The proceeds of refunding bonds shall either be immediately applied to the retirement of the bonds to be refunded or be placed in escrow to be applied to the payment of the bonds upon their presentation; provided, however, any accrued interest and any premium appertaining to a sale of refunding bonds may be applied to the payment of the interest or the principal or both interest and principal or may be deposited in a reserve therefor as the board may determine. The escrow shall not necessarily be limited to refunding bond proceeds but may include other money made available for such purpose. Any escrowed proceeds pending such use may be invested or reinvested in federal securities. Escrowed proceeds and investments, together with any interest to be derived from any such investment, shall be in an amount at all times sufficient as to principal, interest, any prior redemption premium due and any charges of the escrow agent payable therefrom to pay the bonds refunded as they become due at their respective maturities or due at designated prior redemption date or dates upon which the board shall exercise a prior redemption option. Upon establishment of an escrow in accordance with this section, the refunded bonds payable therefrom no longer constitute outstanding indebtedness of the authority.