Using Sanitizers in Wash Water: Part 1 - Reasons for Washing Fresh Produce

This series of three technical videos provides detailed information on best practices to maintain produce quality and safety following the harvest of fresh fruits and vegetables.
Using Sanitizers in Wash Water: Part 1 - Reasons for Washing Fresh Produce - Videos


Food safety considerations are critically important during and immediately after washing fresh fruits and vegetables.

Parts 2 and 3:

Using Sanitizers in Wash Water: Part 2 - Types of Wash Systems

Using Sanitizers in Wash Water: Part 3 - Correct Use of Sanitizers


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More by Luke LaBorde, Ph.D. 

Lee Stivers

View Transcript

- [Voiceover] Using sanitizers in wash water.

To maintain the quality and safety of fresh fruits and vegetables, it's important to understand reasons for specific post harvest procedures and the potential food safety risks associated with them if we conduct them incorrectly.

This is particularly true when washing freshly picked produce.

There are three videos in this wash water video series.

In the first video, we're going to discuss the reasons you might need to wash fresh produce.

In the second video, we will review the main types of washing systems.

And finally, in the last video, we will cover the proper use of sanitizers in wash water.

Reasons for washing fresh produce.

Whether you're selling direct at farm markets, produce auctions, or to a retail food store, you already know that your customers are more likely to buy fresh fruits and vegetables that look wholesome and nutritious.

Supermarkets know what sells and they are increasingly requiring growers to deliver produce that has been washed to remove adhering soils.

Here are a few examples of why we might need to wash fresh produce.

Certainly, it's harder to sell products that are covered with soil, like these carrots, or these root vegetables.

Your customers want fruits and vegetables to look attractive.

Here you have some green onions.

Since they grow in the ground, they're going to pick up some soil.

These washed onions look much nicer and will likely command a higher price at the market.

Here are some melons on display at a produce auction.

Nice, attractive displays are the ones that get bid on.

In general, fruits or vegetables that grow closer to the soil are more likely to become soiled.

In this diagram, we can see the likelihood of contamination by following the arrow.

Starting high up in the apple tree, going all the way down to the spinach which grows close to the ground.

That's because low growing crops easily get splashed with soil.

If raw or incompletely composted manure is added to the soil, if domestic and wild animals and their droppings are close by or if runoff and rain splash can get on the plant.

You can't do too much to change the natural height of a crop plant, but there are some things you can do before harvesting to avoid washing your product.

If your customers prefer or demand produce free from adhering soil, then you should consider growing practices that will minimize soiling of produce.

For example, you can use various mulches like this plastic mulch on raised beds on these tomatoes, or straw mulch on these strawberries.

These are all good methods, but sometimes we just have to wash produce.

Certainly, washing makes the product look better, but does it make produce safer to eat?

Can washing produce actually make it less safe to eat?

Before we continue, let's talk a little bit about what makes a food safe or unsafe to eat.

Whether we are talking about a farm, a retail market, a processing plant, or in the kitchen, food safety issues are primarily connected to microorganisms, or microbes for short.

Most microbes are harmless or even beneficial to us.

Some can cause food to decay or spoil.

Soils are naturally abundant in these kinds of microorganisms and some of them are plant pathogens.

You're probably already familiar with the many bacterial and fungal plant pathogens that reduce crop yields or reduce post harvest shelf life and point of purchase quality.

Like, for instance, this cucumber with rhizoctonia fruit rot or these strawberries with gray mold.

But a few types of microbes can cause people to become sick.

These are called human pathogens.

Maybe you've heard about people becoming sick from eating produce contaminated with salmonella, e.Coli, or listeria bacteria.

Human pathogens are not usually present in soils at levels high enough to get someone sick, but pathogen contamination of produce can occur and cause outbreaks if raw or incompletely composted animal manure is applied, if wild or domesticated animals get into the fields, or if contaminated irrigation water is used.

These outbreaks can really cause a lot of damage, serious illness and even death.

Salmonella from Mexican cucumbers sold by a San Diego based produce distributor in 2015 affected 341 people from 30 states, hospitalizing 70 of them and killed two.

A 2011 outbreak of Listeriosis illness linked to Colorado grown cantaloupe contaminated with Listeria monosytogenes sickened 147 people in 42 states and caused 33 people to die.

In 2011, an e. Coli O157:H7 outbreak found in romaine lettuce infected 58 people from 9 states.

In each of these outbreaks, human pathogens in the soil, in irrigation water, or growing in pack houses, were responsible for the contamination.

Let's take a closer look at pathogenic microorganisms so that we can better understand how washing affects them.

Size is a good place to start.

Just how small are microorganisms?

To get a little context, we can compare microorganisms to a grain of salt.

This diagram is drawn to scale and that large red circle represents the size of a little grain of salt and the three smaller circles represent the relative sizes of various types of microorganisms.

After the grain of salt, the next largest is the yeast cell, which is very small, about five microns.

The mold spores are tinier and the tiniest red dot is a representation of a cell of listeria monocytogenes, a human pathogen.

Compared to the salt, you can see just how tiny these microorganisms are and so how easily they could hide in crevices on the surfaces of fruits and vegetables.

You're always going to find microorganisms on produce, but why are they so hard to remove?

Well, there are many tiny cracks and crevices on the surface of fruits and vegetables that can hide bacteria.

Consider the size of a spinach leaf, shown at left.

It's about 50,000 microns long.

That's about two inches.

On the right is one of the many breathing holes, or stomata, that allows carbon dioxide to enter and oxygen to exit the interior of this leaf.

These holes are anywhere from five to 25 microns long.

That's only a fraction of the width of a human hair.

Look at those gray, rod shapes inside the stomata.

Those are very tiny bacteria hiding inside the stomata.

Some might think that we can kill all harmful bacteria on produce by washing them in water containing a sanitizer.

Because bacteria can become so well hidden within cracks, crevices, and even stomata, we cannot count on water to wash them away.

Nor can we assume that adding a sanitizer to wash water is going to completely kill them.

In fact, if you don't wash produce correctly, it's possible to actually make produce less safe and that's because when we wash produce, we're adding moisture.

If we don't manage and minimize the retention of moisture, we could be creating conditions that promote the growth of any harmful microbes that remain in those cracks and crevices.

Okay, let's get back to the how's and why's of correct washing procedures.

We can break it down into three important steps.

The first is to remove visible soil deposits.

Next is the washing step.

The last step is to remove as much moisture as possible from the surface of produce.

The first thing we can do is to inspect the produce and remove any heavily damaged produce.

It makes no sense to waste time washing produce that we can't sell.

Then we want to remove heavy adhered soil deposits manually with a dry brush or using a dry roller brush system.

We do this to prevent our wash water from getting excessively muddy.

We'll understand why this is important in a minute.

Then, we wash with water to dislodge and carry away any remaining soil on the surface.

In most instances, we recommend adding a chemical sanitizer to the wash water, but it is important to again emphasize what we just learned, that washing, even with a sanitizer added, cannot be completely relied upon to kill all bacteria on the surface of produce.

But then, why do we add sanitizers if it's not possible to sterilize the surface of produce?

Consider an apple before and after washing.

The purple dots represent bacteria cells on the surface of the apple.

As you can see, after washing, we still have some bacteria cells on the apple.

Because not all bacteria are removed, those that remain can still grow during later handling and storage steps.

That's why it's so important to take all possible steps to prevent harmful microbes from getting a chance to contaminate produce before we even think about washing it.

We can do that by using good agricultural practices or GAPs to prevent contamination before it occurs, but even if we follow all of the good agricultural practices that minimize pre-harvest contamination, we can't be 100% sure that we will never have a few contaminated items.

So, what then, is the purpose of adding a sanitizer to wash water?

In our example, the sanitizer prevents movement of bacteria from one contaminated apple to others in the wash tank and, therefore, limits the number of apples becoming contaminated.

We can see this in this animation.

When we put that apple in the wash solution, some, but not all, of the bacteria are dislodged into the solution where they can spread to the other apples.

It's literally one bad apple spoiling the barrel, but when we add the right type of sanitizer at the right concentration, those microbes that are swimming around in the wash solution are quickly killed and cannot spread to other fruits or vegetables in the tank.

So, we can see that we need a two pronged approach.

We need to follow good agricultural practices to keep harmful microbes off produce before and during harvest and, if we are going to wash produce after harvesting, kill any microbes that wash off the produce into the wash water tank.

We will go into more detail on washing methods in the next module.

After you remove visible soil deposits and wash with water, the third washing step is to dry the produce.

Not as dry as a raisin, but dry enough to remove as much surface moisture as possible as you now know there will always be some bacteria left on the produce after washing, even with the sanitizer added.

If we do not minimize the amount of surface moisture, we're creating conditions that will accelerate microbial growth later on during storage and shipping and, remember, we're not just talking about food safety issues.

The same conditions can cause rapid product spoilage and decreased shelf life and point of purchase quality.

Do not use a reusable cloth or towel to dry produce.

So, how do we minimize surface moisture?

We'll show some examples in the next module including allowing air drying of fruits and vegetables and centrifuging leafy greens.

To review, we identified consumer preferences as a major push for washing produce.

We identified a few production practices that could minimize the need for washing and if we feel we must wash, we learned that there is a correct way to do this.

We need to prevent the spread of harmful microbes in the wash tank and we need to prevent microbes from growing after washing.

Our next video discusses various types of wash systems.


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