Preserving Whole Tomato Concentrate with Chemical Additives

Tomatoes are among the most perishable vegetables on the planet. With over 90% water content and active enzymatic activity, fresh tomatoes can begin to deteriorate within days of harvest – losing color, texture, and nutritional value rapidly. Converting tomatoes into concentrate is one of the most effective ways to capture their value post-harvest. But even concentrate needs protection. Without proper preservation, it becomes a substrate for microbial growth within weeks. This is where chemical preservatives – specifically potassium metabisulphite (KMS) and sodium benzoate – play a critical role in extending shelf life and keeping the product safe, flavorful, and market-ready.

Why tomato concentrate is prone to spoilage 🔗

Even after concentration, tomato-based products retain enough moisture and organic compounds to support microbial growth. Bacteria, yeasts, and molds find the nutrient-rich environment favorable, especially at room temperature. According to research published in the Asian Journal of Food and Agro-Industry, tomato pulp and juice are highly susceptible to microbial spoilage, and without chemical intervention, deterioration – including fermentation, sedimentation, and off-flavor development – can begin within 30 to 40 days at room temperature. Color fading and flavor loss accompany microbial spoilage, making preservation both a safety and quality concern.

The natural acidity of tomatoes (pH typically between 4.0 and 4.5) is an advantage. Many preservatives work more effectively in acidic environments, meaning tomato products are actually well-suited to chemical preservation when handled correctly.

The two key chemical preservatives used 🔗

Potassium metabisulphite (KMS) 🔗

Potassium metabisulphite, chemically represented as K₂S₂O₅, is a white crystalline powder with a characteristic sulfur odor. When dissolved in water, it releases sulfur dioxide (SO₂), which acts as an active antimicrobial and antioxidant agent. It is registered as a food additive under the code E224 and is widely used in fruits, vegetables, juices, and concentrates.

In tomato concentrate, KMS serves two important functions. First, it inhibits microbial growth – the released SO₂ disrupts microbial cell walls, interferes with enzyme systems, and converts into sulphur dioxide in aqueous solution, which acts as an active antimicrobial compound by reducing disulphide linkages and inhibiting respiratory mechanisms in bacteria, molds, and yeasts. Second, it prevents oxidative browning. Potassium metabisulphite inhibits enzymatic browning reactions in tomatoes by blocking phenolic compounds from reacting with enzymes, thereby maintaining the bright red color that consumers expect in concentrate.

Its antioxidant properties are equally significant. When KMS is dissolved in water, it releases sulfur dioxide which reacts with oxygen – making it a powerful antioxidant that helps protect both color and delicate flavors during storage. Research confirms that tomato juice preserved with KMS remained acceptable for up to 45 days at room temperature – though sodium benzoate tends to outperform it in longer storage studies.

The European Food Safety Authority (EFSA) has set a maximum recommended daily intake for sulfites at 0.7 mg/kg body weight per day (expressed as SO₂ equivalent), and its use is regulated across food categories. It should not be used on fresh or raw fruits and vegetables or in meat products.

Sodium benzoate 🔗

Sodium benzoate (C₇H₅NaO₂), also labeled as E211, is the sodium salt of benzoic acid. It is a white, odorless crystalline powder that is highly soluble in water, making it easy to incorporate uniformly into liquid and semi-liquid products like tomato concentrate.

Its antimicrobial mechanism is pH-dependent. When sodium benzoate dissolves in water, it dissociates into benzoate ions, which then combine with protons to form undissociated benzoic acid – and it is this undissociated form that carries the inhibiting activity. In acidic environments like tomato products (pH 4.0-4.5), the compound is highly effective. The undissociated benzoic acid penetrates microbial cell membranes, lowers intracellular pH, and inhibits cellular uptake of substrate molecules, resulting in oxidative stress and inhibiting cell growth.

Sodium benzoate is particularly effective against yeasts and molds, which are among the primary spoilage agents in tomato concentrate. Studies comparing different preservatives found that tomato products preserved with sodium benzoate were more stable than those preserved with KMS or sorbic acid, remaining acceptable for the full 60-day observation period at room temperature. The FDA limits its concentration as a food preservative to 0.1% by weight, and it is classified as Generally Recognized as Safe (GRAS) within these limits.

Importantly, a combined use of KMS and sodium benzoate has shown even better results. Research found that tomato paste treated with a combination of sodium metabisulphite and sodium benzoate showed no appreciable changes in chemical properties even after 40 weeks of ambient storage, far outperforming samples treated with either compound alone.

Step-by-step process for preserving whole tomato concentrate 🔗

Producing chemically preserved tomato concentrate requires careful attention at every stage – from selecting raw material to final sealing. Below is a standard procedure used in post-harvest processing.

Step 1: Selection and sorting of tomatoes 🔗

Begin with fully ripe, firm tomatoes free from visible rot, cracks, or insect damage. Overripe or damaged fruit introduces higher microbial loads and reduces the quality of the concentrate. Uniform ripeness ensures consistent color and flavor in the final product.

Step 2: Washing and cleaning 🔗

Wash the selected tomatoes thoroughly under running water to remove soil, pesticide residues, and surface microorganisms. A clean water rinse is critical before any processing begins, as surface contamination can carry spoilage organisms into the concentrate.

Step 3: Blanching 🔗

Blanch the cleaned tomatoes by immersing them in boiling water (100°C) for approximately 3 to 5 minutes. Blanching inactivates naturally occurring enzymes – particularly peroxidase and lipoxygenase – that would otherwise cause color degradation and off-flavors during storage. It also softens the skin for easier peeling. As established in tomato powder processing research, thermal blanching is a standard step used to inactivate enzymes and improve the quality of tomato-derived products.

Step 4: Peeling and pulping 🔗

After blanching, the tomato skins slip off easily. Peel the tomatoes and remove seeds if desired. Crush or blend the peeled tomatoes into a smooth pulp using a blender or pulper. At this stage, the tomato is essentially a raw juice or slurry with full microbial and enzymatic activity still possible.

Step 5: Straining 🔗

Pass the pulp through a fine mesh sieve or strainer to remove remaining seeds, skin fragments, and fibrous material. This produces a smooth, uniform concentrate base with a consistent texture.

Step 6: Heating and concentration 🔗

Transfer the strained pulp into a clean stainless steel pan and heat it over medium flame, stirring continuously to prevent scorching. The goal is to evaporate excess moisture and reduce the volume by roughly 50-60%, resulting in a thick, concentrated paste. This step increases the total soluble solids (TSS) content, which itself provides some protection by reducing water activity. Heat the concentrate to at least 85-90°C before removing from flame.

Step 7: Cooling 🔗

Allow the concentrate to cool to approximately 60-65°C before adding chemical preservatives. Adding preservatives while the product is still too hot can degrade them – particularly KMS, which can lose sulfur dioxide through volatilization at high temperatures, reducing its preservative efficacy.

Step 8: Adding chemical preservatives 🔗

This is the most critical step. Dissolve the preservatives in a small amount of the concentrate or distilled water before adding to the bulk, to ensure uniform distribution.

• Potassium metabisulphite (KMS): Add at a rate of 500-700 ppm (0.05-0.07% of the total weight). Stir thoroughly to ensure even distribution.
• Sodium benzoate: Add at 0.05-0.1% of total weight (500-1000 ppm). Stir well until completely dissolved.

Using both preservatives in combination is recommended, as they operate through different mechanisms and together provide broader antimicrobial coverage. Research comparing tomato puree samples treated with different preservative combinations found that samples treated with 0.05% potassium metabisulphite plus 0.05% sodium benzoate, along with citric acid, showed the best physicochemical and sensory performance over the storage period. Citric acid may optionally be added (0.1-0.2%) to further lower pH and enhance the activity of both preservatives.

Step 9: Hot-filling and sealing 🔗

While still hot (above 80°C), ladle the preserved concentrate into pre-sterilized glass jars or bottles. Fill to the brim to minimize headspace and reduce oxygen exposure, which could otherwise trigger oxidative deterioration even in the presence of preservatives. Seal the containers immediately with airtight lids. Invert the sealed jars for 2-3 minutes to sterilize the lid and create a vacuum on cooling.

Step 10: Labeling and storage 🔗

Once fully cooled, label each container with the date of preparation and preservative content. Store in a cool, dry location away from direct sunlight. While chemical preservatives allow room-temperature storage, storage at lower temperatures (4°C or below) further reduces changes in lycopene, beta-carotene, and ascorbic acid content, preserving nutritional quality alongside microbial stability.

Effect of preservatives on quality parameters 🔗

Chemical preservatives do more than prevent spoilage – they actively stabilize key quality attributes of tomato concentrate throughout its shelf life.

Color retention 🔗

The red color of tomato concentrate is primarily due to lycopene and beta-carotene. Oxidative reactions during storage can degrade these pigments, leading to a dull, brownish appearance that reduces consumer acceptability. KMS specifically prevents oxidative and enzymatic browning, stabilizing the vibrant red color expected in quality concentrate. Studies confirm that color fades gradually in unpresevered samples, while preserved samples maintain better visual quality throughout the storage period.

Flavor stability 🔗

Off-flavors in stored tomato concentrate typically develop from microbial activity or oxidation of volatile flavor compounds. Both KMS and sodium benzoate suppress the microbial populations responsible for fermentation and off-flavor production. Research tracking tomato juice and concentrate samples noted that color faded and off-flavor developed at the end of storage at room temperature, with the progression being more pronounced in samples with weaker preservatives or none at all – confirming the role of sodium benzoate in maintaining flavor integrity for longer periods.

Nutritional quality 🔗

One trade-off of chemical preservation is the gradual decline of heat-sensitive nutrients, particularly vitamin C (ascorbic acid). Research consistently shows that negligible changes in chemical constituents except vitamin C were observed in preserved tomato juices throughout a 60-day storage period. Lycopene and beta-carotene are relatively more stable under preservation conditions, particularly at lower storage temperatures. This underscores the importance of combining chemical preservation with appropriate temperature control to retain maximum nutritional value.

Regulatory considerations and safe use 🔗

Both KMS and sodium benzoate are approved food additives recognized by international regulatory bodies including the U.S. Food and Drug Administration (FDA) and the European Food Safety Authority (EFSA). Their use is subject to maximum concentration limits precisely because overuse carries risks. Sodium benzoate, when combined with ascorbic acid under certain conditions, can form benzene – a known carcinogen – making careful formulation important when adding vitamin C alongside this preservative. KMS can cause allergic reactions in sulfite-sensitive individuals, particularly those with asthma. Processors must strictly follow recommended dosage limits and label products accordingly, especially for markets with mandatory allergen declaration requirements.

As with all chemical additives, the principle is straightforward: use the minimum effective concentration needed to achieve preservation, combined with good manufacturing practices like proper sterilization, hygiene, and appropriate packaging.

What do you think? Given that sodium benzoate outperforms potassium metabisulphite in extended storage studies, would using sodium benzoate alone be sufficient for commercial tomato concentrate, or is a combination approach always preferable? And as consumers increasingly scrutinize food labels, how should processors balance chemical preservation with the growing demand for clean-label products?

References

1. https://scialert.net/fulltext/?doi=ajft.2011.914.923
2. https://foodadditives.net/preservatives/potassium-metabisulfite/
3. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/potassium-metabisulfite
4. https://lingyue-foodchem.com/potassium-metabisulfite/
5. https://annexechem.com/guide-to-potassium-metabisulphite/
6. https://infocons.org/blog/2024/07/16/e224-potassium-metabisulphite/
7. https://foodadditives.net/preservatives/sodium-benzoate/
8. https://foodmicrobiology.academy/2020/05/09/use-of-sodium-benzoate-e211-in-food-preservation/
9. https://www.researchgate.net/publication/338471839_Effect_of_chemical_preservatives_on_the_shelf_life_of_tomato_juice
10. https://en.wikipedia.org/wiki/Sodium_benzoate
11. https://www.researchgate.net/publication/303248263_Effect_of_Chemical_Preservatives_and_Storage_Conditions_on_the_Nutritional_Quality_of_Tomato_Pulp
12. https://www.foodscijournal.com/articles/afns-aid1038.php
13. https://www.researchgate.net/publication/269645189_Effect_of_Chemical_Additives_on_the_Shelf_Life_of_Tomato_Juice
14. https://www.fda.gov/