What Is Titratable Acidity and Why Is It So Important in Fresh Produce?

• Titratable acidity (TA) is measured as it is an indicator of quality, maturity, and taste in fresh produce.
• Measuring titratable acidity in the fresh produce supply chain can ensure quality, shelf-life, consistency, and consumer satisfaction.
• It is used to determine harvest time, quality control, sorting, grading, and regulatory compliance.

Titratable acidity is a crucial intrinsic chemical parameter that is monitored in the fresh produce supply chain. It is beneficial for vegetables and fruits, whether eaten fresh or processed into products such as juices and wines. Please find out more about titrable acidity, its importance, and why it is measured.

What is Titrable Acidity?

Table 1: “Level of titrable acidity and pH for various fruits,” Omar & MatJafri (2013). (Credits: DOI 10.3920/QAS2012.0175)

Titrable acidity (TA) is the total acid content in a food, usually of organic acids and, to a lesser extent, of inorganic acids. The common organic acids are citric, tartaric, and malic acids:

• Citric acid and malic acid are commonly found in fruits and vegetables.
• Leafy vegetables contain oxalic acid.

Titrable acidity content changes in plants and the fresh produce supply chain due to various processes.

The processes producing acids can be one of the following:

• Plants are complex and have several chemical cycles that produce acids, such as the Krebs cycle, which is part of cellular respiration in all organisms.
• During reaction, fruits accumulate organic acids.
• During the processing of fresh produce into juices, wines, and other alcoholic drinks, acids are produced as byproducts of fermentation.
• Inorganic acids, such as phosphoric and carbonic acids, can also be present due to the dissolution of carbon dioxide in water.

Titrable acidity is reduced when acids are oxidized during respiration, yielding carbon dioxide and water. An increase in TA reflects an increase in production, whereas a decrease in TA demonstrates an increase in oxidation.

Titrable Acidity vs pH

More than one acid can be present in the fresh produce simultaneously or at different times of maturity. For example, malic acid is high before maturity, but tartaric acid predominates in ripe grapes. The dominant acid present can identify TA, but this is not necessary to measure TA accurately, as TA measurement methods cannot differentiate between the acids present. However, this is not an issue, as the equivalent weights of common acids in the fresh produce are similar.

Titrable acidity measures the total concentration of the acids in fresh produce and is expressed as grams per liter (g/L) and milliequivalents per liter (meq/L). TA, however, does not estimate the acid’s strength or the hydrogen ion concentration; these are measured by pH. There is no direct correlation between TA and pH. However, higher levels of TA can lower pH. The average levels of TA and pH of organic acids usually found in common fruits are listed in Table 1.

TA is a standard chemical parameter measured from farms to retailers at various points along the fresh produce supply chain for several reasons.

Importance of Titrable Acidity

TA is measured because it is a crucial indicator for quality, flavor, ripeness, and maturity.

Influences Taste and Flavor

Acidity is responsible for the sourness and tartness in fruits. As fruits ripen, the concentrations of organic acids reduce, and the soluble sugar contents (SSC) increase. While fruits should be sweet, TA is also necessary to ensure a good taste, a vital component of flavor. Fruits with very high SSC or ^oBrix and low TA can be rejected.

One of the main applications of TA is calculating taste using the SSC/TA ratio. The ratio is a better indicator of taste than either parameter alone, see Table 2.  The SSC/TA ratio ensures that fruits have the characteristic taste and flavor of fresh produce. The optimal ratio of the two parameters will vary by species and cultivar.

As an indicator of taste and flavor, TA has several applications in the supply chain.

Table 2: “Acid and ^oBrix of some commercially important fruits,” Sadler and Murphy (2010). ( Credits:  DOI 10.1007/978-1-4419-1478-1_13)

Quality Indicator

TA is one of the standard intrinsic parameters used to estimate the quality of fresh produce, besides soluble sugar contents, firmness, and dry matter, because of the following reasons:

• A significant decrease in TA during storage can induce fruit senescence or decay, which affects taste, firmness, and color.
• The organic acids measured by TA determine color development through their influence on anthocyanins.
• SSC/ TA ratio is used as a nutritional indicator.
• As a vital component of taste, TA measurements are necessary to meet quality standards for various fresh produce and ensure consumer satisfaction.
• TA can prove that the fresh produce poses no safety or health risks, since low TA can indicate postharvest defect or spoilage.

Indicator of Maturity and Ripeness

Since TA levels decrease and SSC levels rise as fruits mature, TA levels are used alone or as taste indicators to estimate maturity.

• In non-climacteric fruits, TA is used to assess ripeness.
• In climacteric fruits, TA is used to measure maturity and ripeness.

Influences Shelf-life

Acidity is crucial for preserving and maintaining the quality of fresh produce.

• When TA levels are very low, fruit senescence accelerates, reducing shelf life.
• Correct TA levels also help prevent the growth of harmful microbes, keeping fruits fresh.
• Low TA is also associated with over-ripeness.

Hence, TA measurements can be used to predict the shelf-life of fresh produce. Maintaining the correct TA can extend the shelf-life of fresh produce.

Applications of Titratable Acidity in The Fresh Produce Supply Chain

As an indicator of taste, flavor, maturity, quality, and shelf life, TA is used as a harvest index and for sorting, grading, quality control, and regulatory compliance.

Harvest Index

TA is a crucial harvest maturity index for many fruits and vegetables, used to fix the harvest time. As fruits mature, their TA is one of the parameters tracked on farms. Fruits harvested at the correct maturity, with optimal TA levels, will have the required qualities to meet end-consumer expectations, avoid rejection, and allow time for long-distance transport to maximize marketing. Non-climacteric fruits are harvested when ripe, and climacteric fruits are harvested when fully mature but unripe.  Fresh produce harvested before full maturity or ripeness will not develop the required qualities, but picking fruits too late can reduce transport and storage time and increase spoilage.

Harvesting at the wrong time is one of the significant causes of fruit quality problems in the supply chain. Measuring TA objectively and quantitatively provides stakeholders with the information needed to adjust storage conditions, save yields, and maximize profits.

Sorting and Grading

As an indicator of taste and maturity, TA is measured to sort and grade fresh produce. TA measurements can ensure consistency in taste and flavor between and within batches. TA helps ensure that fruits of uniform maturity are packed in a batch so they arrive at the retailer with similar ripeness. It reduces food loss and unscheduled ripening caused by ethylene production and spread from single overripe fruits in a batch.

Quality Control

Failing to maintain quality can harm the brand. Quality measurements can be made throughout the supply chain during transport, storage, and retailing.

• Measuring TA ensures the expected flavor and taste are maintained in fruits and vegetables in each batch.
• The TA of fruits grown for processing, like juice or wine making, are measured to meet required properties, besides SSC and dry matter.
• TA levels are a standard quality parameter measured during the processing to ensure end acidity levels are maintained and to ensure proper chemistry for product development and stability.
• TA is measured during transport, storage, and retailing to ensure the fresh produce meets required quality standards. Poor-quality fresh produce should be culled periodically to prevent ethylene-triggered spoilage.

For Regulatory Compliance

Nowadays, the supply chains for fresh produce are international. Suppliers must meet international marketing standards, which are drafted by international agencies like the UN and OCED, and regional regulations for the USA and the European Union.  These agencies use standards to regulate and maintain the quality of fresh produce, which rely on quantitative data for intrinsic and external parameters.

TA is one of the standard quality and maturity parameters, whose values are set for fresh produce in regulations. Measuring TA is necessary to meet regulatory requirements of quality, maturity, safety, and nutritional standards.

Factors Influencing Titrable Acidity

The hike and fall in TA levels in the supply chain can depend on various factors. These are as follows:

• Acids: Types of acids and their concentrations influence TA.
• Genotype: Within a species, TA can vary among cultivars in the orchard and during storage, as shown in Figure 1.
• Physiology: Higher fruit respiration rates in the postharvest stages reduce TA levels.
• Cultivation practices: Application of biofertilizers and effective micro-organisms can increase TA.
• Postharvest applications: Using treatments, such as chitosan, can preserve quality and retain TA during extended storage.
• Storage: The storage time and conditions are crucial. The longer the storage time, the greater the reduction in TA. Controlled atmosphere conditions with low oxygen and high carbon dioxide, and low temperatures reduce respiration rate to maintain TA levels; see Figure 1.
• Processing: The methods used to process fruits and vegetables to produce value-added products can affect acidity. Sometimes, TA levels are regulated through supplementary products.
• Environment: Seasonal and environmental factors also affect the acidity of fresh produce in the orchard.
• Packaging type: Packaging such as Modified Atmosphere packaging (MAP) is designed to control gas mixtures to lower respiration rates, increasing TA.

Figure 1: “Effect of storage temperature on titratable acidity (TA) of (A) ‘Garmrok’, (B) ‘Hayward’, (C) ‘Goldone’, and (D) ‘Jecy Gold’ kiwifruit,” Cha et al. (2019). (Image credits: https: //doi .org/10.7235/HORT.20190062)

Using a combination of methods, from cultivation to packaging, can reduce TA loss more than using only one corrective measure.

Titratable Acidity Measurements

TA can be measured in the laboratory through titration with sodium hydroxide (NaOH). However, these are destructive, time-consuming, and require specific analytical skills and space. Therefore, the industry is increasingly moving towards the use of near-infrared spectroscopy-based devices for non-destructive, real-time, precise analysis. Felix Instruments Applied Food Science produces several quality meters that can measure titrable acidity. It has one general instrument and several others customized for fruits such as mangos, avocados, grapes, kiwifruit, and melons.

Contact us at Felix Instruments Applied Food Science to learn more about quality meters for TA measurements.

Sources

 

Freshknowledge. (n.d.). Titratable acid. Retrieved from https://www.freshknowledge.eu/en/increase-your-knowledge/how-to-deal-with-fresh-produce/doing-quality-measurements.htm#Titratable_acid_measurements-anchor

 

Cha, G.H., Prathibhani, H.M., Kumarihami, C., Kim, H.L., Kwack, Y.B., & Kim, J.G. (2019). Storage Temperature Influences Fruit Ripening and Changes in Organic Acids of Kiwifruit Treated with Exogenous Ethylene. 원예과학기술지, 37(5), 618-629.

https: //doi .org/10.7235/HORT.20190062

 

Hassan, A. H., & Emam, M. S. (2015). Improving fruit quality and storability of strawberry fruits by using pre and postharvest treatments. Journal of American Science, 11(1), 44-60.

 

Mukherjee, A., Gómez-Sala, B., O’Connor, E. M., Kenny, J. G., & Cotter, P. D. (2022). Global regulatory frameworks for fermented foods: a review. Frontiers in nutrition, 9, 902642.

 

Omar, A. F., & MatJafri, M. Z. (2013). Principles, methodologies and technologies of fresh fruit quality assurance. Quality Assurance and Safety of Crops

 

Paul, V., Singh, A., & Pandey, R. (2010). Determination of titrable acidity (TA). Post-harvest physiology of fruits and flowers, 44.

 

Sadler, G. D., & Murphy, P. A. (2010). pH and titratable acidity. In Food analysis (pp. 219-238). Boston, MA: Springer Us.

 

Semmelmeyer, E. (2006). OECD guidance on objective testing to determine the ripeness of fruit. Journal of fruit and ornamental plant research, 14, 101.

 

Suszek, G., Souza, E. G. D., Nóbrega, L. H. P., Pacheco, F., & Silva, C. T. A. D. C. (2017). Use of yield and total soluble solids/total titratable acidity ratio in orange on group definition for standard DRIS. Revista Brasileira de Fruticultura, 39(4), e-876.