Is there a natural bread preservative?

Lecithin

Helps keep bread fresher longer and works with the gluten
to make a lighter bread. It also helps make the bread moister
and acts as a mild preservative. Made from soy or egg yolks.
Comes in liquid or granular form.
Use: 1 Tablespoon per cup of flour  

Non-Diastatic Malt

Super food source for the yeast which give the bread better
structure & makes the bread softer & tender. Made
from dried sprouted barley and is not the same as malted
milk powder. Comes in liquid or granular form.
Use: 1/4 teaspoon per cup of flour  

Ascorbic Acid

Creates an acidic environment for the yeast which helps
it work better. It also acts as a preservative & deters
mold and bacterial growth. If you can't find pure ascorbic
acid crystals you can use Fruit Fresh (canning isle) or a
crushed/powdered vitamin C tablet.
Use: 1/8 teaspoon per loaf  

Dry Acid Whey

It is the essence of buttermilk with out the milk solids.
Like with Ascorbic Acid it helps create a good environment
for the yeast work quickly and vigorously, giving a maximum
rise in short periods of time. Acts as a preservative & deters
mold and bacterial growth. When buying Dry Acid Whey make
sure it says "acid" on the package. If it doesn't
assume it is sweet whey which isn't the same and won't work
correctly.
Use: 1 teaspoon per cup of flour  

Vital Wheat Gluten

Vital wheat gluten occurs naturally in all wheat and wheat
derived white flours. Some white flours have more or less
than others. Vital wheat gluten only does one thing, it helps
improve the rise and texture of bread. With out it you have
a rock, door stop, paper weight. Generally, if you are using
white bread flour you don't need to add any. However, all-purpose
or whole grain flours need Vital Wheat Gluten.
1 Tablespoon per cup of flour  

Pectin

Pectin adds moistness to the bread and it replaces fat in
the bread. This is the same pectin used to make jams and
jellies. It comes in liquid and granular form. The granules
are easier to work with and store.
Use: 1 teaspoon per loaf  

Ginger

Ginger is a yeast booster it gives it a "quick-start",
and keeps it working. Because of its astringent properties
it also helps keep the bread fresher longer and it deters
mold and bacterial growth. It is best to used powdered ginger
in your bread. You don't have to worry you won't taste it
in the amount used.
Use: 1/4 teaspoon per loaf  

Dry Milk

Milk helps with crust browning, bread moisture, taste and
nutritional value. It also helps the dough to relax for those
times you want to roll it out or shape it. Dry milk or powdered
milk work the same.
Use: 1 Tablespoon per cup of flour  

Gelatin

Gelatin helps with bread texture and moisture. It is also
of nutritional value and is good for the hair and fingernails.
Make sure to use unflavored gelatin.
Use: 1 teaspoon per loaf  

Fats

Fats. Fats help with taste, texture and the moisture of
the bread. Most French bread recipes don't contain fat as
it takes away the chewiness of the bread. You don't need
to be worried about the fat content of most bread. Most recipes
use a tablespoon or two and that is for the whole loaf. A
single slice is very low in fat.
1 Tablespoon per cup of flour  

Eggs

Eggs add rise, color, texture and taste to bread. Also,
if you use the yolk as well you get some of the effects like
using lecithin.
Use: 1 large egg replaces about 1/4 cup of liquid in the
recipe.  

Buttermilk

Buttermilk helps the yeast work quickly and vigorously,
giving maximum rise in the time frame allotted by bread machines.
It also softens the texture of the bread. Like with any acid
type addition it also helps keep the bread fresher longer
and it deters mold and bacterial growth. You may need to
add 1/2 to 1 tsp. of baking soda to the bread to offset the
tartness of the buttermilk. I personally, like the tartness
as it reminds me of sourdough.
Use: 1/2 Cup replaces the same amount of other liquid in
the recipe.  

Garlic

Garlic is a flavoring in larger amounts, but in smaller
amounts it helps the yeast, it makes the dough easier to roll
out and it is a preservative and deters mold and bacterial
growth.
Use: 1 teaspoon per loaf, will affect flavor  

Cake Flour

Cake flour makes for a softer more tender bread. It also
makes a good addition to pizza dough as it helps make rolling
out the dough easier.
Use: Replace up to 1/4 of the flour called for in the recipe
(no more).