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From: http://www.nal.usda.gov/wicworks/Topics/FG/Chapter4_InfantFormulaFeeding.pdf

Discard any infant formula remaining after a feeding. The mixture of infant formula with saliva provides an ideal breeding ground for disease-causing micro-organisms.

Why is this so? Doesn't Saliva contain anti-bacterial properties?

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Doesn't Saliva contain anti-bacterial properties?

Yes, in particular some IgA antibodies and lysozyme, but there are also plenty, and I mean plenty of bacteria in your mouth at any given point; we do, after all, still have to brush our teeth! In fact, animal (including human) bites should be treated lest they lead to sepsis. Those bacteria would love all the magnificent nutrients in infant formula. Contact with an infant's mouth will introduce bacteria to the formula, as will exposure to air, contact with your hand, and anything else it experiences. As an example, in the past few years, there have been some cases (from China, I believe, someone correct me if I'm wrong) of formula contaminated with bacteria, which were more than happy to stick around.

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Consider also that the neonatal immune system isn't yet fully working. In fact, one of the reasons breastfeeding is important is transfer of IgA from the mother. Even when antibody production starts after 4-6 months, they don't bind as well. "In addition to quantitative differences in antibody production during early life versus adulthood, there are also qualitative differences. IgG and IgA responses to pathogens, although inducible, are relatively weak during the first year of life, being short-lived and of low avidity." (Jaspsen et al., 2006) –  Gossar Aug 28 '13 at 14:26
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Whether infant formula or food for adult consumption - should be discarded if there is salivary contamination since there is universal presence of microorganisms in saliva. These act as culture media and the microorgnisms rapidly degrade the food, not to mention the selective overgrowth of pathogenic microorganisms.

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