Home » Correctly engineered applied postharvest technologies

Correctly engineered applied postharvest technologies

By Jan Lievens

This could save a company big money by giving them access to a quality platform – and what the customer is looking for.

Let us look a bit closer at what is really involved in this process…

Fresh horticultural commodities are unique packages of water! In fact, freshness is water, and freshness sells. Water loss is one of the main causes of deterioration that reduces the marketability of fresh fruits and vegetables. Profitability in fresh fruit, vegetable, and flower sales depends on the ability to deliver as much of this water as possible to consumers.

Effects of water loss

Many fruits, vegetables, and flowers become shrivelled after losing only a small percentage of their original weight due to water loss. Severe desiccation results in considerable losses, for example, wilted leafy vegetables may require excessive trimming to make them marketable, and grapes may shatter loose from clusters if their stems are severely dried. Seriously shrivelled fruits, vegetables, and flowers are effectively un-marketable and must be discarded. Water loss represents saleable weight loss and reduced profits.

Factors that affect water loss include relative humidity, the temperature of the product, the products surrounding atmosphere, and air velocity. Water loss from warm products to warm air is particularly serious under windy-conditions, or during transport in an open vehicle.

Humidity

The quality of fresh produce in storage depends to a great extent on the humidity. Humidity is more difficult to control than temperature and often does not receive adequate consideration when storage areas are designed.

If the air is too dry, there may be enough water loss to affect the texture and cause visible shrivelling or wilting. It can even make the product un-sellable. It is known that there are certain varieties of Stone fruit that have deterioration from as little as 0.8% weight loss.

Fruits such as apples and pears are most resistant to moisture loss, but during several months of storage they may lose 2-3% or more in weight because of water loss. A moisture loss of 4-5% results in spongy texture and visible shrivelling of these fruits.

Table grapes get dry stems, brown berries and other problems. Blueberries start shrivelling very quickly which results in big and fast losses for producers.

Excessive humidity, on the other hand, is conducive to growth of mould and decay organisms, particularly when water droplets form on the surface of pome and drupe fruits.

There is increasing evidence that extremely high humidity, particularly in the early part of cold storage, can contribute to physiological disorders in certain cultivars of apple.

With most commoditi es, however, the problem is one of maintaining enough moisture in the storage. (A few vegetables such as onions, garlic, squash, and pumpkin require low relative humidity.)

Vegetables are, in general, very susceptible to moisture loss in storage, with leafy vegetables losing moisture most readily; in an unfavourable environment they can suffer damaging water loss within only a few hours.

A moisture loss of 4% or more may necessitate trimming of the outside wilted leaves. Softening or wilting of root crops or cabbage heads will be apparent when the total moisture loss exceeds 5-6%, whereas moisture loss more than 8% renders the product un-sellable.

Unlike pome and drupe fruit, that are susceptible to increased decay and physiological disorders at high relative humidity, most vegetables requiring storage at high relative humidity are resistant to increased decay or physiological disorders.

For most vegetables that are susceptible to rapid water loss, the incidence of decay is usually not accelerated by the presence of condensation on the surface of the product if storage temperatures are maintained near those recommended for the product.

For a given relative humidity, moisture loss is greater with high produce temperature. Thus, to minimise moisture loss it is essential to cool the produce promptly after harvesting with proper RH control and management.

In a refrigerated storage environment, the best way to maintain high humidity is to use an evaporator coil that is large enough to provide rapid cooling of the air without requiring operation at a low temperature combined, but you will need added professional RH applications.

An undersized cooling coil must be operated with a low surface temperature to cope with demands, especially during loading of the storage, that cause moisture to condense and freeze on the coil and effectively remove water from the storage environment. This lowers the humidity and results in abnormal moisture loss from produce. Also, the accumulation of frost reduces the air flow over the coil and lowers its cooling efficiency even further. There is proof of how humidity of the storage atmosphere is related to the temperature of air leaving the coil.

The use of jacketed storages is one way of providing for a large cooling surface to minimise product moisture loss. However, the main constraints in the application of jacketed storages to fresh produce are a lack of pre-cooling capacity, growth of microorganisms, changes in product flavour and texture in response to high humidity, and additional construction costs.

Where adequate humidity can be obtained in no other way, water should be added to the storage by humidifiers that introduce water in a gas form. It is extremely important not to spray water directly on the produce because any water on the surface of the produce encourages microbial growth. Alternatively, produce stored in bulk bins (about 385 kg) or field boxes (about 20kg) may be enclosed with a perforated polyethylene to maintain an atmospheric humidity of 94-98%.

Please note:

A polyethylene barrier around produce that has not been pre-cooled slows field heat removal and increases deterioration of the product.

Whenever mentioned in this article, humidity is expressed in terms of relative humidity (RH). RH is the actual amount (or percentage) of moisture in the atmosphere at a given time as related to the maximum amount (100%) that could be retained at the same temperature.

The movement of moisture between an object and the atmosphere depends on the relative, not the absolute, humidity. The RH of the atmosphere changes with the temperature.

As the temperature is reduced, the RH increases to 100%, at which level the atmosphere is said to be saturated.

The temperature at which this occurs is called the dew point. And that is why you need controlled humidity management.

And remember maintaining high relative humidity is sometimes difficult because refrigeration removes moisture, think of your own fridge in the kitchen.

In my next article, I will go into detail around the question: what is water and what are the simpler scientific explanations.

About Jan Lievens

Jan Lievens, born in Belgium, is a graduate civil engineering(B) and international senior consultant for engineered applied postharvest technology at UTE South Africa. With over 20 years of experience in this field, he is widely regarded as a specialist in the fruit-, vegetable- and flower industry with regards to humidity, airborne bacteria and ethylene removal, both locally and internationally. Furthermore, he also designed airflow-friendly packaging systems for the industry with proven results.

Jan Lievens