Today is :

Archives (2003)

Ready-to-cook vegetables: How safe are they? by Ma. Lizbeth J. Baroña

April-June 2003 Volume 5 No. 2

The packed, ready to cook vegetables found in local and supermarkets, a product of the ingenuity of our local wet market suki, cuts down preparation time by a considerable amount. For most people caught in the rush of the daily grind, it's supposed to be heaven-sent. However, a study at the Postharvest Horticulture Training and Research Center (PHTRC) in the University of the Philippines Los Banos says if not properly treated, the packed vegetables can pose risks.

It will help to ask a question like: how long have the sliced and packed vegetables been sitting on the market stand? The longer the time between slicing, packing, and actual cooking, the bigger the chance for the packed vegetables to deteriorate, and therefore, not safe to eat. Here's why:

Changes after 'wounding'
Before cooking, vegetables are usually peeled, cut, sliced, or trimmed. The scientists term this actions as "minimal processing". What we do not know is that by doing so, we are actually making a "wound" on the vegetable. This "wound" causes changes inside the vegetable that speeds up its decay.

At the PHTRC, a group of scientists made a study to determine the effects of peeling, cutting, and packing on the vegetables. The scientists also determined which packing material holds off decaying longer and how temperature helps in keeping the vegetables fit for eating. The materials tested are polyethylene (PE) polyvinylchloride (PVC) bag. The PE bags are the plastic bags we commonly use in packing sugar and other food products, while the PVC bag is the plastic material used in wrapping food on a Styrofoam tray.

Scientists observed two main factors that change inside the "wounded" vegetable: the respiration rate, and the ethylene production. The faster the respiration rate, the shorter the shelf life of the vegetable. Ethylene is a natural plant hormone, or substance, that is responsible for the vegetable's ageing. These two factors determine how long a produce can be stored.

Putting the veggies to the test
To determine how wounding would later have an effect on a stored vegetable, they cleaned and disinfected samples of sayote, bell pepper, cabbage, carrot, radish, eggplant, sitao, kangkong, squash, and ampalaya at 20°C. Later, they were dried, cut and stored at 5°C.

Ready-to-cook mixed vegetables packed were samples of three popular dishes: chopsuey, sinigang, and pinakbet. The vegetables were cut, packed, and sealed in PE bags, or in Styrofoam trays wrapped with PVC plastic, They are stored in 5 and 15°C. This is how wet markets and supermarkets pack the vegetables they sell.

Changes from within
Compared to vegetables which have not been cut or peeled, or those that remain whole, the sample vegetables which were peeled, cut and stored, produced more ethylene, the substance that makes them age. Therefore, the peeled and cut vegetables aged faster.

The ageing of the cut vegetables slowed down when it was transferred from a storing place of 20°C to 5°C. This led the scientists to believe that storing the cut vegetables in a chilled place slows down the ageing substance from being produced.

The same effect was observed in the cut, mixed vegetables packed in PE and PVC bags. Likewise, the vegetables produced more ageing substance after they were cut slowing down only when the packed vegetables were transferred to 5°C.

The scientists also observed a lower level of oxygen in the vegetables packed in PE bag as compared to the vegetable packed using PVC bag. They believed it was because the PE bag was sealed tighter, as it was electronically sealed. The PVC bag was sealed using a packing tape. Changing levels of oxygen and carbon dioxide increase respiration rate inside in the vegetable causing diseases to develop in the vegetable.

The study concludes that deterioration of the vegetables can also be caused by it losing moisture, which sometimes depends on the kind of vegetable, and on how thick or thin they were cut.

What you can do
Deterioration in fresh produce is inevitable, even without interference. Regardless of how it was cut or sliced, minimally processed peeled, trimmed, and cut - vegetables or fruits showed hastened signs of decay as compared to the whole or intact vegetables.

The key to a safer and longer shelf- life of a produce, as recommended by the study, is proper temperature and packing treatments. Vegetables like sayote, sitao, eggplant, squash and ampalaya the vegetables which showed higher ethylene production - should be packed 3 hours after cutting when the production of ethylene, the substance causing ageing, has declined.

While unpacked, cut vegetables should be stored at 5°C to arrest moisture loss, ageing, and breaking down. If stored at 15°C, you can still hope to treat your family to a dinner of sinigang even at 5 days after packing. Even better if held at a chilled temperature of 5°C, your packed chopsuey will still be toothsome after 14 days.

Next time you drop by the market, you might want to take a closer look at the packed vegetables sold. With this know-how in mind, you are assured of an instant, worry-free dinner.

Sources: Physiological and microbial changes of packed ready-to-cook vegetables during storage by: Noida B. Flor, Gladys S. Ocfemia, Elda B. Esguerra, Gloria D. Masilungan and Ofelia K. Bautista of Post Harvest Training and Research Center, University of the Philippines Los Banos, College Laguna. For more information, contact: 049-535-3138; 536-2444; fax 049 536-3259

[More 2003 Articles]