids (TA), and TSS/TA. This will keep immature fruit off the market. 

The Texas Marketing Order authorizes quality, research, and promotion programs and marketing, grade, size, maturity, pack, and container regulations in Texas, became effective in 1960. The marketing order committee is comprised of 15 members – nine producers and six handlers. 

Brix Hydrometer. Brix (sugar) measurements are performed with a hydrometer to determine the specific gravity of liquids. Sugars and acids are soluble compounds; the more soluble solids mean, the higher the solution's specific gravity. The measurements are taken at 20 C temperatures. 

Refractometer. Refractometers are instruments that measure the refractive index of juice using light refraction or total internal reflection of light. The sugar content of citrus juice is determined by measuring the refractive index of the juice. Most bench-type refractometers, and some hand-held digital types, have automatic temperature compensation. 

Acid Determinations. In citrus fruits, organic acids, mainly citric acid and malic acid, constitutes 10-20% of the TSS. Acidity is measured by titrating a given amount of citrus juice with a 0.3125 Normal sodium hydroxide (NaOH). A 25 ml sample is placed in a beaker. Three or four drops of phenolphthalein indicator are added to the juice solution. NaOH is added drop by drop; the indicator turns pink when the juice's acid is neutralized with NaOH (titration). A conversion table is used. 

Brix/Acid Ratio. Brix to acid ratio is crucial; it constitutes a measure of the balance between sugars and acids. This ratio is a measure of the palatability of the juice. The Brix/acid ratio is obtained by dividing the % Brix by the % titratable acid. A larger value indicates a sweeter taste.  

Unit of Acidity Measurement. pH is the unit of measure which describes the degree of acidity or alkalinity of a solution. It is measured on a scale of 0 to 14. The "p" is the mathematical symbol for negative logarithm, and "H" the chemical symbol for Hydrogen. Therefore, the formal definition of pH is the negative logarithm of Hydrogen ion activity. It gives a quantitative expression of acid or base in terms of its hydrogen ion activity. If the H+ concentration is greater than the OH-, the material is acidic; i.e., the pH value is less than 7. If the OH concentration is greater than its H+, the material is basic, which means the pH value is greater than 7. If equal numbers of H+ and OH- ions are present, the material is neutral, with a pH of 7. 

Now, I want to give my readers a simpler understanding; therefore, non-technical simplifications. Here, I try to bring some clarity via some known numbers. The total soluble solids in grapefruit increase through the season from October-November to January-February, like 9.0, 10.0, 11.0 to 11.5. And the ratio of TSS to ratio of total soluble solids to anhydrous citric acid in the juice. The minimum ratio concerning the TSS given in the last sentence is 7.2:1. 7.0:1, 6.8:1, and 6.5:1. 

The role of the non-climacteric nature of grapefruit, lime, and orange is an important factor to remember. The fruit is harvested mature. The maturity standard measurements of fruit recorded are constant. 

Climacteric is a fruit ripening stage associated with an increase in ethylene production and increased cell respiration. Ethylene is a natural plant hormone and used in agriculture to force the ripening of fruits. It is a colorless, flammable gas with a faint "sweet and musky" smell. We can measure ethylene gas in parts per million quantities.

Concluding Remarks

  1. Federal regulations govern the maturity standards of citrus fruit when harvested.
  2. Citrus is non-climacteric, which means the maturity standards do not change.
  3. Total soluble solids increase, for example, in grapefruit from October to February.
  4. A market-ready Persian lime is expected to have a minimum of 42% juice in the fruit. 

If you're enjoying US Citrus fruit ( through our CRAFT Citrus Club, in addition to federal regulations, lime on your ceviche, salad, or cocktails, please give a salute to our fruit harvesters - Charlie, David, Martha, Belinda, and Jose try their best to get most uniform fruit to your table.  

Mani Skaria, Ph.D.

Professor Emeritus and Founder of US Citrus

Try our seasonal citrus box while its available...the Craft Citrus Club!

Our Rio box is a 15-lb curated box of our fresh-harvested citrus from South Texas. The fruit is NOT gassed, rather it is tree-ripened. Our season runs from Nov-May for oranges, grapefruit and mandarins.

Our Grande box is a 30-lb box of either fresh-harvested Rio Red Grapefruit or Sweet Oranges from South Texas.

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